Dr. Jeff Masters' WunderBlog

Heavy Rains From 99L Drench Belize and Mexico

By: JeffMasters, 4:41 PM GMT on August 31, 2014

Tropical wave 99L is spreading heavy rains over Belize and Mexico's Yucatan Peninsula as the storm heads west-northwest at about 10 mph. Belize radar shows little rotation to 99L's echoes, and satellite loops on Sunday morning showed the heavy thunderstorm activity was poorly organized, with few low-level spiral bands. Sea Surface Temperatures (SSTs) were very warm, near 29.5°C (85°F), and wind shear was moderate, 10 - 20 knots. These conditions are favorable for development, but 99L will not be able to develop until it finishes crossing Mexico's Yucatan Peninsula and emerges into the Gulf of Mexico's Bay of Campeche on Monday. The 8 am Sunday run of the SHIPS model predicted that conditions will remain favorable for development over Bay of Campeche during the remainder of the week, with mpderate wind shear, a moist atmosphere, and warm SSTs of 29.5°C (85°F.) None of our three reliable computer models for predicting tropical storm formation showed 99L developing in their Sunday morning runs, though. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 30% and 50%, respectively. If a tropical storm does form in the Bay of Campeche, the most likely track would be to the west-northwest or northwest, with landfall occurring on the Mexican coast south of Texas on Wednesday. An Air Force hurricane hunter aircraft has been tasked to investigate 99L on Monday afternoon, if necessary.

Hurricane expert Steve Gregory has a more detailed look at the tropics in his latest blog post.


Figure 1. Latest satellite image of Invest 99L in the Western Caribbean.

Death Valley ‘Sliding Rocks’ Mystery Resolved
Scientists have long puzzled over how huge rocks on Death Valley's Racetrack Playa managed to slide over the dry lake bed, leaving tracks hundred of feet long. The mystery was solved last week, as detailed by wunderground's weather historian, Christopher C. Burt, in his latest blog post.


Figure 2. Mysterious tracks left by sliding rocks in the dry lakebed of Death Valley's Racetrack Playa as captured by wunderphotographer PugetSoundPost on March 25, 2013.

Jeff Masters

Hurricane

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Invest 99L in Western Caribbean a Threat to Develop on Monday

By: JeffMasters, 2:32 PM GMT on August 30, 2014

A tropical wave in the Western Caribbean was designated Invest 99L by the National Hurricane Center on Friday night, and is headed west-northwest at about 15 mph. Satellite loops on Saturday morning showed that 99L had a modest amount of spin and heavy thunderstorms, but these thunderstorms were poorly organized. Sea Surface Temperatures (SSTs) were very warm, near 29°C (84°F), and wind shear was moderate, 10 - 20 knots. Although these conditions are favorable for development, 99L will likely not have enough time to develop before crossing Mexico's Yucatan Peninsula on Sunday. The 8 am Saturday run of the SHIPS model predicted that conditions will be even more favorable for development on Monday when the wave will emerge in the Gulf of Mexico's Bay of Campeche. Wind shear will remain moderate, the atmosphere will be moister, and SSTs will be warmer: 29.5°C (85°F.) One of our three reliable computer models for predicting tropical storm formation, the GFS, showed some weak development occurring in the Bay of Campeche on Tuesday. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave 99L 2-day and 5-day development odds of 10% and 40%, respectively. The semi-circular ring of high terrain along the southern edge of the Bay of Campeche is known to aid in generating the counter-clockwise spiraling winds needed to assist in spinning up a new tropical storm, and given the propensity of tropical storms to quickly spin up in the Bay of Campeche, I'd put the 5-day development odds at 50%. If a tropical storm does form in the Bay of Campeche, the most likely track would be to the west-northwest or northwest towards the Mexican coast south of Texas. An Air Force hurricane hunter aircraft has been tasked to investigate 99L on Monday afternoon, if necessary.


Figure 1. Latest satellite image of Invest 99L in the Western Caribbean.

Jeff Masters

Hurricane

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The Tropics Go Quiet World-Wide

By: JeffMasters, 3:28 PM GMT on August 29, 2014

Hurricane Cristobal ceased to be at 11 am EDT on Friday, as the storm completed its transition to a powerful extratropical storm. Though Cristobal is no longer a hurricane, it still has hurricane-force winds, and will be a threat to marine interests off the Newfoundland coast today, and to Iceland on Sunday night. With Cristobal's transition to an extratropical storm and the demise of the Eastern Pacific's Tropical Storm Marie earlier today, there are now no named tropical cyclones anywhere in the world--an unusual situation for what is traditionally one of the busiest days of the Northern Hemisphere's tropical cyclone season. This quiet period appears likely to extend though the weekend, as I don't expect any new named storms to form anywhere in the world through Sunday.


Figure 1. MODIS true-color image showing Hurricane Cristobal's off the coast of Massachusetts at approximately 11 am EDT on August 28, 2014. At the time, Cristobal had top winds of 75 mph. Image credit: NASA.

Caribbean disturbance 97L headed towards Mexico
In the Central Caribbean, a tropical wave (designated as 97L by NHC earlier this week, but no longer being labeled as such) is generating disorganized heavy thunderstorms, as seen on visible satellite loops. Wind shear was a high 20 - 30 knots on Friday, and will remain high through Saturday. On Sunday, when the wave will be in the Western Caribbean, shear will fall, but the wave will likely not have enough time to develop before crossing Mexico's Yucatan Peninsula. The wave should emerge in the Gulf of Mexico's Bay of Campeche on Monday, and development odds will be higher then. One of our three reliable computer models for predicting tropical storm formation, the GFS, showed some weak development occurring in the Bay of Campeche on Monday and Tuesday. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave the system 2-day and 5-day development odds of 0% and 20%, respectively. Given the propensity of tropical storms to quickly spin up in the Bay of Campeche, I'd put the 5-day development odds at 40%. If a tropical storm does form in the Bay of Campeche, the most likely track would be to the west-northwest or northwest towards the Mexican coast south of Texas.

New tropical wave off the coast of Africa
A large tropical wave moved off the coast of Africa on Friday, and is headed west at about 15 mph. One of the three reliable computer models for predicting tropical storm formation, the UKMET model, predicts some weak development of the wave five days from now, when the storm was predicted to be headed northwest about 700 miles east-northeast of the northernmost Lesser Antilles Islands. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave the system 2-day and 5-day development odds of 0% and 10%, respectively.


Figure 2. A man wearing a tiny life jacket and clutching a neon green noodle and a pet dog floats on the remains of a house in Waveland, MS, during Hurricane Katrina. The photo was taken from the second floor window of a home, and the water is close to the roof line of the first floor. The home was at an elevation of about 17 feet, and the surge is close to ten feet deep here. There are electric lines running down from a pole to a home from left to right. In the distance on the right is a home with water up to the roof line. The eye is probably overhead, as the water is relatively calm and there appears to be little wind or rain, even though the pine trees are bent from the recent force of the eyewall winds. The photo was taken by Judith Bradford. Her husband, Bill Bradford, swam out and rescued the man and his dog, and two other people who floated by. He reported that the water was nothing like white water, but was a gentle, continuous flow. He was lucky. In the nearby Porteaux Bay area, a woman watched her fiance get pulled from a tree by the force of the current. The man was washed out into the Gulf and drowned. The image above is described in more detail in Part 9 of Margie Kieper's Katrina storm surge web page.

Ninth anniversary of Hurricane Katrina
August 29 is a date indelibly etched into the memory of millions, for on this date in 2005, Hurricane Katrina roared ashore on the Mississippi coast, bringing incredible destruction and suffering. The scale and intensity of the destruction the hurricane brought was extraordinary, and can best be appreciated by viewing two of the best chronicles of Katrina's record storm surge--Margie Kieper's remarkable city-by-city aerial tour of the destruction, and extreme weather photographer Mike Thiess' 13-minute video of his storm surge experience in Gulfport, Mississippi. I had only been blogging for four months when Katrina struck, and was happy that hurricane expert Steve Gregory was on hand that summer to share the blogging effort. Steve has not been blogging since that epic season, but is now back as a featured blogger on wunderground. He plans to provide regular updates on the Atlantic tropical weather at least three times per week.

Nola.com has an interesting feature that allows you to swipe the photos taken nine years ago and see Hurricane Katrina disaster dissolve into present-day recovery (thanks go to wunderground member patrap for this link.)

Jeff Masters

Hurricane

Updated: 3:33 PM GMT on August 29, 2014

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Cristobal Headed Towards Iceland; 98L Moving Inland Over Texas

By: JeffMasters, 3:38 PM GMT on August 28, 2014

Hurricane Cristobal continues to churn northeastwards over the Atlantic towards Iceland. Satellite loops show that Cristobal has its most impressive appearance of its lifetime, with a large symmetric area of heavy thunderstorms. Cristobal will merge with a frontal zone on Friday and transition to a powerful extratropical storm that will likely bring tropical storm-force winds and heavy rain to Iceland on Sunday night. The GOES-14 satellite is in rapid-scan mode over Cristobal on Thursday, and you can access an impressive 1-minute resolution satellite loop of the storm from the NOAA/RAMMB website.


Figure 1. MODIS true-color image showing Hurricane Cristobal's off the coast of North Carolina at 2 pm EDT on August 27, 2014. At the time, Cristobal had top winds of 75 mph. Image credit: NASA.

Gulf of Mexico's 98L moving inland over Texas
A weak area of low pressure (Invest 98L) was centered near the coast at the Texas/Mexico border on Thursday morning. Satellite loops and images from the Brownsville, Texas radar showed the low was generating a few areas of heavy thunderstorms that were slowly growing more organized. Wind shear was a moderate 10 - 20 knots and ocean temperature were a very warm 29°C, which favor development, but 98L should move inland over South Texas on Thursday afternoon, before development into a tropical depression can occur. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave 98L 2-day and 5-day development odds of 10%.

Caribbean disturbance 97L headed towards Mexico
In the Eastern Caribbean, a tropical wave (designated as 97L by NHC earlier this week) is generating disorganized heavy thunderstorms as it heads west to west-northwest. Wind shear was a prohibitively high 20 - 40 knots on Thursday, and will remain high through Saturday. On Sunday, when the wave will be in the Western Caribbean, shear will fall, but the wave will likely not have enough time to develop before crossing Mexico's Yucatan Peninsula. The wave is expected to emerge in the Gulf of Mexico's Bay of Campeche on Monday, and development odds will be higher then. Two of our three reliable computer models for predicting tropical storm formation show some weak development in the Bay of Campeche on Tuesday. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave the system 2-day and 5-day development odds of 0% and 20%, respectively.

New tropical wave coming off coast of Africa this weekend
A large and powerful tropical wave will move off the coast of Africa on Friday evening, and will move near or over the Cape Verde Islands on Saturday as the storm moves west at 10 - 15 mph. None of the three reliable computer models for predicting tropical storm formation predict the wave will develop over the next five days, but wind shear will be low enough and ocean temperatures warm enough to allow some slow development. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave the system 2-day and 5-day development odds of 0% and 30%, respectively.


Figure 2. MODIS true-color image of Hurricane Marie in the Eastern Pacific taken at 19:05 UTC (3:05 pm EDT) on August 26, 2014. At the time, Marie was a Category 2 storm with 100 mph winds. The swirl at the lower left of Marie is the remnant of Tropical Storm Karina, which was destroyed by Marie's strong upper-level outflow winds. A must-see video of the high shear from Marie literally ripping Karina apart is posted at the GOES CIMSS Satellite blog. Image credit: NASA.

Tropical Storm Marie still generating huge waves in Eastern Pacific
The Eastern Pacific's Hurricane Marie was downgraded to a tropical storm on Wednesday, had top sustained winds of just 45 mph at 11 am EDT on Thursday, but was still generating huge swells that will pound the coast of Southern California and Mexico's Baja Peninsula through Friday. A High Surf Advisory is in effect for Los Angeles, where maximum waves of 10 - 15 feet with a few sets up to 20 feet high will potentially cause structural damage to piers and beachside property as well as significant beach erosion. The powerful surf will be accompanied by strong rip currents and long-shore currents, making for very hazardous swimming and surfing conditions through Friday. According to the NWS Los Angeles, Marie's high surf event is the most significant southerly swell (swell only) event in Southern California since July 25, 1996. Satellite loops on Thursday morning showed Marie was basically a swirl of low clouds with no heavy thunderstorms. The storm is over waters cooler than 22°C (72°F) and will steadily degenerate into a remnant low by Friday.

Jeff Masters

Hurricane

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CRISTOBAL heads Out to Sea / Invest 98 in GOM / Disturbance approaches CARIB

By: JeffMasters, 5:05 PM GMT on August 27, 2014

(By Steve Gregory - Subbing for Dr. Masters who is on Vacation.)

HURRICANE CRISTOBAL


CRISTOBAL is located about 360NM SE of Cape Hatteras moving North at around 11Kts. The storm remains a minimal hurricane with sustained surface winds around 65Kts. ALL models call for the storm to turn NNE and then NE at an accelerated pace as an upper air, mid-latitude TROF moves into the Northeastern US. While the storm may see some additional intensification from the high level divergent flow ahead of the NE U.S. TROF and associated baroclinic forcing – the storm will be transitioning rather quickly tomorrow to an extratropical storm as it heads northeast into the NORATL shipping lanes. (I for one am glad to see this extremely tough system to forecast departing our area of interest!)

INVEST 98L and ‘EX INVEST 97L’

While NHC dropped 97L early yesterday – it added INVEST 98L earlier this morning. INVEST 98L in the NW GOM originated at the ‘tail end’ of a very strong mid-level Dry Line that surged south/southwest from the SE US across Florida and into the northern Gulf of Mexico (GOM) 2 days ago – triggering a line of very strong T-storms along and ahead of it. While the dry line boundary has begun to fade away (though relatively dry air continues to dominate much of the GOM) a small surface circulation has formed in the NW GOM, centered about 200NM east of Corpus Christi, TX. An area of convection, with some isolated deep convection, is near and to the North and Northeast of the Low itself. The convection and surface circulation is quasi-stationary, and with wind shear of 30Kts over most of the system, significant development is unlikely for at least the next 24 hours. The first run of specialized tropical cyclone models (12Z cycle) are in remarkably good agreement on forecasting the system to move slowly westward into south Texas in 48-72 hours. With a generally anti-cyclonic flow aloft, and some of the warmest SST’s seen in years in this part of the GOM, and falloff in wind shear to under 15Kts could allow this small disturbance to spin-up prior to moving inland. That said - I doubt the system could ever develop beyond depression intensity.

' EX INVEST 97L' STILL WORTH WATCHING’

While yesterday’s Global model runs had forecast what was 97L to develop into a significant cyclone, the last few runs have completely backed away on this. However, most of the global models now forecast this elongated wave near the eastern CARIB to move across the CARIB towards the Yucatan over the weekend, with some models showing a small cyclone formation in the southern GOM next week.

There has been a dramatic increase in moisture both with the westbound wave and with a moisture plume from the deep (Equatorial) tropics in South America (SOAMER) that has been surging northward ahead of the wave for over 24 hours. Though wind shear is over 30Kts over much of the eastern CARIB (and will remain relatively high for the next few days) there is a somewhat anti-cyclonic flow developing between a small upper air Low/TROF in the west central CARIB and the approaching T.W. in the far eastern CARIB. This anti-cyclonic curvature of the high level wind field may develop further and move westward as the tropical wave traverses the CARIB over the next few days – and shear speeds may drop off during the weekend. Clearly a system worth monitoring over the coming days.

STRONG DISTURBANCE STILL WESTBOUND OVER AFRICA

One of the season's strongest ‘Cape Verde’ disturbances is emerging off the West African coast, with virtually all global models forecasting the system to gradually intensify over the weekend as it heads West/Northwest. This strong wave with a cyclonic circulation field has a long history since it developed in the highlands of east-central Africa last weekend and has maintained a low-mid level circulation and significant convection as it crossed north Africa. The most reliable models (especially the GFS) have consistently shown this system developing into a strong cyclone this weekend and early next week – but also shows the system turning Northwestward and eventually northward as it approaches the central Atlantic. The latest (12Z GFS) model run has shown this recurvature occurring a bit further west than earlier runs - but the ‘theme’ of ultimately turning this system out to sea before it can impact the CARIB or US remains unchanged.

ONCE POWERFUL HURRICANE MARIE POUNDING CALIFORNIA COAST WITH LARGE SWELLS

Hurricane MARIE should soon be downgraded to a Tropical storm – and then Depression - within 24 hours as the storm’s rapidly warming cloud tops clearly indicate the storm is moving over much cooler SST’s (under 24°C). The storm is about 900NM SW of Los Angeles and continues to move W/NW at about 13Kts, and will turn more Northwestward in 24-36 hrs and gradually fade away.

The very large and damaging swells from Marie along the Baja and southern CA coast beaches (and to a lessor degree, the original wave action from Karina which became coherent with the large swells from Marie) appear to have peaked this morning, with reported swell heights overnight reaching 10-15 feet, and isolated 20ft heights in some ‘coves’ along the south/southwest facing beach’s. The NWS has maintained advisories for beaches from Ventura to San Diego for “damaging high surf, very strong rip currents and minor coastal flooding.” The LAX areas expecting the highest surf are Long Beach through the Palos Verdes Peninsula, including Cabrillo Beach and Point Fermin, as well as Malibu and Zuma beaches. The biggest waves are expected to hit in Orange County, especially the Huntington and Newport Beach areas. Wave heights should gradually decline over the next 48 hours, with heights expected to fall off to near normal on Saturday.



Fig 1: Early morning imagery shows departing CRISTOBAL, along with the small Low pressure area in the far NW GOM (INVEST 98L) - the large, elongated Tropical Wave near the eastern CARIB (EX 97L) – and the strong tropical wave/Low near the west African coast. The forecast tracks for these system are based on the consensus of the 00Z Global Model solutions and the 12Z Early Model runs in the case of 98L.



Fig 2: The above Water Vapor image from earlier this AM shows a small convective ‘ball’ with isolated deep convection, and a surface Low that is INVEST 98L. This formed at the tail end of a remarkably strong ‘Dry Line’ that pushed S/SW into the northern GOM 2 days ago. Much of the Gulf remains under a very dry environment – in stark contrast to the rapidly moistening CARIB as a moisture plume continues to develop in the central CARIB along with expansive moisture (and isolated to scattered deep convection) associated with the elongated Tropical Wave in the far eastern CARIB. Based on imagery loops, moisture from the ITCZ south of the Tropical Wave also appears to be moving northward towards the wave itself.



Fig 3: The early morning wind shear analysis shows high shear values of 30Kts+ over 98L and 20-30Kts over the eastern CARIB. There is a chance that the anti-cyclone over the central GOM may shift further northward allowing shear values to fall, allowing for a more favorable venting environment for the system to develop before it moves inland on Saturday.



Fig 4: Should atmospheric conditions become more favorable for development for 98L, SST’s of 30°C or higher will certainly provide more than enough energy for quick development.



Fig 5: The IR image with satellite derived winds shows the still quite strong Tropical Low/Wave complex emerging off the West African coast. It also shows the large convective complex that was over east Africa that developed just west of southernmost Saudi Arabia several days ago. However, unlike the leading wave near the Cape Verdes, the central African wave does not have a well defined rotation – though a fairly strong African Easterly Jet (AEJ) with E/NE winds over 60Kts (not shown) from south of Saudi Arabia and nosing east/southeast into the central African Wave, may yet help to develop this wave. At this point though, none of the models show significant development of this system.



Fig 6: Hurricane MARIE is weakening rapidly, but the large swells generated by the storm earlier this week continue to move into the Baja and southern CA coast. Offshore Buoys show swells heights of 8-11 ft. However, under water topography – and the possibility of ‘rogue’ waves - mean swell heights could still reach 20+ ft along some southern CA locations today and early tonight before swell wave train heights lower significantly.

The next update will be by Dr. Jeff Masters tomorrow.

For those interested , you can follow my Tropical updates on my own blog tomorrow (located Here).

Steve Gregory

Updated: 5:22 PM GMT on August 27, 2014

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Cristobal a Hurricane; Little Change to 97L

By: JeffMasters, 12:25 PM GMT on August 26, 2014

It doesn't look much like hurricane, but the Hurricane Hunters measured surface winds around 75 mph on Monday evening and Tuesday morning in Hurricane Cristobal, making it the third hurricane of the 2014 Atlantic hurricane season. These missions proved the value of hurricane hunter flights, since there is no way that we would have known Cristobal was a hurricane based on satellite data. The storm is stretched out in a long line of heavy thunderstorms, has no eye or low-level spiral bands, and is giving early August's Hurricane Bertha some stiff competition for ugliest Atlantic hurricane of the century. Along with Hurricane Arthur and Hurricane Bertha, Cristobal gives us three Atlantic hurricanes so far this year, exceeding the entire 2013 Atlantic hurricane season total. The second (and final) hurricane of the 2013 season (Ingrid) did not arrive until September 14. On average, the third hurricane of the Atlantic season arrives on September 9, and the third named storm of the year on August 13. The last time the first three named storms in the Atlantic became hurricanes was in 1983, when Alicia, Barry and Chantal all became hurricanes (if we exclude 1992, when an unnamed subtropical storm formed prior to the arrival of Hurricanes Andrew, Bonnie, and Charley.) Cristobal continues to dump heavy rains over the Central and Southeast Bahamas and Turks and Caicos Islands as the storm heads northeastwards out to sea. Satellite loops show that Cristobal is struggling with wind shear, with a center of circulation partially exposed to view, and all the heavy thunderstorms pushed to the south and east sides of the center. The only land area at risk from Cristobal is Bermuda, and the 5 am EDT Tuesday Wind Probability Forecast from NHC gave that island a 27% chance of experiencing tropical storm force winds of 39+ mph. The GOES-14 satellite is in rapid-scan mode over Cristobal on Tuesday, and you can access an impressive 1-minute resolution satellite loop of the storm from the NOAA/RAMMB website.


Figure 1. MODIS true-color image showing Tropical Storm Cristobal's intense thunderstorms stretching from the Southeast Bahamas to Bermuda at 2 pm EDT on August 25, 2014. At the time, Cristobal had top winds of 60 mph. Image credit: NASA.

Little change to 97L headed towards the Lesser Antilles
A tropical wave (Invest 97L) was near 13°N, 47°W on Tuesday morning, about 900 miles east of the Lesser Antilles Islands, and was headed west to west-northwest at about 15 mph. Satellite loops show the wave has changed little since Monday, and has a modest amount of spin but only a small amount of heavy thunderstorms. Water vapor satellite images and the Saharan Air Layer analysis show that 97L is located in a dry environment, which is keeping development slow. Wind shear was a moderate 10 - 20 knots, which should allow some slow development. Sea Surface Temperatures are near 27.5°C, which is warm enough to allow some slow development. The wave should arrive in the Lesser Antilles Islands by Friday and be near Puerto Rico on Saturday, according to the Tuesday morning runs of the GFS model. None of the three reliable computer models for predicting tropical storm formation predict 97L will develop over the next five days. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 97L 2-day and 5-day development odds of 0% and 20%, respectively. These odds are 10% lower than their previous advisory, and NHC has stopped running their suite of models on 97L.

New tropical wave coming off coast of Africa this weekend
A large and powerful tropical wave will move off the coast of Africa on Friday evening, and the GFS model has been very aggressive in recent runs about developing this wave into a tropical storm within a day of its emergence. The other reliable models for tropical cyclone genesis, the European and UKMET models, have not been developing this wave right away. Residents of the Cape Verde Islands should anticipate the possibility of heavy rain and strong winds on Saturday as the wave moves west at 10 - 15 mph across the islands. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 97L 2-day and 5-day development odds of 0% and 30%, respectively.

The Gulf of Mexico is worth watching
In the Gulf of Mexico, heavy thunderstorm activity has diminished since Monday along a weak cold front stretching from South Florida to the Louisiana coastal waters. Some models show a weak area of low pressure developing along this front and moving westwards over Texas by Friday, and we should keep an eye on this region for development.


Figure 2. MODIS true-color image of Hurricane Marie in the Eastern Pacific taken at approximately 18:15 UTC (2:15 pm EDT) on August 25, 2014. At the time, Marie was a Category 4 storm with 145 mph winds. Image credit: NASA.

Powerful Hurricane Marie generating huge waves in Eastern Pacific
The Eastern Pacific's Hurricane Marie had weakened to a Category 2 storm with 105 mph winds on Tuesday morning, but was still generating huge swells that were bringing large waves to the coasts of Southern California and Mexico's Baja Peninsula. At 5 am EDT on Tuesday, Marie's tropical storm-force winds covered a huge area of ocean, up to 275 miles from the center, and 12-foot high seas extended up to 550 miles from the center. A High Surf Advisory is in effect for Los Angeles, where waves of 10 - 15 feet will potentially cause structural damage to piers and beachside property as well as significant beach erosion. The powerful surf will be accompanied by strong rip currents and long-shore currents, making for very hazardous swimming and surfing conditions through Thursday. Satellite loops on Tuesday morning showed a steady degradation of Marie's cloud pattern, with the eyewall cloud tops warming and the areal coverage of the strongest thunderstorms decreasing. The storm is headed to the northwest over cooler waters and into drier air, and will not affect any land areas.

You can see a spectacular loop of infrared satellite images of Marie as it intensified into a Category 5 storm on Sunday at the CIMSS University of Wisconsin.

Hurricane expert Steve Gregory, who subbed for me while I was on vacation last week, is now providing regular updates on the Atlantic in his blog, three times per week. Steve will sub for me again on Wednesday.

Jeff Masters

Hurricane

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Strengthening Cristobal Headed Out to Sea; 97L and Gulf of Mexico Worth Watching

By: JeffMasters, 3:23 PM GMT on August 25, 2014

Tropical Storm Cristobal continues to dump heavy rains over the Central and Southeast Bahamas and Turks and Caicos Islands as the storm heads slowly north-northeastwards out to sea. Satellite loops show that Cristobal is struggling with wind shear, with a center of circulation that is completely exposed to view, and all the heavy thunderstorms pushed to the south and east sides of the center. The shear is expected to relax by Wednesday as a trough of low pressure captures the storm and accelerates it to the northeast, out to sea. Cristobal will likely be intensifying into a Category 1 hurricane as it brushes Bermuda on Wednesday, and the 11 am EDT Monday Wind Probability Forecast from NHC gave that island a 41% chance of experiencing tropical storm force winds of 39+ mph.


Figure 1. MODIS true-color image of Tropical Storm Cristobal over the Southeast Bahamas at 15:55 UTC (11:55 am EDT) on August 24, 2014. At the time, Cristobal had top winds of 45 mph Image credit: NASA.

Keeping an eye on 97L headed towards the Lesser Antilles
A tropical wave (Invest 97L) was near 12°N, 40°W on Monday morning, midway between the coast of Africa and the Lesser Antilles Islands, and was headed west to west-northwest at about 15 mph. Satellite loops show the wave has a modest amount of spin but only a small amount of heavy thunderstorms. Water vapor satellite images and the Saharan Air Layer analysis show that 97L is located in a dry environment, which is keeping development slow. Wind shear was a moderate 10 - 20 knots, and the 8 am EDT Monday run of the SHIPS model predicted that wind shear would mostly stay in the moderate range for the next five days, which should allow some slow development. Sea Surface Temperatures are near 27.5°C, which is warm enough to allow some slow development. The wave should arrive in the Lesser Antilles Islands by Friday, according to the Monday morning runs of the GFS and European models. One of the 00Z Monday runs of the three reliable computer models for predicting tropical storm formation, the UKMET model, showed some weak development of 97L by Friday as it passes just north of the northernmost Lesser Antilles Islands. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 97L 2-day and 5-day development odds of 0% and 30%, respectively.

Keeping an eye on the Gulf of Mexico
In the Gulf of Mexico, a weak cold front is kicking up some heavy thunderstorms in the Louisiana coastal waters. This activity will spread to the Texas coastal waters by Wednesday. With wind shear a moderate 10 - 20 knots, we should monitor this area for development. About 1/3 of the 20 members of the 06Z Monday GFS model ensemble showed some development in the Gulf on Thursday or Friday. (The GFS ensemble is a set of 20 runs of the GFS model done at lower resolution with slightly different initial conditions to generate an uncertainty "plume" of model runs.) The preferred track of the system was to the west towards Texas.


Figure 2. MODIS true-color image of Hurricane Marie in the Eastern Pacific taken at 20:40 UTC (4:40 pm EDT) on August 24, 2014. At the time, Marie was a Category 5 storm with 160 mph winds. Image credit: NASA.

Mighty Hurricane Marie generating huge waves in Eastern Pacific
Hurricane Marie exploded into the Eastern Pacific's first Category 5 hurricane in four years on Sunday, maintaining Category 5 winds of 160 mph for six hours before an eyewall replacement cycle weakened the storm slightly. The hurricane was still a very powerful Category 4 storm with 145 mph winds on Monday morning, though satellite loops showed a steady degradation of the cloud pattern, with the eyewall cloud tops warming and the areal coverage of the strongest thunderstorms decreasing. The storm is headed northwest over cooler waters and into drier air, and will not affect any land areas. However, Marie's tropical storm-force winds cover a huge area of ocean, up to 310 miles from the center, and these winds are generating massive swells that are bringing high surf to the shores of Mexico's Baja Peninsula. These swells will pound the shores of Southern California Tuesday through Thursday. A High Surf Advisory is in effect for Los Angeles, where waves of 10 - 15 feet will potentially cause structural damage to piers and beachside property as well as significant beach erosion. The powerful surf will be accompanied by strong rip currents and long-shore currents, making for very hazardous swimming and surfing conditions. According to NWS in San Diego, Marie's ascension to Category 4 status on August 24 is the earliest in the year the Eastern Pacific has seen its 5th major hurricane since reliable records began in 1949.

You can see a spectacular loop of infrared satellite images of Marie as it intensified into a Category 5 storm on Sunday at the CIMSS University of Wisconsin.

The GOES-West Satellite will be in rapid scan mode over Marie on Monday, and loops will be available at the NOAA/RAMMB website.

The Western Pacific remains quiet, with no new named storms expected to develop over the next five days. The Madden Julian Oscillation (MJO), a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, is currently in a phase that creates sinking air of the Western Pacific. This discourages tropical cyclone formation. By late next week, this suppressed phase of the MJO will likely shift so that tropical storm formation is suppressed over the Atlantic, keeping the traditional peak portion of the Atlantic hurricane season quieter than usual.

Jeff Masters

Hurricane

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Tropical Storm Cristobal Slowly Intensifying – Very Little Threat to US

By: JeffMasters, 4:45 PM GMT on August 24, 2014

(By Steve Gregory - Substituting for Dr. Masters who is on Vacation.)

Edited for latest Fix data, VIS image and Naval FCST Chart - shown in BOLD

Slowly intensifying Tropical STORM Cristobal with MAX sustained surface winds near 45Kts was near 23.2°N / 72.8°W at 18:00Z,or about 320NM E of Nassau, Bahamas now appears to be heading just west of due North (350°) at around 5Kts as it has become caught up in the generally southerly flow just east of the weak, N-S orientated upper level TROF. This TROF off the east coast is located between the sub-tropical High over the central Atlantic to the east, and another sub-tropical high over the Gulf coast states. This synoptic scale environment that we’ve been discussing for the past 2 days, has apparently ‘shown it’s hand’ already – and has GREATLY lowered the chances that CRISTOBAL will have any significant impact on the US mainland.

Additional RECON data also shows a better defined core with a 'thermal eye wall' Temp differential of 4°C - although no eye wall has been reported or seen on imagery. In addition, most significant convection remains to SE and south of the center.


Although wind shear has lowered considerably during the past 18 hours in the immediate vicinity of the vortex center to under 5Kts(!) – normally quite conducive to intensification – the outflow pattern for the storm that was quite symmetrical during the past 2 days has now degraded somewhat, with ‘good’ outflow to the east and far to the south, but poor to fair outflow in the west/northwest quadrant. In addition, there still does not appear to be a well defined jet outflow channel, though this may yet 'connect up' with a fairly strong northerly jet far to the east of the storm. And while dry air is not an immediate problem for the storm, water vapor imagery loops clearly show drier air starting to impinge on the overall circulation field just to the west (near Florida) and further east (northeastern CARIB) of the storm related to a closed upper LOW approaching the NE CARIB. In addition, a weak east-west frontal band along 32N is still sinking very slowly southward, adding yet another potential complication for the intensity forecast, and to a lessor degree, the track forecast. Bottom line, unless / until CRISTOBAL develops a stronger and far better organized core, the areas of drier air will may soon impinge on the storm's inner circulation which will impede development of the storm, or halt it altogether during the next 12-48 hours.

On the ‘plus’ side (depending on your point of view) SST’s are very warm (near 29°C / 86°F) and should remain above 28°C over CRISTOBAL’S projected track for the next 2-3 days - which is quite supportive of intensification, though the very slow movement of the storm may induce some upwelling of cooler sub-surface water, which in turn, would slow the positive impact on convectively driven intensification.

The latest global and specialized hurricane forecast models, while still showing significant spread in forecast track solutions, have all shifted much further to the east (even further eastward then most of the models originally depicted just a couple days ago), with all solutions showing the storm tracking far to the east of the US mainland. The only significant model divergences of note at this time is a clustering of several model solutions that show the storm taking a more North-Northwest track Monday thru Wednesday before finally turning northeastward and then out to sea about 300NM or more east of Cape Hatteras later in the week. This is a reasonable solution and is due to the initial weakening of the TROF now off the east coast, which allows the sub-tropical ridge over the Atlantic to build westward, forcing a more northwesterly track early in the week ahead. But by late Wednesday and Thursday, a mid-latitude TROF now in the PAC NW will be advancing to the Northeastern US, turning the steering flow to the southwest, which will force CRISTOBAL to turn northeastward and head out to sea.

Unlike the track forecasts, the intensity forecasts have remained very consistent over the last several days, with most calling for a very slow intensification to CAT 1 intensity in a couple days. While there is no clear reason to disagree, the above mentioned ‘negatives’ may tilt the odds to keeping CRISTOBAL just below hurricane strength.



Fig 1: Early morning VIS imagery shows a significantly more consolidated and classical looking storm system, with most of the stronger convection (and higher winds) to the east of the storm center.



Fig 2: Water Vapor imagery shows dry air approaching from the west, and to a lessor degree, from the east, associated with an upper Low near the Leeward Islands. Unless CRISTOBAL intensifies soon, the drier air may become entrained in the inner circulation - hindering intensification.



Fig 3: The area of very low winds shear that has consistently been to the S-SW of the developing system for the last 2 days has now orientated itself over the center of the storm with very low values near or even below 5Kts. This is one of the most 'positive' metrics, along with the very warm SST's, for the intensification of the storm.



Fig 4: Outflow associated with a very high level anti-cyclone is quite good in the eastern semi-circle, and may soon be able to 'latch on' to the northerly jet further to the east of the storm. However, outflow is relatively poor to the west and NW. Outflow is a critical factor that determines storm intensity.



Fig 5: The steering level winds clearly show the break in the east-west orientated ridge line that extended from the High pressure center in the central Atlantic to the second center over the SE U.S. The southerly winds ahead of the weak TROF that 'broke through' the ridge are steering the storm poleward. This TROF is expected to weaken on Monday, allowing the ridge to build westward, steering CRISTOBAL on a more Northwestward course for a couple days, before the winds back around to the SW ahead of the next mid-latitude TROF.



Fig 6: Early cycle model runs are in good agreement that CRISTOBAL will NOT significantly impact the US mainland - and confidence in this outlook is now much higher than yesterday.



Fig 7: Intensity forecasts have remained very consistent over the past few days, with slow intensification expected during the next 72 hrs to CAT 1 Hurricane force in about 48-72 hours - though the approaching drier air may prevent or delay this.



Fig 8: Latest NAVY Track/Intensity Forecast for Cristobal


ELSEWHERE in the Tropical Atlantic - a fairly strong disturbance (97L) now S/SW of the Cape Verdes with isolated convection is westbound - but is unlikely to show significant development for at least the next 4 days as the system is embedded within the dry and warm Saharan Air Layer (SAL), By Friday, the system will have a notable chance for development. An even stronger wave now over Africa should emerge off the coast next weekend.

Dr. Masters will return tomorrow with the next full update; but if there is a significant change, I’ll issue another brief update late today.

Note: For those interested , I will once again be providing my own Weather Bog Updates starting tomorrow located HERE



Steve Gregory

Updated: 8:23 PM GMT on August 24, 2014

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Atlantic Disturbance 96L (Interim Update)

By: JeffMasters, 8:46 PM GMT on August 23, 2014

(By Steve Gregory - Substituting for Dr. Masters who is on Vacation.)

Over the past few hours, a low level circulation appears to be developing in the vicinity of 21°N/73°W (near Great Inagua Island) based on both RECON reports and high resolution VIS imagery loops.

While there is some convection developing close to this circulation feature, and there have been A/C sampled wind reports of gale force– it’s probably a ‘toss-up’ whether NHC will officially ‘call’ this Tropical Storm Cristobal within the next couple of hours – or wait for additional convection and confirmation that this is in fact, the development of a definitive circulation center. Regardless of classification, the central circulation is moving Northwestward at about 15Kts, and will be moving into the central Bahamas on Sunday with tropical storm conditions.

There continues to be significant variations among all the models on the future evolution of this cyclone, but the general consensus is that the storms’ forward motion will slow during the next 24-48 hours as steering currents weaken in response to the weak TROF off the east coast that extends southward towards the NW Bahamas. At the same time, a slow but steady increase in intensity appears likely, with the storm reaching the NW Bahamas by late Monday.

The threat of CAT 1 hurricane conditions anywhere from southern Florida to the Mid-Atlantic coast continues – though this is certainly not a forgone conclusion as several reliable models continue to forecast the storm to turn northward and then northeastward by Monday and Tues, remaining well offshore without making landfall - paralleling the east coast from Florida to North Carolina.

Elsewhere in the Tropical Atlantic...

A broad, large scale easterly wave located in the central Atlantic is moving westward with little shower activity, while a strong disturbance has moved off the African coast and is also moving westward at ~20Kts. This second disturbance bears monitoring – but at this time, is unlikely to develop for at least the next 5 or more days.



Fig 1: VIS imagery loops show a general low level circulation center near 21N°/73°W and has been confirmed by RECON over the past 2 hours. The system is now moving Northwestward at 18Kts.


I’ll have a complete update late Sunday morning.

Steve Gregory


Updated: 8:54 PM GMT on August 23, 2014

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Atlantic Disturbance 96L Still 96L But Threat to US Increases

By: JeffMasters, 3:41 PM GMT on August 23, 2014

(By Steve Gregory - Substituting for Dr. Masters who is on Vacation.)

There’s been little change overnight in 96L, with RECON reports indicating a broad, disorganized circulation center with a surface pressure near 1007mb along the north coast of Hispaniola near 20.8N/71.7W. Mid-upper level winds also indicate that the larger scale circulation field was not significantly impacted by the mountainous terrain of Hispaniola, and has been moving on a generally West/West-Northwest course at around 20Kts, though a somewhat more Northwesterly (310°) heading seems to have developed over the last few hours. Although there continues to be large scale outflow associated with the high level anti-cyclone sitting atop the developing system,, there are no distinct outflow channel jets; so any initial intensification of the system later today or on Sunday will likely start out on the slow side. While wind shear of 15-20Kts continues to be of moderate intensity across large areas of the system’s structure, an area of lighter shear (<10Kts) close to the circulation center continues to move in tandem with the systems primary circulatory vortex.

The overall satellite signature continues to slowly improve, with a somewhat more symmetrical appearance, and large scale curvature especially noteworthy in the E-SE quadrant where hints of a ‘feeder’ band type structure appears to be developing.

With the system embedded in a moist environment and no significantly drier air noted on the periphery of the system, dry air should not be a hindrance to intensification during the next 48 hours. In addition, the developing system is, or soon will be, moving over very warm waters, with SST’s over 29°C (~85°F) – well above that needed to support hurricane intensities. However, if the system should slow to a crawl or even go stationary later Sunday as forecast by many models, upwelling of cooler sub-surface waters could prove significant, slowing the rate of intensification appreciably.

Since yesterday, there have been major shifts to the forecast tracks – especially after Sunday – raising the risk level for the entire eastern US from Florida to New England – with about equal odds that the potential storm will impact the coast or turn out to sea. This is not a surprise since track and intensity forecasts are notorious for major shifts for systems that are still in the formative stages. Ironically, the CMC forecast, which was consistently forecasting the system to track across south Florida, has now shifted dramatically to that shown by many of the major model suites, while the more reliable track forecast models have now shifted westward, much closer to the coast. The spread of forecast tracks is now quite large – again, a typical feature of forecasts for systems still forming. The forecast challenge has been unusually high for this system, not just because it has yet to really develop and has tracked across Hispaniola (a landmass notorious for ‘destroying’ even the most well developed and intense hurricanes) but because of the unusually high track sensitivity to a ‘weakness’ in the east-west sub-tropical ridgeline that extends from the central Atlantic to the Gulf coast. While the models continue to show a break between the high pressure center in the central Atlantic and the one near the Gulf coast during the next 36 hours, the resulting TROF within this break will be relatively weak, and will begin to dissipate staring late Monday. The exact timing of this ‘break’ – and the exact location and strength of 96L during the next 72 hours will determine exactly where and when (if ?) the system turns northward. Prior model runs were in generally good agreement that the system would turn northward when it was still about 300NM east of Florida, while the most recent model runs show a far more gradual recurvature – with some of the more reliable models showing the storm getting very close to the coast before beginning to turn northward, and are all calling for the storm to be moving much slower, with the system not expected to be near the US coast for another 4 or even 5 days.

The intensity forecasts are equally difficult, although they have continued to be quite consistent with each other and between succeeding model runs. The odds are relatively high (70%-80%) the system will become a tropical storm either late today or on Sunday, with a fairly slow rate of intensification to near CAT 1 intensity on Monday or Tuesday. This continues to be a reasonably good forecast scenario all things considered, and a CAT 1 intensity threat for the east coast of Florida northward to the Carolinas is quite real – albeit a still very uncertain one.

All things considered, I must admit this is one of the more ‘challenging’ forecasts I’ve come across in my many decades of forecasting.



Fig 1: Early morning VIS imagery shows a system still trying to get ‘its act together’ with a better organized signature than yesterday, but a still poorly organized low level circulation. Some hints of ‘feeder’ type bands are seen extending into the southern CARIB south and east of the ‘center’ witch should provide an ample flow of moisture into the system assuming it finally intensifies into a strong cyclone.



Fig 2: The Total Precipitable water (TPW) analysis show 96L surrounded by moist air, with a deep moisture plume extending from the deep tropics over northern SOAMER across the CARIB into the mid-level vortex circulation that is 96L. This should continue to supply high atmospheric moisture content to the system as it develops during the next few days.



Fig 3: Wind Shear analysis (mid levels) shows moderate shear conditions in the 15-20Kt range in the vicinity of the primary low-mid level circulation. Much lower shear conditions are seen just south-southwest of the mid level circulation field. The shear analysis is automated, so the lower values shown may not be very precise in the delineation of its aerial extent. However, this wind shear ‘couplet’ has been moving in tandem with the disturbance, and since none of the models are forecasting adverse shear conditions ahead of the systems’ projected track – shear should not be a major hindrance to intensification for the next 72 hours.



Fig 4: There’s been no significant change to yesterday’s development of a high level (~200mb) anti-cyclone which is providing decent outflow for the developing system. Though no distinct outflow jet channels have developed, the current level of outflow does support a CAT 1 intensity storm.



Fig 5: The lower level steering winds clearly show the east-west orientated sub-tropical ridge from the central Atlantic westward to the Gulf coast region, along with a break (TROF) between the two separate High pressure centers. This TROF / Ridge-line is the primary feature set that will ultimately determine the track taken by the developing cyclone. Both the intensity of the developing system and the exact location and impact of the TROF on the sub-tropical ridge will determine how the steering currents evolve and ultimately guide the the cyclone over the next several days. If the TROF weakens quickly enough, allowing the ridge to rebuild westward into the SE US – the cyclone will track into the SE coast of the US. If the TROF/ridge weakness remains in place – the cyclone will recurve and turn northward before reaching the coast.



Fig 6: Early cycle model runs are in general agreement on the track of the developing cyclone for the next 36-48 hours, but the spread between solutions beyond then has grown quite large since yesterday; indicative of the difficulty the models are having in determining the subtle-changes and inter-play between the sub-tropical ridge, the TROF off the east coast, and the cyclone itself.



Fig 7: Intensity forecasts have been fairly consistent over the past few days - seemingly unaffected by the track changes - with slow intensification expected during the next 72 hrs to CAT 1 Hurricane force by early next week.

I’ll have another brief update late this afternoon after additional RECON and SAT imagery can be reviewed, along with all the 12Z global model run output.

Steve

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Atlantic Disturbance 96L (Interim Update)

By: JeffMasters, 7:35 PM GMT on August 22, 2014

(By Steve Gregory - Substituting for Dr. Masters who is on Vacation.)

Aside from the increase in convection along with the development of a high level anti-cyclone above the lower level circulation today, there has been little additional development of 96L.

Based on the latest RECON data, along with radar and satellite imagery, the broad area of Low pressure is currently located over Mona Pass near 19N/68W and will be approaching the northeastern tip of the Dominican Republic soon as the system continues on an essentially westward track at near 20Kts. The lowest surface pressure is estimated near 1007mb and the strongest observed winds are near tropical storm (gale) force in a small area to the northeast of the primary circulation.

The latest global model runs continue to call for the developing cyclone to move along or just off the northern coast of Hispaniola overnight, and then slow its forward movement on Saturday as it moves into the lower Bahamas. Once in this region, and some distance away from the high terrain of Hispaniola, the system should be able to spin-up a core circulation and attain tropical storm intensity in 24-36 hours.

With the east-west orientated sub-tropical ridge over the SW Atlantic still forecast to weaken in vicinity of the Bahamas by early Sunday, the system should see it’s forward movement slow even further as it begins to turn northwestward and then northward by late Sunday or by Monday. While there continues to be significant uncertainty on this development, the fairly good continuity among the more reliable models on both the track and intensity forecasts suggests reasonably good confidence that the cyclone will turn northward and ultimately take a track that takes the system out to sea without serious impact to the US mainland next week. Of course, as with any tropical system still in the formative stages, along with the potential for subtle changes in the actual location and strength of the sub-tropical ridge & steering flow – significant uncertainty in the ultimate track and intensity remains.

Elsewhere across the Tropical Atlantic

There is another large scale easterly wave - but with little convection - approaching the central Atlantic along 35W, and a somewhat stronger system about to emerge off the west African coast. However, no significant development of either wave is expected during the next 5 or more days.

I’ll have a complete update late Saturday morning

Steve Gregory

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Atlantic Disturbance 96L Remains Disorganized - For Now…

By: JeffMasters, 3:39 PM GMT on August 22, 2014

(By Steve Gregory - Substituting for Dr. Masters who is on Vacation.)

There has generally been little change overnight in disorganized tropical disturbance 96L, though the most recent upper level analysis and VIS imagery loops suggest the environment is becoming more conducive to cyclone formation within the next 12 hours, with the potential for further development to a strong Tropical Storm force cyclone later this weekend.

While the overall appearance of the disturbance remains poorly organized (with the appearance of at least 2 separate low level vortices during the last 36 hours} the most recent visible Satellite imagery loop suggests a primary surface circulation is located to the N/NE of Puerto Rico NEAR 19N/65.5W . However, at this point, limited surface/buoy/ship reports do not yet seem to clearly reflect this possible development.

While the overall satellite signature shows a rather elongated system, with convection widely dispersed, a burst of deep convection has developed over the last few hours near and across the NW and SE quadrants of the low level circulation center. San Juan Radar also depicts some 'banding' features developing to the SE of the newly forming center.

In addition, an upper level anticyclone has apparently formed over the last 3-6 hours near and just SW of the developing cyclone center. That said – wind shear still appears to be of moderate intensity near the center of the system (15Kt-20Kts) – though much lower shear values are clearly seen just SW of the center near the developing upper level anti-cyclone. Though these developments show a significant environmental improvement supporting cyclone development since yesterday – the nearby mountainous terrain of Hispaniola and Puerto Rico are likely to inhibit any rapid type of development during the next 12-36 hours.

The more reliable global model runs over the past 24 hours continue to call for a general track into the central Bahamas this weekend before turning northward Sunday as a break in the east-west orientated subtropical ridge to the north of the system develops further into a TROF over the next 48-72 hours, steering the system Northwestward and then Northward. Most every model (with 2 distinct exceptions) keep the cyclone away from the US mainland. (The CMC continues to track the system across south Florida – but this seems unlikely unless the system does NOT develop significantly for at least the next 72 hours – while the last OPNL ECMWF and GFDL model suite shows the storm first turning northward as the other major models do, but then suddenly turns the storm NW towards the Mid-Atlantic coast towards the middle of next week. However, the ECMWF Ensembles - which are typically much more reliable for developing systems - do NOT show this turn towards the coast.) Since the system has yet to become a well established cyclone, and upper level wind flow forecasts at these longer ranges during the warm season can be quite unreliable - all of these ‘outlier’ track forecasts remain highly suspect.

In addition, overall confidence in the intensity forecasts remains relatively low, though most of the more reliable, specialized hurricane models do call for a slow intensification to a CAT 1 storm after 72 hours as the storm begins to track northward away from the central Bahamas. Sea Surface Temps (SST’s) are very warm across the SW Atlantic near the Bahamas, with readings near or just over 29°C (~85°F), solidly supporting hurricane intensity, though the depth of the warm water remains relatively shallow (and a very slow moving storm would tend to upwell the somewhat cooler sub-surface water). All things considered, however, the model intensity forecasts subjectively look reasonable given the current stage of development.

A RECON is still planned for this afternoon, and I’ll have another update on this developing system late today.




Fig 1: The above VIS depicts an elongated, poorly organized disturbance with widely dispersed, though locally heavy, convection. Over the last few hours, a surface circulation appears to be developing NE of Puerto Rico with a strong convective burst noted to the N-NW of this circulation.



Fig 2: Wind Shear analysis (mid levels) shows moderate shear conditions in the 15-20Kt range in the vicinity of the primary low-mid level circulation. Much lower shear conditions are seen just south-southwest of the center. Since the shear analysis is automated, the values shown may not be very precise, though the very slow development is likely due in part to the less than optimal shear environment.




Fig 3: The most significant development since last night is the apparent development of an anti-cyclone at high levels, centered fairly close to the primary mid-level circulation center. Weak but distinct outflow channels are also apparent to the N-NW and South of the developing cyclone.



Fig 4: The lower level steering winds (used for a shallow / developing cyclones) clearly shows the east-west orientated sub-tropical ridge from the central Atlantic westward to the Gulf coast region, along with a developing weakness between the two separate High pressure centers. If the system does NOT develop, it will likely tend to continue W-NW towards Florida. However, any significant development, along with the global model forecasts for a break in the ridge line near the central Bahamas, should lead to the cyclone turning northward late this weekend as it tracks around the western periphery of the central Atlantic High pressure center.



Fig 5: Early cycle model runs are in very good agreement on the track of the cyclone ultimately turning away from the US mainland, though there is that 'sudden turn towards the coast' by a couple models that cannot be totally discounted.



Fig 6: Intensity forecasts have been fairly consistent over the past couple days, with slow intensification expected during the next 24-72 hrs, with an increase in intensity to CAT 1 Hurricane force during the 3-5 day period.


Steve

Updated: 3:45 PM GMT on August 22, 2014

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Which Hurricane Model Should You Trust?

By: JeffMasters, 6:25 PM GMT on August 21, 2014

Afternoon satellite images and data from the 2 pm EDT Thursday hurricane hunter mission into Invest 96L found that the storm had developed a closed surface circulation, but the storm had little in the way of organized heavy thunderstorm activity, and was not yet classifiable as a tropical depression. As we wait to see if 96L will develop or not, it is worth reviewing the performance of the models we use to forecast the track and intensity of tropical cyclones. Remember that all of these results are for storms that reached at least tropical depression status. For tropical disturbances that have not yet become a tropical depression, the intensity forecast models should not be trusted at all, and the track models will have much larger errors than analyzed here.

The National Hurricane Center (NHC) did a decent job forecasting Atlantic hurricane and tropical storm tracks during 2013, with errors similar to or slightly larger than the previous 5-year means for 12 to 96 hour forecasts, according to the 2013 National Hurricane Center Forecast Verification Report, issued in May 2014. A new record accuracy for 5-day forecasts was set, though forecasts for 12 to 96 hours were slightly worse than in 2012. One possible reason for the increase in track errors in 2013 was the small number of hurricanes we had (just two.) On average, the NHC track errors steadily decrease as the intensity of a tropical cyclone increases. The average track error in an official NHC 1-day forecast in 2013 was 57 miles, and was 118 miles for 2 days, 162 miles for 3 days, 190 miles for 4 days, and 190 miles for 5 days. The official track forecasts tended to have a westward to northwestward bias of 55 - 82 miles for 3 - 5 day forecasts (i.e., the official forecast tended to fall to the west or northwest of the verifying position.)


Figure 1. Verification of official NHC hurricane track forecasts for the Atlantic, 1990 - 2013. Over the past 24 years, 1 - 3 day track forecast errors have been reduced by about 60%. Track forecast error reductions of about 40% have occurred over the past thirteen years for 4- and 5-day forecasts. Image credit: 2013 National Hurricane Center Forecast Verification Report.

NHC Intensity Forecasts: a Notable Improvement in 2013
Official NHC intensity forecasts did better than usual in 2013, and had errors lower than the 5-year average error for all forecast periods (12 hours through 5 days.) Mean forecast errors in 2013 ranged from about 5 knots (6 mph) at 12 hours to about 13 knots (15 mph) at 5 days. Errors for 72 and 96 hour intensity forecasts were nearly 50% smaller than the 5-year average. The official forecasts were biased too high at most forecast times. There has been a noticeable improvement in intensity forecasts in the past few years, but this is likely due to the lack of rapidly intensifying hurricanes. These rapid intensifiers are typically the source of the largest forecast errors.


Figure 2. Verification of official NHC hurricane intensity forecasts for the Atlantic, 1990 - 2013. Intensity forecasts have shown little to no improvement since 1990. Image credit: 2013 National Hurricane Center Forecast Verification Report.

Which Track Model Should You Trust?
As usual, in 2013 the official NHC forecast for Atlantic storms was better than any individual computer models at most forecast time periods, although NOAA's HWRF model did slightly better than the NHC official forecast for 5-day forecasts. Once again, the European Center (ECMWF) and GFS models were the top performers, when summing up all track forecasts made for all Atlantic named storms. The two models were about equal in performance for 12-hour through 72-hour forecast, with the GFS model besting the European model for 3-day, 4-day, and 5-day forecasts. NOAA's specialized regional hurricane models, the GFDL and HWRF, were also respectable in accuracy for 12-hour through 48-hour forecast accuracy in 2013, though not quite as good as the GFS and European models. The simple BAMM model and CMC models did quite poorly compared to the others.

The best-performing model averaged over the past three years has been the European Center model, with the GFS model a close second. Wunderground provides a web page with computer model forecasts for many of the best-performing track models used to predict hurricane tracks. The European Center does not permit public display of tropical storm positions from their hurricane tracking module of their model, so we are unable to put ECMWF forecasts on this page. Here are some of the better models NHC regularly looks at:

ECMWF: The European Center's global forecast model
GFS: NOAA's global forecast model
UKMET: The United Kingdom Met Office's global forecast model (not evaluated by NHC in 2013)
GFDL: The Geophysical Fluid Dynamics Laboratory's hurricane model, initialized using GFS data
HWRF: The intended successor for the the GFDL hurricane model, also initialized using GFS data
NAVGEM: The Navy's global forecast model (which replaced the defunct NOGAPS model in 2013)
CMC: The Canadian GEM model
BAMM: The very old Beta and Advection Model (Medium layer), which is still useful at longer ranges

If one averages together the track forecasts from the first five of these models, the NHC official forecast will rarely depart much from it. These are the five models used to formulate the TVCA consensus model seen in Figure 3; the TVCA model was very close to the official NHC forecast.


Figure 3. Skill of computer model forecasts of Atlantic named storms in 2013, compared to a "no skill" model called "CLIPER5" that uses just climatology and persistence to make a hurricane track forecast (persistence means that a storm will tend to keep going in the direction it's current going.) OFCL=Official NHC forecast; GFS=Global Forecast System model; GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; ECMWF=European Center for Medium Range Weather Forecasting model; TVCA=one of the consensus models that blends together up to four of the above models, plus the UKMET model (not shown); CMC=Canadian Meteorological Center (GEM) model; BAMM=Beta Advection Model (Medium depth.) The UKMET model was not evaluated. Data taken from the National Hurricane Center 2013 verification report.

Which Intensity Model Should You Trust?
Over the past three years, the official NHC intensity forecast has generally outperformed their four main intensity models. These four models were the dynamical HWRF and GFDL models, which subdivide the atmosphere into a 3-D grid around the storm and solve the atmospheric equations of fluid flow at each point on the grid, and the statistics-based LGEM and DSHP models (DSHP is the SHIPS model with inland decay of a storm factored in.) The top-performing global dynamical models for hurricane track, the GFS and European (ECMWF) models, are typically not considered by NHC forecasters when making intensity forecasts. The GFS model has done a decent job at making intensity forecasts over the past three years, but the European model has made poor intensity forecasts. In 2013 and for the period 2011 - 2013, the HWRF model was the best-performing intensity model for forecasts of 48 hours or less. The LGEM statistical model was the best one at longer-term intensity forecasts of 3 - 5 days.


Figure 4. Skill of computer model intensity forecasts of Atlantic named storms in 2011 - 2013, compared to a "no skill" model called "Decay-SHIFOR5" that uses just climatology and persistence to make a hurricane intensity forecast (persistence means that a storm will tend to maintain its current behavior.) NHC=Official NHC forecast; GFS=Global Forecast System model; GFDL=Geophysical Fluid Dynamic Laboratory model; HWRF=Hurricane Weather Research Forecasting model; ECMWF=European Center for Medium Range Weather Forecasting model; LGEM=Logistic Growth Equation Model; DSHP=Statistical Hurricane Intensity Prediction Scheme with inland Decay. Image credit: 2013 National Hurricane Center Forecast Verification Report.

Some Promising New Models From the Hurricane Forecast Improvement Project (HFIP)
Last year was the fifth year of a ten-year project, called the Hurricane Forecast Improvement Project (HFIP), aimed at reducing hurricane track and intensity errors by 50%. HFIP model performance in 2013 was improved over 2012. Notably, the 15-km resolution global FIM9 model was competitive with the top-tier GFS, European, GFDL, and HWRF dynamical models for track. The University of Wisconsin non-hydrostatic intensity model (UWN) performed well for intensity forecasts. Also, as detailed in a July 11 guest blog post by Dr. Morris Bender of NOAA's Geophysical Fluid Dynamics Laboratory (GFDL) in Princeton, New Jersey, and Dr. Vijay Tallapragada of NOAA's National Center of Environmental Prediction Environmental Modeling Center (NCEP/EMC), the intensity forecasts of the latest versions of the GFDL and HWRF hurricane models, when averaged together, make an intensity forecast that is one of the best available.

For those interested in learning more about the hurricane forecast models, NOAA has a 45-minute training video (updated for 2011.) Additional information about the guidance models used at the NHC can be found at NHC (updated in 2009) and the NOAA/HRD Hurricane FAQ (updated in 2013).

Sources of Model Data
You can view 7-day ECMWF and 16-day GFS forecasts on wunderground's wundermap with the model layer turned on.
Longer ten-day ECMWF forecasts are available from the ECMWF web site.
FSU's experimental hurricane forecast page (CMC, ECMWF, GFDL, GFS, HWRF, and NAVGEM models)
NOAA's HFIP model comparison page (GFS, ECMWF, FIM, FIM9, UKMET, and CMC models.)
Experimental HFIP models

This will be my last post until Monday. Hurricane expert Steve Gregory will be posting in my blog while I am gone the next three days.

Jeff Masters

Hurricane

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Disorganized 96L Bringing Heavy Rains to Lesser Antilles

By: JeffMasters, 3:05 PM GMT on August 21, 2014

Heavy rain showers are sweeping through the Lesser Antilles Islands as a strong tropical wave (Invest 96L) heads west-northwestwards at about 15 - 20 mph through the islands. Satellite loops on Thursday morning showed a pretty unimpressive system, with a broad, elongated surface circulation and a modest amount of heavy thunderstorm activity that had changed little in intensity or organization since Wednesday. The storm was poorly organized, with a clumpy appearance and just a few small low-level spiral bands. Radar loops from Barbados and Martinique showed only a slight amount of rotation in the radar echoes. An 8:37 pm EDT Wednesday pass from the ASCAT satellite showed top surface winds near 35 mph on the east side of 96L. Wind shear is moderate, 10 - 20 knots, and water vapor satellite images and the Saharan Air Layer analysis show that 96L has moistened its environment considerably since Wednesday, though some dry pockets persist in the vicinity, particularly to the west. Sea Surface Temperatures (SSTs) are near 28°C, which favors development.


Figure 1. Radar image of 96L from 10:35 am EDT August 21, 2014. Image credit: Barbados Meteorological Services.

Forecast for 96L
Despite 96L's disorganized appearance on satellite imagery, the Thursday afternoon flight of the Hurricane Hunters is underway, but the earliest I would expect 96L to become a tropical depression would be Friday morning. The 0Z Thursday runs of our three most reliable models for predicting tropical cyclone genesis, the UKMET, GFS, and European models, had one model, the UKMET, predicting potential development into a tropical depression by Saturday. The storm will pass through the Lesser Antilles Islands Thursday and Friday, bringing heavy rain showers and strong winds, which will also affect the Virgin Islands, Puerto Rico, and Dominican Republic Friday through Saturday. A Flash Flood Watch is in effect from late Thursday night through Saturday morning for the Virgin Islands, Puerto Rico, Culebra, and Vieques. Rainfall amounts of 4 - 6" with locally higher amounts are expected.


Figure 2. Latest satellite image of 96L.

The circulation center of 96L has jumped considerably to the northwest over the past day, resulting in northward shifts in the expected track of the system from all of the major models. On Saturday, 96L may pass near or over the islands of Puerto Rico and Hispaniola, whose rugged terrain would likely disrupt the storm. By Sunday, 96L is expected to be near the Southeast Bahamas and Turks and Caicos Islands, and both the GFS and UKMET models predict that 96L will be able to develop into a tropical depression by Sunday or Monday. The 8 am EDT Thursday run of the SHIPS model predicted that wind shear would stay in the moderate range, 10 - 20 knots, though Sunday, then rise on Monday. With dry air expected to be in the region, wind shear would likely be able to drive the dry air into the circulation of 96L, keeping any development slow. A trough of low pressure is expected to be over the U.S. East Coast early next week, and the GFS and European models predict that this trough will be strong enough to turn 96L north and then northeast, keeping the storm away from the Southeast U.S. coast. However, long-range model forecasts of disturbances that haven't formed into a tropical depression yet are unreliable, and we should not be confident that 96L will miss the Mainland U.S. yet. In their 8 am EDT Thursday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day development odds of 50% and 70%, respectively.

Active in the Eastern Pacific
In the Eastern Pacific, Hurricane Lowell became the 7th hurricane of season this morning, giving the Eastern Pacific 12 named storms, 7 hurricanes, and 5 intense hurricanes so far this year. Typically, the Eastern Pacific sees just 8 named storms, 4 hurricanes, and 1 intense hurricane by August 21. Lowell and and Tropical Storm Karina are expected to become entangled with each other early next week and die in the cool waters well to the west of Baja Mexico. The models have been consistently predicting that a another named storm (Marie) will form late this week from a tropical wave (Invest 92E) that crossed Central America on Monday that is moving parallel to the Mexican coast a few hundred miles offshore. NHC is giving 5-day odds of development of 90%. Current model runs show the storm staying well offshore and not affecting any land areas. Ocean temperatures in the waters just west of the Baja Peninsula are unusually warm---30°C (86°F), which is about 3°C (5°F) above average--so Marie will have plenty of heat energy available to power it. Satellite images are showing the the disturbance already has a pronounced spin to it and a growing area of heavy thunderstorms.

The Western Pacific remains quiet, with no new named storms expected to develop over the next five days.

There is a unique sunrise-to-sunset 1-minute resolution satellite loop of Tropical Storm Lowell from August 19 available from NOAA/NESDIS.

Jeff Masters

Hurricane

Updated: 4:19 PM GMT on August 21, 2014

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96L Slowly Organizing on its Way to the Lesser Antilles

By: JeffMasters, 2:14 PM GMT on August 20, 2014

A tropical wave (96L) located near 11°N 53°W, several hundred miles east of the Lesser Antilles Islands, is headed west-northwestwards at about 10 - 15 mph. Satellite loops on Wednesday morning showed the wave had a broad, elongated surface circulation and a modest amount of heavy thunderstorm activity that was steadily increasing in areal coverage and intensity. Decent upper-level outflow channels were present on the storm's west and south sides. The storm was poorly organized, though, with a clumpy appearance and just a few small low-level spiral bands. An 8:57 pm EDT Tuesday pass from the ASCAT satellite showed top surface winds near 35 mph in a clump of thunderstorms a few hundred miles to the east of center of 96L. Wind shear is moderate, 10 - 20 knots, and water vapor satellite images and the Saharan Air Layer analysis show that while the wave has plenty of dry air to its north to contend with, it has managed to moisten its environment considerably since Monday. Sea Surface Temperatures (SSTs) are near 28°C, which favors development. The outermost thunderstorms of 96L had appeared on Barbados radar by Wednesday morning.


Figure 1. Latest satellite image of 96L.

Forecast for 96L
The wave should continue to organize over the next two days, and pass through the Lesser Antilles Islands Thursday night and Friday morning, bringing heavy rain showers and strong winds--particularly to the southern islands in the chain. The wave will then track west-northwest through the Caribbean a few hundred miles south of Puerto Rico. The 0Z Wednesday runs of our three most reliable models for predicting tropical cyclone genesis, the UKMET, GFS, and European models, had one model, the UKMET, predicting development into a tropical depression south of Puerto Rico. All three models show that on Saturday, 96L will pass over or just south of the island of Hispaniola, whose rugged terrain would likely disrupt the storm. The 8 am EDT Wednesday run of the SHIPS model predicted that wind shear would stay in the moderate range, 10 - 20 knots, for the next five days. With dry air expected to be in the Caribbean, the moderate levels of wind shear would likely be able to drive the dry air into the circulation of 96L, keeping any development slow. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day development odds of 30% and 50%, respectively. The Hurricane Hunters are on call to investigate the disturbance on Thursday afternoon, if necessary. If 96L does develop, it would likely be similar to Tropical Storm Bertha of early August while in the Caribbean--a disorganized system that struggles against dry air. The most likely day for development into a tropical depression is Friday, when the storm will be south of Puerto Rico.

A second disturbance near 14°N, 46°W, about 1000 miles east of the Lesser Antilles Islands, has a small area of disorganized heavy thunderstorms with some modest rotation. In their 8 am EDT Wednesday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 10% and 10%, respectively. None of our three reliable models for predicting tropical cyclone genesis predict that this disturbance will develop over the next five days as it heads west-northwest at about 10 mph.

Active in the Eastern Pacific
In the Eastern Pacific, we have Tropical Storm Lowell and Tropical Storm Karina , which are expected to become entangled with each other early next week and die in the cool waters well to the west of Baja Mexico. The models have been consistently predicting that a another named storm (Marie) will form late this week from a tropical wave (Invest 92E) that crossed Central America on Monday and will move parallel to the Mexican coast a few hundred miles offshore. This storm, which NHC is giving 5-day odds of development of 80%, is something residents of the Baja Peninsula should monitor next week, though current model runs show the storm staying well offshore. Ocean temperatures in the waters just west of the Baja Peninsula are unusually warm---30°C (86°F), which is about 3°C (5°F) above average--so Marie will have plenty of heat energy available to power it. Satellite images are showing the the disturbance already has a pronounced spin to it and a growing area of heavy thunderstorms.

The Western Pacific remains mercifully quiet, with no new named storms expected to develop over the next five days.

Jeff Masters

Hurricane

Updated: 2:15 PM GMT on August 20, 2014

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96L Approaching Lesser Antilles a Threat to Develop

By: JeffMasters, 7:58 PM GMT on August 19, 2014

A tropical wave located in the Central Atlantic near 10°N 50.5°W, about 800 miles east of the Lesser Antilles Islands, was designated Invest 96L by NHC on Tuesday afternoon, and is headed westwards to west-northwestwards at about 10 - 15 mph. Satellite loops show the wave has a broad, elongated surface circulation and modest amount of heavy thunderstorm activity that is poorly organized. Thunderstorm activity was beginning to increase slightly in areal coverage early Tuesday afternoon, though, and a solid outflow channel to the north had developed on 96L's west side. An 8:12 am EDT Tuesday pass from the ASCAT satellite showed top surface winds near 35 mph. Wind shear is moderate, 10 - 20 knots, and water vapor satellite images and the Saharan Air Layer analysis show that the wave has plenty of dry air to contend with. Sea Surface Temperatures (SSTs) have warmed, and are now near 28°C, which favors development.


Figure 1. MODIS true-color image of Invest 96L northeast of the coast of South America, at approximately 10:30 am EDT August 19, 2014. Image credit: NASA.

Forecast for 96L
The wave should pass through the Lesser Antilles Islands Thursday night or Friday morning, then track west to west-northwest through the Caribbean a few hundred miles south of Puerto Rico. The 12Z Tuesday run of the GFS and European models showed 96L tracking through the Central Caribbean early next week, and arriving in the Western Caribbean on Tuesday. The UKMET model showed a more northerly component of motion, with a path over Hispaniola. Two of our three reliable models for predicting tropical cyclone genesis, the UKMET and GFS models, predicted that 96L would develop into a tropical storm after passing through the Lesser Antilles. When both of these models show development, the odds increase that development will occur. The UKMET model showed 96L developing on Saturday, and the GFS showed it developing on Friday. Given the presence of so much dry air near the disturbance, the risk of development is low through Wednesday, but development odds will increase on Thursday as ocean temperatures warm and the atmosphere becomes moister. The 2 pm EDT Tuesday run of the SHIPS model predicted that wind shear would stay in the moderate range, 10 - 20 knots, for the next five days. In their 2 pm EDT Tuesday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day development odds of 30% and 40%, respectively. The Hurricane Hunters are on call to investigate the disturbance on Thursday afternoon, if necessary. If 96L does develop, it would likely be similar to Tropical Storm Bertha of early August while it is in the Caribbean--a weak and disorganized system that struggles against dry air.

A second disturbance near 13°N, 39°W, midway between Africa and the Lesser Antilles Islands, also has a small area of disorganized heavy thunderstorms with some modest rotation. In their 2 pm EDT Tuesday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 10% and 20%, respectively. None of our three reliable models for predicting tropical cyclone genesis predict that this disturbance will develop over the next five days as it heads west-northwest at about 10 mph.


Figure 2. Tropical Storm Lowell as seen at 11 am EDT Tuesday, August 19, 2014. At the time, Lowell had top winds of 50 mph. Image taken from a super-rapid scan mode loop from the NOAA/RAMMB website.

The Eastern Pacific heating up
In the Eastern Pacific, we have a new named storm, Tropical Storm Lowell, which formed at 03 UTC on Tuesday. The GOES-West satellite is in super-rapid scan mode over Lowell today, and you can access some very impressive one-minute time resolution loops of Lowell at the NOAA/RAMMB website. Lowell's formation gives the Eastern Pacific 12 named storms, 7 hurricanes, and 5 intense hurricane so far this season. On average, the Eastern Pacific sees 8 named storms, 4 hurricanes, and 1 intense hurricane by August 18, so it has been a very active year in the basin. Tropical Storm Karina is also spinning away in the Eastern Pacific today, and Karina and Lowell are expected to become entangled with each other early next week and die in the cool waters well to the west of Baja Mexico. The models have been consistently predicting that a another named storm (Marie) will form late this week from a tropical wave that crossed Central America on Monday and will move parallel to the Mexican coast a few hundred miles offshore. This storm, which NHC is giving 5-day odds of development of 70%, is something residents of the Baja Peninsula should monitor next week. Ocean temperatures in the waters just west of the Baja Peninsula are unusually warm---30°C (86°F), which is about 3°C (5°F) above average--so Marie will have plenty of heat energy available to power it.

The Western Pacific remains mercifully quiet, with no new named storms expected to develop over the next five days.

Jeff Masters

Hurricane

Updated: 8:25 PM GMT on August 19, 2014

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Tropical Atlantic Remains Quiet; Eastern Pacific Heating Up

By: JeffMasters, 3:25 PM GMT on August 19, 2014

A tropical wave located in the Central Atlantic near 11°N 48°W, about 1,000 miles east of the Lesser Antilles Islands, is headed westwards at about 10 mph. Satellite loops show the wave has a broad, elongated surface circulation and a small amount of heavy thunderstorm activity that is poorly organized. Wind shear is moderate, 10 - 20 knots, and water vapor satellite images and the Saharan Air Layer analysis show that the wave is surrounded by dry air, though the amount of dryness has lessened over the past two days. Sea Surface Temperatures (SSTs) are marginal for development, about 27°C. The Tuesday morning run of one of our three reliable models for predicting tropical cyclone genesis, the UKMET model, did show this wave developing into a tropical storm on Friday as it passed through the Lesser Antilles. Given the presence of so much dry air near the disturbance, the risk of development is low Tuesday and Wednesday, but development odds will increase on Thursday as the wave nears the Lesser Antilles, where ocean temperatures will be warmer and the atmosphere a little moister. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day development odds of 10% and 20%, respectively. The Hurricane Hunters are on call to investigate the disturbance on Thursday afternoon, if necessary.

A second disturbance near 13°N, 37°W, midway between Africa and the Lesser Antilles Islands, also has a small area of disorganized heavy thunderstorms with some modest rotation. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave this disturbance 2-day and 5-day development odds of 10% and 20%, respectively. None of our three reliable models for predicting tropical cyclone genesis predict that this disturbance will develop over the next five days as it heads west-northwest at about 10 mph.


Figure 1. Tropical Storm Lowell as seen at 11 am EDT Tuesday, August 19, 2014. At the time, Lowell had top winds of 50 mph. Image taken from a super-rapid scan mode loop from the NOAA/RAMMB website.

The Eastern Pacific heating up
In the Eastern Pacific, we have a new named storm, Tropical Storm Lowell, which formed at 03 UTC on Tuesday. The GOES-West satellite is in super-rapid scan mode over Lowell today, and you can access some very impressive one-minute time resolution loops of Lowell at the NOAA/RAMMB website. Lowell's formation gives the Eastern Pacific 12 named storms, 6 hurricanes, and 5 intense hurricane so far this season. On average, the Eastern Pacific sees 8 named storms, 4 hurricanes, and 1 intense hurricane by August 18, so it has been a very active year in the basin. Tropical Storm Karina is also spinning away in the Eastern Pacific today, and Karina and Lowell are expected to become entangled with each other early next week and die in the cool waters well to the west of Baja Mexico. The models have been consistently predicting that a another named storm (Marie) will form late this week from a tropical wave that crossed Central America on Monday and will move parallel to the Mexican coast a few hundred miles offshore. This storm, which NHC is giving 5-day odds of development of 70%, is something residents of the Baja Peninsula should monitor next week. Ocean temperatures in the waters just west of the Baja Peninsula are unusually warm---30°C (86°F), which is about 3°C (5°F) above average--so Marie will have plenty of heat energy available to power it.

The Western Pacific remains mercifully quiet, with no new named storms expected to develop over the next five days.

Jeff Masters

Hurricane

Updated: 3:26 PM GMT on August 19, 2014

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July 2014: Earth's 4th Warmest July on Record

By: JeffMasters, 4:53 PM GMT on August 18, 2014

July 2014 was Earth's fourth warmest July since records began in 1880, said NOAA's National Climatic Data Center (NCDC) today. NASA rated July 2014 cooler: the 11th warmest July on record. The July ranking by NOAA broke a streak of two consecutive warmest months on record--May and June of 2014 (they originally ranked April 2014 as tied for warmest April on record, but have since revised it to the second warmest April on record.) Global ocean temperatures during July 2014 were tied with July 2009 for the warmest July on record, and global land temperatures in July 2014 were the 10th warmest on record. The year-to-date January - July period was the 3rd warmest on record for the globe. Global satellite-measured temperatures in July 2014 for the lowest 8 km of the atmosphere were the 4th or 5th warmest in the 36-year record, according to Remote Sensing Systems and the University of Alabama Huntsville (UAH), respectively.


Figure 1. Departure of temperature from average for July 2014, the 4th warmest July for the globe since record keeping began in 1880. Record-warm conditions were experienced over portions of five continents, most notably Northern Europe and Northwest Africa. The Central U.S. and Central Russia had much cooler than average temperatures. Overall, 32 countries across every continent except Antarctica had at least one station reporting a record high temperature for July. The United States and the Russian Federation each had several stations that reported record warm temperatures as well as several stations with record cold temperatures for the month. No other countries had stations that reported a record cold July temperature. The period of record varies by station. Image credit: National Climatic Data Center (NCDC) .



Three billion-dollar weather disasters in July 2014
Three billion-dollar weather-related disasters hit the Earth during July 2014, all in China, according to the July 2014 Catastrophe Report from insurance broker Aon Benfield. The total number of billion-dollar weather disasters for the seven months of 2014 is fourteen, which is well behind the record-setting pace of 2013, which had 22 such disasters by the end of July, and ended up with a record 41 by the end of the year.


Disaster 1. With a name meaning “thunder of God,” Super Typhoon Rammasun was the strongest typhoon on record to hit China, as measured by observed sea level pressure at landfall (899.2 mb measured at Qizhou Island, about 20 miles east of Hainan Island, according to the National Meteorological Center of China Meteorological Administration.) According to the Joint Typhoon Warning Center, Rammasun peaked as a Category 4 super typhoon with 155 mph winds shortly before hitting China's Hainan Island. Rammasun killed 206 and did $6.5 billion in damage, making it the most expensive weather disaster so far in 2014. Approximately $249 million of the damage was done in the Philippines, making it that nation's 8th most expensive typhoon on record. The image above was taken by the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Aqua satellite at 1:35 p.m. local time (0535 UTC) on July 18, 2014. Image credit: NASA Natural Hazards.


Disaster 2. Drought conditions worsened across portions of northern and eastern China in July, with nine provinces enduring some of their lowest rain totals since 1961. Among the worst-hit areas were Shandong, Shaanxi, Henan, and Inner Mongolia, where the lack of rainfall has caused severe damage to crops and limited the availability of drinking water. In this photo, we see a farmer standing in dried and cracked earth that used to be the bottom of Zhifang Reservoir on July 29, 2014 in Dengfeng, China. Photo credit: ChinaFotoPress/ChinaFotoPress via Getty Images.


Disaster 3. Torrential monsoon rains over southern China July 13 - 18, 2014, killed 66 people and did $1.25 billion in damage. In this photo, a bridge in Fenghuang Ancient Town is submerged by flood waters on July 15, 2014 in Jishou, China. Image credit: ChinaFotoPress via Getty Images.

An El Niño Watch continues
July 2014 featured neutral El Niño conditions in the equatorial Eastern Pacific, and sea surface temperatures were near average in late July and the first half of August in the so-called Niño 3.4 region (5°S - 5°N, 120°W - 170°W), where SSTs must be at least 0.5°C above average for five consecutive months for an El Niño event to be declared. NOAA is continuing its El Niño Watch, but in early August dropped their odds of a fall El Niño from 80% to 65%.

Arctic sea ice falls to 4th lowest July extent on record
Arctic sea ice extent during July was the 4th lowest in the 36-year satellite record, according to the National Snow and Ice Data Center (NSIDC). The Northern Sea Route (also known as the Northeast Passage)--the shipping lane along the north coast of Russia in Arctic waters--was open by mid-August, according to ice edge analyses by the National Snow and Ice Data Center and the University of Illinois Cryosphere Today. The Northwest Passage through the Arctic waters of Canada was ice-choked, and will likely stay closed in 2014. Mariners have been attempting to sail these passages since 1497. The Northeast Passage opened to ice-free navigation for the first time in recorded history in 2005, with the Northwest Passage following suit during the summer of 2007. Both passages have been open multiple summers since then, as long-term melting of the ice has continued. However, this summer's weather in the Arctic has featured winds favorable for not letting sea ice drift out through Fram Strait, and we have seen the total volume of sea sea as estimated by the University of Washington PIOMAS model pull back from the record low set in 2012. It appears that some of the 2nd-year ice that survived the summer of 2013 will also survive the summer of 2014, so the ice pack is armoring itself a bit going into 2015 with a modest amount of multi-year ice compared to what we went into 2012 with (2012 set the record for lowest Arctic sea ice extent.)

Quiet in the tropics
A tropical wave located in the Central Atlantic, midway between the Lesser Antilles Islands and the Lesser Antilles Islands, is headed westwards at about 10 mph. Satellite loops show the wave has a broad, elongated surface circulation that has become less defined since Sunday, and heavy thunderstorm activity is almost entirely lacking due to moderate wind shear of 10 - 20 knots and dry air. Water vapor satellite images and the Saharan Air Layer analysis show that the wave is surrounded by a very dry airmass. Sea Surface Temperatures (SSTs) are marginal for development, about 27°C. The Monday morning run of one of our three reliable models for predicting tropical cyclone genesis, the UKMET model, did show some weak development of the wave by late in the week, with the wave arriving in the Lesser Antilles on Friday. Given the presence of so much dry air near the disturbance, the risk of development is low. Development odds will rise a little as the wave nears the Lesser Antilles late in the week, when ocean temperatures will be warmer and the atmosphere a little moister. In their 2 pm EDT Monday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day development odds of 0% and 10%, respectively.

Jeff Masters

Climate Summaries

Updated: 6:14 PM GMT on August 18, 2014

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Disaster Movie 'Into the Storm' is a Disaster

By: JeffMasters, 4:30 PM GMT on August 17, 2014

From the beginning of tornado disaster movie, "Into the Storm", we are treated to the most magnificent special effects that Hollywood can create. In the eighteen years since the infamous "Twister" of 1996, computer graphics technology has progressed remarkably. The spinning fire tornado and jetliners hurtling through the air in "Into the Storm" make the flying cows and clumsily rendered tornadoes of Twister seem quaint. Unfortunately, as is the case in nearly all disaster movies, the plot, dialogue, and acting of "Into the Storm" are a disaster. The movie opens humorously, with a scene featuring two low-wisdom yahoo storm chasers who look like they came straight out of "Jackass". They put on the most believable acting job of anyone in the movie, but unfortunately, are only minor characters. The rest of the movie features main characters whose acting ranges from mediocre to bad. Not only is the acting bad, but none of the characters are sympathetic, and there is very little character development. As a result, we have no one to root for. The characters range from bland (meteorologist Allison) to annoying (Vice Principal Gary, whose school gets devastated by a tornado) to drab (his son Donnie, who gets trapped in rubble with his wanna-be-girlfriend Kaitlyn), to downright obnoxious (storm chaser Pete.) The scene of Donnie and Caitlin trapped in tornado rubble and recording their final words on their cell phones for posterity is quite possibly the most melodramatic and painful disaster movie scene in cinema history. I had to shut my eyes and think about how good "Sharknado" was by comparison to shut out the interminably long debacle of dialogue and acting.


Figure 1. The cast of "Into the Storm" contemplate the very bad day they are having thanks to an onslaught of destructive tornadoes. Image credit: Official Into the Storm website.

Meteorologically, "Into the Storm" had less to complain about than other major weather disasters movies like "Twister", "The Day After Tomorrow", and "Sharknado". The tornadoes were believably rendered in most cases, and the damage they did was fairly realistically portrayed. Still, there were a lot of problems with the movie's meteorology. The winds of the tornadoes were able to hurl impressively heavy vehicles incredibly long distances, yet not blow the characters around much. A string of five separate tornadoes (not a multi-vortex tornado) were able to spin in very close proximity, something not observed in nature. In one scene, we are treated to the view up the inside of a tornado, which is remarkably symmetrical and light at the top. It would certainly be very dark at the top inside a tornado, and not so perfectly symmetrical.

The main characters of the movie were its tornadoes, and they certainly put on an impressive performance that was thrilling at times. But great special effects can't make up for awful plot, dialogue, and acting, and I give "Into the Storm" one and-a-half stars out of four. Aggregate critic ratings of the five major weather disaster movies of the past twenty years from the movie ratings site, Rottentomatoes.com, agree that "Into the Storm" was the worst one of the lot, with only 20% of critics liking the movie. The percentage of critics liking these movies:

82%: Sharknado
58%: Twister
54%: Sharknado 2
45%: The Day After Tomorrow
20%: Into the Storm

\
Video 1. Official trailer for "Into the Storm." The most impressive special effects are shown here, so save yourself $10 and the painful melodrama of the movie and just watch the trailer.

Quiet in the tropics
A tropical wave located in the middle Atlantic, midway between the Lesser Antilles Islands and the coast of Africa, is headed westwards at about 10 mph. Satellite loops show the wave has a broad, elongated surface circulation, but heavy thunderstorm activity is almost entirely lacking due to high wind shear of 20 knots and dry air. Water vapor satellite images and the Saharan Air Layer analysis show that the wave is surrounded by a very dry airmass. Sea Surface Temperatures (SSTs) are marginal for development, about 26°C, but will be warmer late in the week as the wave approaches the Lesser Antilles Islands. The Sunday morning runs of two of our three reliable models for predicting tropical cyclone genesis, the European and UKMET models, did show some weak development of the wave by late in the week. The models differed wildly in the forward speed of the disturbance, with the UKMET model predicting the wave would pass a few hundred miles northeast of the Lesser Antilles on Thursday night, and the European model predicting that this would not occur until Sunday night. Given the rather divergent opinions of the models and the presence of so much dry air, the risk of development this week is low. In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day development odds of 10% and 20%, respectively.

I'll have a new post by Monday afternoon at the latest.

Jeff Masters

Book and Movie Reviews

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Little Development Threat in Atlantic; The Story of This Week's Remarkable Floods

By: JeffMasters, 3:40 PM GMT on August 16, 2014

A tropical wave (Invest 95L) that moved off the coast of Africa on Friday was between the Cape Verde Islands and Africa on Saturday morning, but is headed west-northwest into drier air. Satellite loops show the wave has a closed surface circulation, but heavy thunderstorm activity is lacking due to high wind shear of 25 knots and dry air. Water vapor satellite images and the Saharan Air Layer analysis show that 95L is entering a dry airmass. Sea Surface Temperatures beneath 95L were a marginal 26°C on Saturday, but were predicted to fall to 25.5°C by Sunday, limiting the potential for development. The wave is drifting slowly west-northwest, and will affect the weather over the Cape Verde Islands Saturday and Sunday. None of the reliable computer models for predicting tropical storm formation (GFS, European, and UKMET) develop 95L. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave 94L 2-day and 5-day development odds of 20%.

A second tropical wave midway between the Cape Verdes Islands and the Lesser Antilles Islands is headed west at about 15 mph. This wave has a decent amount of spin, but has very limited heavy thunderstorms due to dry air. Some of the members of the GFS and European ensemble model predict that this wave could develop, but the atmosphere is likely far too dry for this to occur.


Figure 1. MODIS true-color image of 95L between the coast of Africa and the Cape Verde Islands at approximately 8:00 am EDT August 16, 2014. Image credit: NASA.

The story of this week's remarkable floods in Detroit, Baltimore, Long Island, and Maine
This week saw one of the most remarkable summer rainstorms in U.S. history, as Detroit, Baltimore, Long Island, and Maine all suffered top-five rainiest days in their history. Wunderground's weather historian, Christopher C. Burt, offers his take on the storm in his latest post, Incredible East Coast Rainfall Event of August 12-14. Dr. Marshall Shepherd, who is Director of the Atmospheric Sciences Program at University of Georgia and 2013 President of the American Meteorological Society (and hosts a noon EDT Sunday weather talk show on The Weather Channel called "Weather Geeks"), has a post discussing this topic in more detail, Recent Urban Floods: A simple equation. His equation:

Urban Flooding = Increase in intensity of top 1% rain events + expanding urban impervious land cover + storm water management engineered for rainstorms of "last century"


Figure 2. Flooding near Islip, New York, on August 13, 2014. Islip set an all-time New York state record for 24-hour precipitation with 13.57". Image credit: wunderphotographer Hurricane765.

Wunderground has an excellent new infographic on floods.

Wunderground's climate change expert, Dr. Ricky Rood, offers his thoughts on this week's remarkable floods.

Jeff Masters

Hurricane Flood

Updated: 1:56 AM GMT on August 17, 2014

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Dry Air Dominates the Atlantic

By: JeffMasters, 2:57 PM GMT on August 15, 2014

There are two tropical waves in the Eastern Atlantic worth mentioning today--one right at the coast of Africa, and another about 700 miles to its west, a few hundred miles southwest of the Cape Verde Islands. Both of these waves are headed west-northwest towards much drier air, and none of the reliable models for tropical cyclone formation is predicting development during the coming five days--though the wave farther from the coast of Africa has the UKMET model and a few members of the GFS and European model ensembles showing some weak development. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave the disturbance near the coast of Africa 2-day and 5-day odds of development of 0% and 20%, respectively. Dry air dominates the tropical Atlantic, and it will be difficult for a tropical storm to form in the coming week.


Figure 1. The satellite-based Saharan Air Layer (SAL) analysis for 8 am EDT Friday, August 15, 2014, showed two tropical waves off the coast of Africa that were headed west-northwest towards very dry air. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Tropical Storm Karina not threatening Hawaii
Tropical Storm Karina, which formed in the waters south of Mexico's Baja Peninsula on Wednesday, intensified into a hurricane for 12 hours on Thursday, but is now back down to tropical storm strength. None of the computer models are currently predicting the Karina will affect Hawaii, as the storm is expected to become entangled early next week with tropical disturbance 94C, which lies 1100 miles east-southeast of the Big Island of Hawaii. This tropical disturbance could also become a tropical storm this weekend; in their 8 am EDT Tropical Weather Outlook, the Central Pacific Hurricane Center gave 94C 2-day odds of development of 60%. Tropical Storm Julio, located about 770 miles north of Honolulu, Hawaii, is steadily disintegrating in the face of high wind shear, and should be dead by Saturday morning. Karina's intensification in a hurricane on Thursday gives the Eastern Pacific 11 named storms, 7 hurricanes, and 5 intense hurricane so far this season, which is well above average. In a typical Eastern Pacific hurricane season, there should have been 7 named storms, 4 hurricanes, and 1 intense hurricane by August 14.

Jeff Masters

Hurricane

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Quiet in the Atlantic; Karina Forms, Julio Dying in the Pacific

By: JeffMasters, 3:00 PM GMT on August 14, 2014

In the Central Pacific, Tropical Storm Julio, located about 700 miles north of Honolulu, Hawaii, is nearing its end as high wind shear rips away at it. Julio was a hurricane most of this week in the waters north of Hawaii where no hurricane had ever been recorded before. Ordinarily, hurricanes cannot exist in those waters because of sea surface temperatures that are near 25°C, which is too cold to support a hurricane. However, ocean temperatures have been near 26 - 26.5°C this week, which is about 1°C above average, and warm enough to support a hurricane.


Figure 1. Hurricane Julio in the North Pacific 600 miles north of Hawaii at 21:10 UTC August 13, 2014. Image credit: NASA.

Hawaii should keep an eye on Tropical Storm Karina, which formed in the waters south of Mexico's Baja Peninsula on Wednesday. Karina is expected to intensify into a hurricane by Friday, and is headed west towards Hawaii. However, none of the computer models are currently predicting the Karina will affect Hawaii, as the storm is expected to become entangled early next week with tropical disturbance 90E, which lies 1300 miles east-southeast of the Big Island of Hawaii. This tropical disturbance could also become a tropical storm this weekend; in their 8 am EDT Tropical Weather Outlook NHC gave 90E 5-day odds of development of 70%. Karina's formation gives the Eastern Pacific 11 named storms, 6 hurricanes, and 5 intense hurricane so far this season, which is well above average. In a typical Eastern Pacific hurricane season, there should have been 7 named storms, 4 hurricanes, and 1 intense hurricane by August 14.

Quiet in the Atlantic
In a typical Atlantic hurricane season, there should have been 3 named storms, 1 hurricane, and 0 intense hurricanes by August 14. So far this season, we have had 2 named storms (Arthur and Bertha), both of which became hurricanes. The typical formation date of the season's fourth named storm is August 23. There are no tropical cyclone threat areas in the Atlantic to discuss today, and none of the reliable models for tropical cyclone formation is predicting development during the coming five days. The next chance for tropical storm formation would appear to be from a tropical wave coming off the coast of Africa early next week, but dry air will continue to interfere with development of any potential Atlantic systems for at least the next week.

Jeff Masters

Hurricane

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Extreme Rains Swamp Baltimore and Long Island

By: JeffMasters, 1:59 PM GMT on August 13, 2014

An extreme deluge nearly unprecedented in Baltimore history swamped the city in flood waters that closed multiple expressways on Tuesday afternoon. Officially, 6.30" of rain fell at the Baltimore Airport on Tuesday. This was their second wettest calendar day in history, behind only the 7.62" that fell on August 23, 1933 during the Chesapeake-Potomac Hurricane. Remarkably, 3.95" of yesterday's Baltimore deluge fell in just 73 minutes. According to the NOAA Precipitation Frequency server, we would expect such a heavy 2-hour rainfall event to happen only once every 100 years. A 6.30" rainfall in 24 hours has a recurrence interval of once every 25 years.


Figure 1. Flooding in Baltimore, Maryland on Shell Avenue between Curtis Bay and Route 2 on August 12, 2014. Image posted to Instagram by blueagavebalto.

Extreme rains swamp Long Island and Connecticut
Dangerous flash flooding is occurring this Wednesday morning across Central Long Island, New York and Southern Connecticut as a low pressure area centered near New York City brings bands of heavy rain to its east. Rainfall rates as high as 5.34" per hour were observed in Islip, New York, triggering flooding that has forced the closure of multiple freeways, including the Long Island Expressway. Thunderstorms that repeatedly trained over the same point brought Islip 5.34" of rain between 5 - 6 am EDT, then another 4.37" between 6 - 7 am. The NWS reported that 13.20" of rain had fallen in Islip so far this morning, as of 10 am EDT. The record rainfall total for the entire month of August in Islip is 13.78".


Figure 2. Radar-estimated rain in Long Island, New York on August 13, 2014 exceeded 8" along a narrow swath.

Why such heavy rains?
Baltimore and Long Island's deluges comes on the heels of Monday's torrential rains in Detroit, whose roads were virtually shut down when the city's 2nd heaviest 24-hour rain since 1874, 4.57", fell. Portions of four major expressways remain closed in Detroit today due to flood damage, and states of emergency remain in effect for much of Detroit and some of its northern suburbs. All of these floods had two things in common: an unusually high level of water vapor in the atmosphere, and an unusually amplified jet stream. Precipitable water (a measure of water vapor) in Detroit on Monday and near Long Island last night was in the 99th percentile historically. The jet stream was in an unusually contorted configuration, with a strong trough of low pressure over the Eastern U.S., and sharp ridge of high pressure over the West. This allowed colder air than usual to move in aloft, increasing the instability of the atmosphere, causing stronger thunderstorm updrafts and heavier rains.


Figure 3. I-94 East in Detroit at Livernois on August 11, 2014. Image posted to Twitter by Ali B. (@AABaydoun.)

Monday's rains meant that four of Detroit's top ten rainiest days since 1874 have occurred in the past seventeen years. Yesterday's rains in Baltimore means that three of Baltimore's top ten rainiest days since 1871 have occurred during the past five years, and four of the top ten rainiest days have occurred in the past 15 years:

1) 7.62" August 23, 1933 (the great Chesapeake-Potomac Hurricane
2) 6.30" August 12, 2014
3) 6.02" September 30, 2010
4) 5.97" September 24, 1912
5) 5.85" July 8, 1952
6) 5.51" October 29, 2012 Hurricane Sandy)
7) 5.02" September 16, 1999 (Hurricane Floyd)
8) 5.00" September 27, 1985 (Hurricane Gloria)
9) 4.91" August 12, 1955 (Hurricane Connie)
10) 4.76" September 5, 1895


Figure 4. Percent changes in the amount of precipitation falling in very heavy events (the heaviest 1%) from 1958 to 2012 for each region. There is a clear national trend toward a greater amount of precipitation being concentrated in very heavy events, particularly in the Northeast and Midwest. Image credit: NCA Overview, updated from Karl et al. 2009.

If these numbers make you suspect that record heavy rains may be occurring more frequently in these cities due to a changing climate, then you're in good company. The U.S. National Climate Assessment, issued every four years by NOAA, is an effort by more than 300 U.S. scientists to assess how the climate is changing in the U.S. The just-released 2014 report said: “Heavy downpours are increasing nationally, especially over the last three to five decades. Largest increases are in the Midwest and Northeast. Increases in the frequency and intensity of extreme precipitation events are projected for all U.S. regions.” Fundamentally, a warmer atmosphere will evaporate more moisture from the oceans, resulting in more days with 99th percentile water vapor in the atmosphere, and increased chances of very heavy rainfall events like this week's deluges in Detroit, Baltimore, and Islip.

Quiet in the Atlantic
There are no tropical cyclone threat areas in the Atlantic to discuss today, and none of the reliable models for tropical cyclone formation is predicting development during the coming five days.

Jeff Masters

Flood Climate Change

Updated: 2:12 PM GMT on August 13, 2014

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Extreme Rains Swamp Detroit

By: JeffMasters, 5:43 PM GMT on August 12, 2014

An extreme deluge nearly unprecedented in Detroit history brought the Motor City to a virtual standstill during the evening rush hour on Monday, with 4.57" of rain falling at the official measurement site at Detroit Metro Airport. The only wetter day in Detroit history (4.75") occurred on July 31 - August 1, 1925--over two years before the Ford Model A went into production. A slow-moving low pressure system tracked over Michigan on Monday, bringing thunderstorms that dumped 4 - 6" of rain--nearly two months' worth--over Metro Detroit in just four hours. Many major expressways in the city remain closed today due to floodwaters and flood damage. The northern Detroit suburb of Warren, where 5" of rain fell, has declared a state of emergency. Over 1,000 vehicles have been abandoned on major thoroughfares, and many more in neighborhoods. One person died in the storm--a heart attack victim found dead in her flooded car. All of Detroit's major expressways were closed due to flooding during the deluge; in my 40 years living in the area, I've never seen a flood do that. Detroit radar shows that a new round of showers is affecting the area this afternoon, which will slow efforts to drain the water off of blocked roads.


Figure 1. I-94 East in Detroit at Livernois on August 11, 2014. Image posted to Twitter by Ali B. (@AABaydoun.)


Figure 2. Erosion damage to I-75 South near I-696 in Detroit on August 12, 2014. Image credit: @MDOT_MetroDet via Twitter.


Figure 3. Rainfall amounts in the 24-hour period ending at noon EDT August 12, 2014, for Southeast Michigan. A wide area of 4+ inches of rain fell across Detroit and its northern suburbs. Image credit: National Weather Service.


Figure 4. The Rouge River in Detroit crested 4.5' above flood stage on Tuesday morning, August 12, 2014. This was the 5th highest crest since 1950. Fortunately, there are not many homes and businesses in the Rouge River flood plain in Detroit. Image credit: NOAA.

Thanks go to TWC's Jon Erdman and Nick Wiltgen for some of the links in this post. TWC has a good article with many images of the historic flood.

Jeff Masters

Flood

Updated: 5:51 PM GMT on August 12, 2014

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Dry Air Dominating the Tropical Atlantic; the Pacific Quieting Down

By: JeffMasters, 3:09 PM GMT on August 12, 2014

Tropical wave 94L was located near 14°N, 35°W on Tuesday morning, but is no longer a threat to develop. Satellite loops show that the wave has lost nearly all of its heavy thunderstorms, and water vapor satellite images and the Saharan Air Layer analysis show that 94L is now embedded in a very dry environment. The wave should arrive in the Lesser Antilles Islands on Saturday, bringing gusty winds and heavy rain showers. None of the reliable computer models for predicting tropical storm formation (the GFS, European, and UKMET) develop 94L or anything else in the Atlantic during the coming five days. In their 8 am EDT Tuesday Tropical Weather Outlook, NHC gave 94L a 0% chance of developing. Like last year, the tropical Atlantic and Caribbean continue to be dominated by high pressure and dry, sinking air, which discourages tropical storm formation. The latest 2-week run of the GFS model shows this pattern continuing into at least the last week of August.


Figure 1. Satellite analysis of the Saharan Air Layer (SAL) done at 8 am EDT August 12, 2014, shows a large area of dry air covering much of the Atlantic. The dry air comes from both the Sahara and from sinking air from an unusually strong area of high pressure over the Atlantic. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.


Figure 2. Vertical instability over the tropical Atlantic between the Lesser Antilles Islands and coast of Africa in 2014. The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Normal instability is the black line, and this year's instability levels are in blue. The atmosphere has been dominated by high pressure and dry, sinking air since February, which has made it difficult for tropical storms to develop. Instability has also been unusually low over the Caribbean, but has been near average over the rest of the Atlantic. Image credit: NOAA/CIRA.

The Pacific quiets down
In the Pacific, we have only one active named storm: Tropical Storm Julio, with top winds of 65 mph, which is about 700 miles north of Hawaii and headed northwards out to sea. Julio passed well north of the Hawaiian islands over the weekend, maintaining hurricane strength over a portion of the ocean where no hurricane had ever been recorded before.

NHC is tracking two disturbances in the Eastern Pacific: Invest 99E, located a few hundred miles south-southwest of Manzanillo, Mexico, and an area of disturbed weather located about 1500 miles east-southeast of the Big Island of Hawaii. Residents of Hawaii should keep an eye on this second disturbance, which NHC gave 2-day and 5-day odds of development of 10% and 30%, respectively, in their 8 am EDT Tuesday Tropical Weather Outlook. The GFS model predicts this disturbance will develop by this weekend and come within 500 miles of Hawaii in 7 - 8 days. The model also predicts that 99E will develop, but stay well away from any land areas.


Figure 3. Enormous tree damage was done to forests on Hawaii's Big Island by Tropical Storm Iselle. Image credit: wunderphotographer mountainwx.

Hawaii cleans up after Tropical Storm Iselle
Power is still out to about 10% of the people on Hawaii's Big Island from Tropical Storm Iselle, which hit the southeast shore of the island on Friday as a tropical storm with 60 mph winds. Iselle is only the second tropical storm on record to hit the Big Island, and was the strongest. About 22,000 customers lost power during the height of the storm on the Big Island, and many of the 8,000 customers still without power may not see their electricity restored until next week, said Hawaii Electric Light. The power company said, "the extent of damage is worse than anything we’ve ever seen here." Iselle damaged about 150 homes and businesses on the Big Island.

Jeff Masters

Hurricane

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LIttle Change to African Tropical Wave 94L Headed Towards Lesser Antilles

By: JeffMasters, 2:58 PM GMT on August 11, 2014

A tropical wave (Invest 94L) that moved off the coast of Africa on Saturday was located near 12°N, 26°W on Monday morning, and could potentially be a tropical storm when it arrives in the Lesser Antilles Islands on Saturday. Satellite loops show the wave has a modest amount of spin and respectable amount of heavy thunderstorms. Water vapor satellite images and the Saharan Air Layer analysis show that 94L is located in a fairly moist environment, with the dry air coming off or Africa located well to the north and west of the disturbance. Wind shear was a high 25 knots, but the 8 am EDT Monday run of the SHIPS model predicted that wind shear would fall to the low range, 5 - 10 knots, by Wednesday. Sea Surface Temperatures beneath 94L were 27°C on Monday, but were predicted to fall to 25.5°C by Wednesday, limiting the potential for development through Wednesday. The wave is headed west at 15 - 20 mph, and by Friday, will move over warmer waters of 27°C, as the storm nears the Lesser Antilles Islands. However, 94L will gain some latitude as it approaches the islands, and move into increasingly dry air to the northwest. These conditions are similar to what Bertha encountered as it approached the islands. If 94L does develop, the odds are that it will be a storm similar to Bertha--struggling against dry air, never reaching hurricane strength in the islands. Arrival in the islands should occur on Saturday, according to the Monday morning runs of the GFS and European models. None of the reliable computer models for predicting tropical storm formation (GFS, European, and UKMET) develop 94L, but about 1/3 of the 20 members of the GFS model ensemble show development late this week (the GFS ensemble is a set of 20 runs of the GFS model done at lower resolution with slightly different initial conditions to generate an uncertainty "plume" of model runs.) None of these ensemble forecasts showed 94L reaching hurricane strength. In their 8 am EDT Monday Tropical Weather Outlook, NHC gave 94L 2-day and 5-day development odds of 0% and 30%, respectively.


Figure 1. Satellite analysis of the Saharan Air Layer (SAL) done at 8 am EDT August 11, 2014, showing Invest 94L lying just south of a large area of dry air that covered much of the Atlantic. Image credit: University of Wisconsin CIMSS/NOAA Hurricane Research Division.

Jeff Masters

Hurricane

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African Wave 94L Worth Watching; Halong Kills 9 in Japan

By: JeffMasters, 2:58 PM GMT on August 10, 2014

A tropical wave that moved off the coast of Africa on Saturday was located near 11°N, 21°W on Sunday morning, and was designated Invest 94L by NHC . Satellite loops show the wave has a modest amount of spin and respectable amount of heavy thunderstorms. Water vapor satellite images and the Saharan Air Layer analysis show that 94L is located in a fairly moist environment, with the dry air coming off of Africa located well to the north and west of the disturbance. Wind shear was a high 25 - 30 knots, but the 8 am EDT Sunday run of the SHIPS model predicted that wind shear would fall to the moderate range on Monday afternoon, then to the low range on Tuesday afternoon. The wave is headed west at 15 - 20 mph, and should arrive in the Lesser Antilles Islands by Saturday, according to the Sunday morning runs of the GFS and European models. None of the reliable computer models for predicting tropical storm formation develop 94L, but about 1/3 of the 20 members of the GFS model ensemble show development late this week (the GFS ensemble is a set of 20 runs of the GFS model done at lower resolution with slightly different initial conditions to generate an uncertainty "plume" of model runs.) In their 8 am EDT Sunday Tropical Weather Outlook, NHC gave 94L 2-day and 5-day development odds of 10% and 30%, respectively. Mid-August is the time when the Atlantic hurricane season kicks into high gear, and 94L is definitely a disturbance we need to watch.


Figure 1. True-color MODIS image from approximately 9 am EDT August 10, 2014, showing Invest 94L off the coast of Africa, south of the Cape Verde Islands. Image credit: NASA.

Tropical Storm Halong hits Japan
Slow-moving Tropical Storm Halong finally made landfall in Southern Japan near Aki city, Kōchi Prefecture, at approximately 5 pm EDT Saturday (6 am Sunday in Japan.) Despite weakening to a 70 mph tropical storm before landfall, Halong dumped extremely dangerous heavy rains over Southern Japan, with storm total rainfall amounts in excess of one meter in some locations. A rare "emergency weather warning" (tokubetsu keihō) for the Mie Prefecture was issued on Saturday by the Japan Meteorological Agency (JMA). Hakusan in the Mie Prefecture had nearly 17 inches of rain on Saturday, breaking its all-time 24-hour rainfall record set just last year in Typhoon Man-yi. The top winds near landfall, reported at Cape Muroto, were 94 mph, gusting to 117 mph (42.1 m/s gusting to 52.5 m/s). At least nine people were killed and 70 injured in the floods, according to the Japan Times.


Figure 2. Radar image of Tropical Storm Halong making landfall on the coast of Japan at 6:50 pm EDT August 9, 2014 (05:50 JST August 10) . Halong had 70 mph winds at the time. Image credit: Japan Met Agency.

Jeff Masters

Hurricane

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Iselle Dissipates; Rare Emergency Warning for Tropical Storm Halong in Japan

By: JeffMasters, 2:59 PM GMT on August 09, 2014

Tropical Storm Iselle has dissipated after making landfall along the southeast shore of Hawaii's Big Island near 9 am EDT (3 am HST) Friday as a tropical storm with 60 mph winds. Iselle is only the second tropical storm on record to hit the Big Island, and was the strongest. Iselle brought torrential rains of up to 4" per hour to the Big Island; two locations received over 14" of rain. Iselle did considerable damage on the Big Island, downing trees, knocking down power lines, and damaging a few homes in Hawaiian Paradise Park in Puna. About 22,000 customers lost power during the height of the storm on the Big Island; power had been restored to all but 9,000 by Saturday morning. About 1900 customers lost power on Oahu, and 8,000 on Maui.


Figure 1. Damage to power lines on the Big Island on August 8, 2014, from Tropical Storm Iselle. Image credit: Hawaiian Electric Companies.

Some peak wind gusts and rainfall amounts from Iselle:

72 mph gust at Oahu Forest mesonet site on Oahu (2300')
68 mph gust at Kaneola, HI (815')
91 mph gust at Mauna Kea on the Big Island (13,700')
62 mph gust at Lanai
61 mph gust at Kula, Maui
57 mph gust at Molokai
54 mph gust at Hilo Airport

14.51", Kulani NWR
14.28", Saddle Quarry
13.90", Glenwood
12.52", Hakalu
12.19", Pua Akala
3.61", Hilo Airport

Wunderground's weather historian, Christopher C. Burt, has an interesting post showing how Iselle's rainfall on the Big Island fell in climatologically favored regions that receive heavy rains at other times of year.


Figure 2. True-color MODIS image of Hurricane Iselle from 23:15 UTC (7:15 pm EDT) August 7, 2014. At the time, the outer spiral bands of the 80 mph Category 1 hurricane were spreading over the Big Island of Hawaii. Image credit: NASA.


Figure 3. Score: Mauna Loa 1, Iselle 0. The twin 13,000' peaks of the Big Island, Mauna Loa and Mauna Kea, seriously disrupted Tropical Storm Iselle as its center crossed the southern portion of the Big Island, as seen in this true-color MODIS image from approximately 21:15 UTC (5:15 pm EDT) August 8, 2014. At the time, Iselle had top winds of 50 mph. Image credit: NASA.

Hurricane Julio expected to skirt Hawaii
Hurricane Julio continues to steadily weaken, and was a Category 2 storm with 100 mph winds at 11 am EDT Saturday morning. Satellite loops show that the cloud tops of Julio's heavy thunderstorms have warmed and the eye is no longer distinct. The storm should be able to take advantage of light to moderate wind shear and marginally warm sea surface temperatures near 26°C and maintain at least Category 1 status until Sunday morning. Fortunately, it is looking increasingly likely that Julio will not have a major impact on the Hawaiian Islands. The Saturday morning runs of our top track models all predicted that the center of Julio would pass 100 - 400 miles northeast of the Hawaiian Islands on Sunday and Monday. On this path, Julio's core of heavy rains and wind would miss the islands, and high surf would be the main impact of the storm. The edge of Julio's cone of uncertainly for Sunday no longer lies over the islands.



Figure 4. Super Typhoon Genevieve as seen by the VIIRS instrument on the Suomi satellite on 01:30 UTC August 8, 2014. In the infrared image (top), note how the temperature in the eye was as warm as 25°C, while the coldest cloud tops of the eyewall thunderstorms were -80°C, indicating that they had risen very high into the atmosphere where the air is cold. At the time, Genevieve was a Category 5 storm with 160 mph winds. Image credit: Dan Lindsey, NOAA/CIRA.

Typhoon Genevieve not a threat to land
What was formerly Hurricane Genevieve is now Typhoon Genevieve, after the storm crossed the International Date Line from east to west early Thursday. There is no difference between a North Pacific hurricane and a typhoon other than its location--if the storm is west of the Date Line, it is called a typhoon, and if it is east of the Date Line, it is called a hurricane. Genevieve put on a spectacular display of rapid intensification, going from a tropical storm with 60 mph winds to a Category 5 super typhoon with 160 mph winds in just 27 hours, from 09 UTC August 6 to 12 UTC August 7. Genevieve spent 24 hours as a Category 5 storm, but weakened to a Category 3 storm with 120 mph winds as of Saturday at 8 am EDT. Satellite images show that Genevieve is still an impressive storm with a large eye surrounded by intense eyewall thunderstorms with cold cloud tops. Fortunately, Genevieve is not expected to threaten any land areas.


Figure 5. Radar image of Tropical Storm Halong nearing the coast of Japan at 9:55 am EDT (22:55 JST) August 9, 2014. Halong had 70 mph winds at the time. Image credit: Japan Met Agency.

Rare emergency warning for Tropical Storm Halong in Japan
In the Western Pacific, slow-moving Tropical Storm Halong weakened to a 70 mph tropical storm Saturday, but is dumping dangerous heavy rains into Southern Japan. A rare "emergency weather warning" (tokubetsu keihō) for the Mie Prefecture was issued on Saturday by the Japan Meteorological Agency (JMA). Hakusan in the Mie Prefecture had nearly 17 inches of rain on Saturday, breaking its all-time 24-hour rainfall record set just last year in Typhoon Man-yi. The Sukumo observation site in Kōchi prefecture set an all-time calendar-day record rainfall today of 327.5 mm (12.89 inches) with records dating back to 1943. Rainfall rates have been 1 to 2 inches pre hour across most of Kōchi Prefecture on Saturday. As of 9 pm JST Saturday, the center of Halong was 60 km (35 mi) south of Cape Ashizuri, Kōchi Prefecture; the Shimizu observation site, 5 miles northwest of that cape, clocked an 83-mph gust at 9:23pm JST (8:23am US EDT). That's the highest gust anywhere in Japan (including the smaller southern islands) so far Saturday their time. Farther east, Cape Muroto clocked a sustained wind of 27.0 m/s (60 mph) at 9:58pm JST (8:58am US EDT), the top sustained wind for all of Japan today. Source: JMA (thanks to TWC's Nick Wiltgen for these stats.) Satellite loops show that Halong is a very large system, and the rains from this massive, slow-moving storm are going to cause serious flooding problems in Japan.

The Atlantic is quiet
In the Atlantic, there are no threat areas to discuss, and none of the reliable models for predicting tropical cyclone genesis predicts development over the next five days. The African Monsoon will crank out strong tropical waves that will emerge from the coast of Africa on Wednesday and next Saturday. The peak part of the Atlantic hurricane season usually begins in mid-August, so we will need to start paying extra attention to these tropical waves.

Jeff Masters

Hurricane

Updated: 3:00 PM GMT on August 09, 2014

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Tropical Storm Iselle Hits Hawaii's Big Island

By: JeffMasters, 3:50 PM GMT on August 08, 2014

Tropical Storm Iselle made landfall along the southeast shore of Hawaii's Big Island near 9 am EDT (3 am HST) as a tropical storm with 60 mph winds. Iselle is only the second tropical storm on record to hit the Big Island, and was the strongest. The Big Island's other tropical storm was an unnamed 1958 storm that had sustained winds of 50 mph at landfall. Iselle is just the fourth tropical storm or hurricane to make a direct hit on any Hawaiian Island since accurate records began in 1949. Iselle is bringing torrential rains to the Big Island, where a rain gauge near Pahala indicated rain rates at nearly 4 inches per hour. A Flash Flood Warning is in effect for this area, and all of the Hawaiian Islands are under a Flash Flood Watch today. It is too early to assess what damage Iselle may have done, but the NWS reported roofs flying off and downed trees in Hawaiian Paradise Park, and at least 21,000 customers were without power early Friday morning on the Big Island. Some peak wind gusts and rainfall amounts on the Big Island so far from Iselle, as of 11 am EDT (5 am HST) Friday:

Hilo Airport: 32 mph gusting to 54 mph at 11 pm HST, 2.33" of rain
Kona Airport: 32 mph gusting to 45 mph at 1:53 pm HST
Bradshaw AFB: 23 mph gusting to 43 mph at 5 am HST, .23" of rain

The winds on top of the highest point in Hawaii, the Big Island's Mauna Kea, elevation 13,796' (4,205 m), gusted up to 72 mph this morning at the University of Hawaii 88" telescope.

Five stations on the windward side of the Big Island had received at least 10" of rain in 24 hours as of 5 am HST Friday, according the NWS Hawaii rainfall summary:

Hakalau: 10.70"
Pua Akala: 10.19"
Saddle Quarry: 11.39"
Glenwood: 10.63"
Kulani NWR: 11.19"


Figure 1. Radar image from the South Hawaii radar at 7:49 am EDT August 8, 2014 of Tropical Storm Iselle near landfall on the Big Island. The radar beam is being intercepted by the high mountains of Hawaii, and cannot "see" to the northwest.


Figure 2. True-color MODIS image of Hurricane Iselle from 23:15 UTC (7:15 pm EDT) August 7, 2014. At the time, the outer spiral bands of the 80 mph Category 1 hurricane were spreading over the Big Island of Hawaii. Image credit: NASA.

Forecast for Iselle
Friday morning satellite images showed that Iselle's thunderstorms continued to be very vigorous with cold cloud tops, but interaction with the high peaks of Mauna Loa and Mauna Kea had severely disrupted the circulation. The storm will have difficultly re-organizing once its center emerges over the ocean, since wind shear is a very high 25 - 30 knots, and water vapor satellite images are showing a lot of dry air on the west side of the Big Island. The shear and dry air should be enough to destroy Iselle by Saturday afternoon.


Figure 3. True-color MODIS image of Hurricane Julio from 19:30 UTC (3:30 pm EDT) August 7, 2014. At the time, Julio was a Category 2 hurricane with 105 mph winds. Image credit: NASA.

Hurricane Julio expected to skirt Hawaii
Hurricane Julio intensified into a Category 3 hurricane with 115 mph winds overnight, becoming the fifth major hurricane in the Eastern Pacific so far in 2014. This is an inordinately high number of major hurricanes--usually, the Eastern Pacific has only three major hurricanes in an entire season, and just one by August 8. Though Julio had weakened to a Category 2 storm with 105 mph winds on Friday morning, satellite loops show that Julio still has an impressive area of heavy thunderstorms and well-formed eye, though the cloud tops are warming, indicating weakening. The storm should be able to take advantage of light to moderate wind shear and marginally warm sea surface temperatures near 26°C and maintain at least Category 1 status until Sunday morning. Fortunately, it is looking increasingly likely that Julio will not have a major impact on the Hawaiian Islands. The Friday morning runs of our top track models all predicted that the center of Julio would pass 100 - 300 miles northeast of the Hawaiian Islands on Sunday. On this path, Julio's core of heavy rains and wind would miss the islands, and high surf would be the main impact of the storm. The edge of Julio's cone of uncertainly for Sunday no longer lies over the islands.

Super Typhoon Genevieve not a threat to land
Farther west in the Pacific, what was formerly Hurricane Genevieve is now Super Typhoon Genevieve, after the storm crossed the International Date Line from east to west early Thursday. There is no difference between a North Pacific hurricane and a typhoon other than its location--if the storm is west of the Date Line, it is called a typhoon, and if it is east of the Date Line, it is called a hurricane. This only applies to storms in the Pacific in the Northern Hemisphere; in the Southern Hemisphere's Pacific Ocean, everything is called a Tropical Cyclone regardless of which side of the Date Line it falls on. Genevieve put on an amazing display of rapid intensification, going from a tropical storm with 60 mph winds to a Category 5 super typhoon with 160 mph winds in just 27 hours, from 09 UTC August 6 to 12 UTC August 7. Genevieve spent 24 hours as a Category 5 storm, before weakening slightly to a 150 mph Category 4 storm at 8 am EDT Friday. Satellite images still show an very impressive storm with a large eye surrounded by a giant area of intense eyewall thunderstorms with very cold cloud tops. Fortunately, Genevieve is not expected to threaten any land areas.


Figure 4. Typhoon Halong as photographed and tweeted by astronaut Alexander Gerst at 4 pm EDT August 7, 2014. At the time, Halong was a Category 1 storm with 85 mph winds.

Typhoon Halong drenching Japan
In the Western Pacific, Typhoon Halong was a Category 1 storm with 80 mph winds at 8 am EDT Friday, and was spreading heavy rains into Southern Japan. Satellite loops show that Halong is a very large system. Halong is moving north at just 5 mph, and will bring Southern Japan an extended period of heavy rain today through Saturday as a Category 1 typhoon.

The Atlantic is quiet
In the Atlantic, there are no threat areas to discuss, and none of the reliable models for predicting tropical cyclone genesis predicts development over the next five days.

Jeff Masters

Hurricane

Updated: 6:38 PM GMT on August 08, 2014

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Hurricane Iselle Lashing Hawaii's Big Island With Heavy Rain

By: JeffMasters, 12:31 AM GMT on August 08, 2014

Hurricane warnings continue for the Big Island as Hurricane Iselle closes in as a Category 1 hurricane with 80 mph winds. The latest center fix from the Hurricane Hunters at 22:55 UTC Thursday (6:55 pm EDT or 12:55 HST) found the pressure had risen to 992 mb, up 7 mb since this morning. They also noted that the eyewall had collapsed, so Iselle is weakening. However, this weakening had yet to translate to a reduction in the maximum sustained surface winds, which were still 75 - 80 mph during their last two passes through the center. Thursday afternoon satellite images showed that Iselle's eyewall thunderstorms continued to be very vigorous with cold cloud tops. The outer spiral bands of Iselle were lashing the Big Island, as seen on Hawaii radar.


Figure 1. Radar image from the South Hawaii radar showing the outer spiral bands of Hurricane Iselle affecting the Big Island. The North Hawaii radar will be intermittent today, due to high winds blowing the radome hatch open, according to the NWS.

Forecast for Iselle
There isn't much time for Iselle to change much before the center makes landfall on the Big Island, which should occur near 10 pm HST (4 am EDT.) Iselle's top winds at landfall will likely be between 65 - 75 mph. The main threat from Iselle will be heavy rains leading to flash flooding and mudslides. The Thursday morning 12Z run of the GFDL model predicted that Iselle would dump widespread rains of 4 - 8" over the islands, with some regions seeing 8 - 16". Wind damage is also a concern from Iselle; the 5 pm EDT Thursday Wind Probability Forecast from Central Pacific Hurricane Center (CPHC) gave Hilo on the Big Island a 94% chance of experiencing tropical storm-force winds of 39+ mph, and a 12% chance of hurricane-force winds. These odds were 35% and 0%, respectively, for Honolulu. On the higher terrain of the islands, winds will be up to 30% stronger than what is observed at sea level. High surf of 10 - 20' and higher will also pound the islands, causing erosion problems and coastal flooding. Since accurate landfall records began in 1949, only one tropical storm (an unnamed storm in 1958) and no hurricanes have ever hit the Big Island.


Figure 2. True-color MODIS image of Hurricane Julio from 19:30 UTC (3:30 pm EDT) August 7, 2014. At the time, Julio was a Category 2 hurricane with 105 mph winds. Image credit: NASA.

Category 2 Hurricane Julio expected to skirt Hawaii
Hawaii's other hurricane threat is Hurricane Julio, which remained a Category 2 hurricane with 105 mph winds at 5 pm EDT on Thursday, but appears to be intensifying. Satellite loops show that Julio has maintained a respectable area of heavy thunderstorms and well-formed eye. The storm should be able to take advantage of light to moderate wind shear and marginally warm sea surface temperatures near 26°C and maintain at least Category 2 status until Friday, when a slow weakening trend should begin. With the notable exception of the GFDL model, which forecast a direct hit on Kauai, the 12Z (8 am EDT) Thursday runs of our top track models predicted that the center of Julio would pass 50 - 200 miles northeast of the Hawaiian Islands on Sunday. On this path, Julio's core of heavy rains and wind would miss the islands, and high surf would be the main impact of the storm. The edge of Julio's cone of uncertainly for Sunday still lies over the islands, so we cannot yet be confident of this track, but at this time it appears that Hawaii will avoid torrential rains from Julio falling upon soils already saturated by Iselle. The 5 pm EDT Thursday wind probability forecast from NHC gave Honolulu a 25% chance of receiving tropical storm-force winds of 39+ mph from Julio, and a 1% chance of hurricane-force winds.


Figure 3. Webcam view from the top of Mauna Kea at 1:40 pm HST August 7, 2014, as the rains from Hurricane Iselle were falling. Image credit: Mauna Kea Weather Center.

The winds at 13,000 feet on Mauna Kea
The weather on top of the highest point in Hawaii, the Big Island's Mauna Kea, elevation 13,796' (4,205 m), will be interesting to follow as Iselle makes landfall. Winds have risen steadily today, and were 25 mph, gusting to 40 mph, at 2:10 pm HST (8:10 pm EDT) at the University of Hawaii 88" telescope (UH88.) There are six anemometers reporting winds on top of Mauna Kea today, but beware of the data from the Canada - France - Hawaii Telescope (CFHT). The Mauna Kea webcam page says that those winds are highly exaggerated due to location of the anemometer tower between two large telescope domes. You can see this tower on Google Maps.

If the center of Iselle passes directly over Mauna Kea, the mountain may set a new record for highest location ever to encounter a direct hit by a tropical storm or hurricane. There are no peaks in the Caribbean as high as Mauna Kea--the highest being Pico Duarte in the Dominican Republic, at 10,161' (3,098 meters). Taiwan's highest point is Yu Shan (Jade Mountain) at 12,963' (3,952 meters). In the Indian Ocean, no tropical cyclone has ever come close to reaching the Himalayas.  The closest approach was either Aila (2009) or an unnamed storm in 1925, both of which only reached maximum elevations of about 400 meters before dissipating--near Mal Bazar, India and Sanischare, Nepal, respectively. ‪Pico de Orizaba (18,486', 5,636 meters) in Mexico, about 70 miles west of Veracruz, had Hurricane Karl (2010) pass within 25 miles of the peak (to the south) as a Category 2 storm, according to the ‪NOAA/CSC hurricane database. However,‬ I doubt the peak experienced the calm of the eye. Thanks go to Phil Klotzbach, Will Komaromi, and Brad Barrett for helping with these stats.

Links
Weather on Mauna Kea
Live stream from KHON2 TV in Honolulu
Central Pacific Hurricane Center
2-km resolution WRF model output from the University of Hawaii for Hawaii
Storm surge maps for Oahu
Storm info from Tropical Tidbits
NWS Honolulu

Jeff Masters

Hurricane

Updated: 1:15 AM GMT on August 08, 2014

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Hurricane Iselle Bearing Down on Hawaii

By: JeffMasters, 4:13 PM GMT on August 07, 2014

Hurricane warnings are flying for the Big Island as Hurricane Iselle bears down on Hawaii as a Category 1 hurricane with 80 mph winds. This is the first hurricane warning issued for the main Hawaiian Islands in 21 years--since Hurricane Fernanda in 1993 (which ended up missing.) Thursday morning satellite images showed very little weakening of Iselle. The eyewall's heavy thunderstorms continued to maintain their intensity and areal coverage, and the eye was still prominent. The outer spiral bands of Iselle had already arrived on the Big Island as of early Thursday morning, as seen on Hawaii radar. Wind shear was moderate, about 15 knots, due to strong upper-level winds out of the north, but this shear is having only a minor impact on Iselle.


Figure 1. Hurricane Iselle, with 90 mph winds, and Hurricane Julio, with 75 mph winds, steam west-northwest towards the Hawaiian Islands in this GOES-West image taken at 8 pm EDT Wednesday, August 6, 2014. Image credit: NASA/Goddard Space Flight Center.

Forecast for Iselle
Wind shear is expected to stay moderate until Iselle reaches the Big Island Thursday evening. Ocean temperatures will remain a marginal 26 - 26.5°C, and the atmosphere surrounding the storm will be fairly dry, resulting in continued slow weakening of Iselle. Our top intensity models are in decent agreement on Iselle's strength, predicting that the storm will weaken by 5 - 10 mph, to top winds of 70 - 75 mph, before landfall. The official CPHC forecast for a 75 mph minimal Category 1 hurricane at 8 pm EDT Thursday is reasonable. The main threat from Iselle will be heavy rains leading to flash flooding and mudslides. The Thursday morning 06Z run of the GFDL model predicted that Iselle would dump widespread rains of 4 - 8" over the islands, with some regions seeing 8 - 16". Wind damage is also a concern from Iselle; the 11 am EDT Thursday Wind Probability Forecast from Central Pacific Hurricane Center (CPHC) gave Hilo on the Big Island a 88% chance of experiencing tropical storm-force winds of 39+ mph, and a 16% chance of hurricane-force winds. These odds were 45% and 2%, respectively, for Honolulu. On the higher terrain of the islands, winds will be up to 30% stronger than what is observed at sea level. High surf of 10 - 20' and higher will also pound the islands, causing erosion problems and coastal flooding. Since accurate landfall records began in 1949, only one tropical storm (an unnamed storm in 1958) and no hurricanes have ever hit the Big Island.


Figure 2. Predicted rainfall along the track of Hurricane Iselle from the 06Z (2 am EDT) Thursday August 7, 2014 run of the GFDL model. The model forecast that Iselle would be a strong tropical storm with 65 mph winds at landfall Thursday evening. Widespread rains of 4 - 8" were predicted over the islands, with some regions seeing 8 - 16". Image credit: NOAA/GFDL.

Hurricane Julio intensifies, expected to skirt Hawaii
Hawaii's other hurricane threat is Hurricane Julio, which intensified to a Category 2 hurricane with 105 mph winds at 11 am EDT on Thursday. Satellite loops show that Julio now has a respectable area of heavy thunderstorms and well-formed eye. The storm should be able to take advantage of light to moderate wind shear and marginally warm sea surface temperatures near 26°C and maintain Category 2 status until Friday, when a slow weakening trend should begin. With the notable exception of the GFDL model, our top track models continue to predict that the center of the storm will pass about 100 miles northeast of the Hawaiian Islands on Sunday. On this path, Julio's core of heavy rains and wind would miss the islands, and high surf would be the main impact of the storm. The edge of Julio's cone of uncertainly for Sunday still lies over the islands, so we cannot yet be confident of this track, but at this time it appears that Hawaii will avoid torrential rains from Julio falling upon soils already saturated by Iselle.


Figure 3. True-color MODIS image of Hurricane Genevieve from 22:45 UTC (6:45 pm EDT) August 6, 2014. At the time, Genevieve was intensifying into a Category 4 hurricane with 135 mph winds. Image credit: NASA.

Hurricane Genevieve becomes Super Typhoon Genevieve
Farther west in the Pacific, what was formerly Hurricane Genevieve has now become Super Typhoon Genevieve, after the storm crossed the International Date Line from east to west early Thursday. Genevieve put on an amazing display of rapid intensification, going from a tropical storm with 60 mph winds to a Category 5 super typhoon with 160 mph winds in just 27 hours, from 09 UTC August 6 to 12 UTC August 7. Satellite images show an very impressive storm with a large eye surrounded by a giant area of intense eyewall thunderstorms with very cold cloud tops. Fortunately, Genevieve is not expected to threaten any land areas. Genevieve reached major hurricane status shortly before crossing the Date Line, so the Eastern Pacific has now had 10 named storms, 6 hurricanes, and 4 intense hurricanes so far in 2014. On average, we expect to see just 7 named storms, 3 hurricanes, and 1 intense hurricane by August 7. It is rare for a tropical cyclone to cross the Date Line; the last storm I'm aware of to do so was Category 5 Hurricane Ioke of 2006, which was renamed Super Typhoon Ioke when it crossed the Date Line on August 27. There is no difference between a North Pacific hurricane and a typhoon other than its location--if the storm is west of the Date Line, it is called a typhoon, and if it is east of the Date Line, it is called a hurricane. This only applies to storms in the Pacific in the Northern Hemisphere; in the Southern Hemisphere's Pacific Ocean, everything is called a Tropical Cyclone regardless of which side of the Date Line it falls on.


Figure 4. Typhoon Halong as photographed and tweeted by astronaut Reid Wiseman at 09 UTC August 7, 2014. At the time, Halong was a Category 1 storm with 85 mph winds.

Typhoon Halong drenching Japan
In the Western Pacific, Typhoon Halong was a Category 1 storm with 85 mph winds at 8 am EDT Thursday, and was spreading heavy rains into Southern Japan. Halong's center passed 25 miles east of Minamidaitojima (South Daito Island) between 3 and 5 pm JST (2 and 4 am US EDT) today. According to Japan's AMeDAS observation network, the peak 10-minute sustained wind at Kitadaito (North Daito) Island was 72 mph from the NNE, with a peak gust of 106 mph also from the NNE, both close to 3 pm JST. Minamidaito reported a peak gust of 100 mph and a minimum pressure of 953.7 millibars (28.17 inches of mercury) along with about 3 inches of rain so far. Satellite loops show that Halong has lost most of the heavy thunderstorms along the northern side of its circulation, but the typhoon still has a large eye. Halong is moving north at just 5 mph, and will bring Southern Japan an extended period of heavy rain today through Saturday as a Category 1 typhoon. Thanks go to TWC's Nick Wiltgen for Halong wind stats.

The Atlantic is quiet
In the Atlantic, there are no threat areas to discuss, and none of the reliable models for predicting tropical cyclone genesis predicts development over the next five days.

Interesting links
Storm surge history of Hawaii from Dr. Hal Needham
Central Pacific Hurricane Center
2-km resolution WRF model output from the University of Hawaii for Hawaii
Storm surge maps for Oahu
Storm info from Tropical Tidbits
NWS Honolulu

I'll have a new post later today.

Jeff Masters

Hurricane

Updated: 4:34 PM GMT on August 07, 2014

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Rare Twin Hurricanes Headed Towards Hawaii

By: JeffMasters, 5:05 PM GMT on August 06, 2014

Rare twin Category 1 hurricanes are nearing Hawaii as Hurricane Iselle, with 85 mph winds, and Hurricane Julio, with 75 mph winds, steam west-northwest towards the islands. Both hurricanes are expected to weaken to tropical storms before they affect Hawaii. Iselle is the bigger danger, as it is expected to make a direct hit on Thursday evening, while Julio's center is expected to pass about 100 miles to the northeast of the islands on Sunday. Satellite images show that Iselle has weakened significantly over the past day. Though the hurricane still has a prominent eye, the eyewall's heavy thunderstorms are much reduced in intensity and areal coverage, due to wind shear and dry air. A new Air Force hurricane hunter aircraft is scheduled to be in the storm near 1 pm EDT Wednesday, and the NOAA jet will fly another dropsonde mission Wednesday evening. The outer spiral bands of Iselle were not yet visible on Hawaii radar on Wednesday morning, but should come into view late Wednesday night.


Figure 1. Official forecast and uncertainty cones for Hurricane Iselle and Hurricane Julio, made at 11 am EDT Wednesday August 6, 2014. Image taken from our wundermap with the Tropical layer turned on.

Forecast for Iselle
Wind shear is expected to stay moderate until Iselle reaches the islands Thursday evening. Ocean temperatures will remain a marginal 26°C, and the atmosphere surrounding the storm will steadily dry, resulting in continued weakening of Iselle. Just how weak the storm will be when it arrives in the islands Thursday afternoon and evening is a matter of considerable disagreement amount our top intensity models; the Wednesday morning runs of the LGEM, GFDL, and HWRF models predicted a strong tropical storm or weak Category 1 hurricane with 65 - 75 mph winds at 8 pm EDT Thursday, while the DSHIPS model predicted a much weaker system with 45 - 50 mph winds. Historically, only one tropical storm and no hurricanes approaching from the east have ever hit the islands, and this climatology would argue for a weaker Iselle on Thursday evening. The official CPHC forecast for a 60 - 65 mph tropical storm is reasonable, but Iselle could easily be a 50 - 55 mph storm Thursday evening. Regardless, the main threat from Iselle will be heavy rains leading to flash flooding and mudslides, and the storm will be capable of generating dangerous heavy rains when it reaches the islands. The Wednesday morning 12Z run of the GFDL model predicted that Iselle would dump widespread rains of 4 - 8" over the islands, with some regions seeing 8 - 16". Wind damage is also a concern from Iselle; the 5 pm EDT Wednesday Wind Probability Forecast from Central Pacific Hurricane Center (CPHC) gave Hilo on the Big Island a 72% chance of experiencing tropical storm-force winds of 39+ mph, and a 8% chance of hurricane-force winds. These odds were 30% and 2%, respectively, for Honolulu. High surf of 10 - 20' and higher will also pound the islands, causing erosion problems and coastal flooding.


Figure 2. Predicted rainfall along the track of Hurricane Iselle from the 12Z (8 am EDT) Wednesday August 6, 2014 run of the GFDL model. The model forecasts that Iselle will be a strong tropical storm with 60 - 65 mph winds when it hits the islands Thursday evening. Widespread rains of 4 - 8" are predicted over the islands, with some regions seeing 8 - 16". Image credit: NOAA/GFDL.

Julio becomes a hurricane
Hawaii's other hurricane threat is Hurricane Julio, which intensified to a Category 1 hurricane with 75 mph winds at 5 am EDT on Wednesday. Julio's ascension to hurricane status now gives the Eastern Pacific 10 named storms, 5 hurricanes, and 3 intense hurricanes so far in 2014. On average, we expect to see just 6 named storms, 3 hurricanes, and 1 intense hurricane by August 6. Satellite loops show that Julio is not very impressive yet, with only a modest area of heavy thunderstorms and no eye. The storm should be able to take advantage of light to moderate wind shear and marginally warm sea surface temperatures near 26°C to intensify 5 - 15 mph by Thursday. Higher wind shear and drier air should induce weakening beginning on Friday. The models have been increasingly enthusiastic about Julio taking a bend to the right in its track this weekend, putting the center of the storm about 100 miles northeast of the Hawaiian Islands on Sunday. On this path, Julio's core of heavy rains of 8 - 16" would miss the islands, and high surf would be the main impact of the storm. The edge of Julio's cone of uncertainly for Sunday still lies over the islands, so we cannot yet be confident of this track, but I am cautiously optimistic that Hawaii will avoid torrential rains from Julio falling upon soils already saturated by Iselle.


Figure 3. Tracks of all tropical cyclones (tropical depressions, tropical storms, and hurricanes) to pass within 100 miles of the Hawaiian Islands, 1949 - 2013. Hurricanes approaching from the east typically fall apart before they reach Hawaii due to the cool waters and dry air that lie to the east of the islands. Only one named storm approaching from the east has hit the islands since 1949, an unnamed 1958 tropical storm that hit the Big Island. Hurricanes approaching from the south represent the biggest danger to the islands, due to the warmer waters and more unstable air present to the south. The only two major hurricanes to have affected the islands since 1949, Hurricane Iniki of 1992 and Hurricane Dot of 1959, both came from the south. Image credit: NOAA/CSC.

Hawaii's hurricane history
On average, between four and five tropical cyclones are observed in the Central Pacific every year. This number has ranged from zero, most recently as 1979, to as many as eleven in 1992 and 1994. August is the peak month, followed by July, then September. Tropical storms and hurricanes are rare in the Hawaiian Islands. Since 1949, the Hawaiian Islands have received a direct hit from just two hurricanes--Dot in 1959, and Iniki in 1992. Both hit the island of Kauai. Only one tropical storm has hit the islands since 1949--an unnamed 1958 storm that hit the Big Island. A brief summary of the three most significant hurricanes to affect Hawaii in modern times:

September 1992: Hurricane Iniki was the strongest, deadliest, and most damaging hurricane to affect Hawaii since records began. It hit the island of Kauai as a Category 4 on September 11, killing six and causing $2 billion in damage.

November 1982: Hurricane Iwa was one of Hawaii's most damaging hurricanes. Although it was only a Category 1 storm, it passed just miles west of Kauai, moving at a speed of nearly 50 miles per hour (80 km/h). Iwa killed one person and did $250 million in damage, making it the second most damaging hurricane to ever hit Hawaii. All the islands reported some surf damage along their southwest facing shores, and wind damage was widespread on Kauai.

August 1959: Hurricane Dot entered the Central Pacific as a Category 4 hurricane just south of Hawaii, but weakened to a Category 1 storm before making landfall on Kauai. Dot brought sustained winds of 81 mph with gusts to 103 mph to Kilauea Light. Damage was in excess of $6 million. No Dot-related deaths were recorded.


Figure 4. Typhoon Halong as photographed and tweeted by astronaut Reid Wiseman at 09 UTC August 6, 2014. At the time, Halong was a Category 2 storm with 100 mph winds.

Typhoon Halong headed towards Japan
In the Western Pacific, Typhoon Halong was a Category 2 storm with 100 mph winds at 8 am EDT Wednesday. Satellite loops show that Halong has intensified since Tuesday, with concentric eyewalls and a prominent eye now visible. Halong is expected to affect Southern Japan as a Category 1 typhoon late this week.

Bertha declared extratropical
In the Atlantic, Tropical Storm Bertha has lost its tropical characteristics over the cold waters south of Canada, and NHC has issued its last advisory on the storm. There are no threat areas to discuss in the Atlantic, and none of the reliable models for predicting tropical cyclone genesis forecasts anything to develop in the next five days.


Video 1. The Discovery Channel's "Destroyed in Seconds" chronicles the devastating impact of 1992's Category 4 Hurricane Iniki on Kauai, Hawaii. Iniki was the strongest hurricane ever recorded to hit the Hawaiian Islands.

Jeff Masters

Hurricane

Updated: 9:54 PM GMT on August 06, 2014

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Climate Change May Increase the Number of Hawaiian Hurricanes

By: JeffMasters, 1:02 AM GMT on August 06, 2014

The Eastern Pacific is a busy place for tropical storms and hurricanes, with an average of 15 named storms, 8 hurricanes, and 3 intense hurricanes forming each year. However, these plentiful storms rarely affect Hawaii. The predominant storm track is well to the south of the Hawaiian Islands, and the air tends to be dry and ocean temperatures relatively cool near the islands, making it difficult for a storm to make it there intact. But with two tropical storms potentially threatening the islands in the coming week, and Tropical Storm Flossie having passed with 100 miles of the islands in 2013, it is fair to ask, could climate change be increasing the odds of tropical storms and hurricanes affecting the Hawaiian Islands? A 2013 modeling study published in Nature Climate Change, "Projected increase in tropical cyclones near Hawaii", found that global warming is expected to increase the incidence of tropical storms and hurricanes in Hawaii. Lead author Hiroyuki Murakami, from the International Pacific Research Center at the University of Hawaii at Manoa, commented in a press release accompanying the paper: "In our study, we looked at all tropical cyclones, which range in intensity from tropical storms to full-blown Category 5 hurricanes. From 1979 to 2003, both observational records and our model document that only every four years on average did a tropical cyclone come near Hawaii. Our projections for the end of this century show a two-to-three-fold increase for this region."


Figure 1. Projected change in number of tropical cyclones per year by the last quarter of this century in the 2013 Murakami et al. modeling study published in Nature Climate Change, "Projected increase in tropical cyclones near Hawaii" (in this study, tropical cyclones were defined as only tropical storms and hurricanes, though the general term "tropical cyclones" usually includes tropical depressions as well.) The frequency of a tropical cyclone in a 5°x5° area over the Hawaiian Islands increased from about 0.7 - 1.2 storms per year to about 2 - 3 storms per year. Note that the research projects that the heavily populated Mexican Pacific coast will see a decrease in tropical storms and hurricanes--about one less storm per year. The green stippling indicates statistical significance at the 99 percent confidence level. Image credit: Press release from the University of Hawaii, Hiroyuki Murakami, and Nature Climate Change (2013).

Why an Increase for Hawaii?
Even though their model predicted that fewer tropical cyclones would form in the Eastern Pacific in a future climate with global temperatures 2°C (3.6°F) warmer than at present, more of these storms made their way to Hawaii. This occurred because of three factors:

1) A shift in the upper air steering currents, caused by movement of the upper-level westerly subtropical jet poleward so that the mean steering flow near Hawaii became more east-to-west.

2) A tendency for storms near Hawaii to be stronger (stronger hurricanes tend to move more to the northwestward in the Northern Hemisphere, due to a phenomenon known as beta drift, caused by the variation in the Coriolis parameter across the width of the storm.)

3) A northwards shift in the genesis location where Eastern Pacific tropical storms formed, due to warming of the ocean waters.

"Our finding that more tropical cyclones will approach Hawaii as Earth continues to warm is fairly robust because we ran our experiments with different model versions and under varying conditions. The yearly number we project, however, still remains very low," reassured study co-author Wang in the press release. Only three tropical storms or hurricanes have made landfall in the islands since 1949, an average of one every 27 years, so an increase by a factor of 2 - 3 would imply a landfall every 9 - 14 years. With such a low incidence of storms, it will be very difficult to determine if they are indeed changing due to a changing climate without several decades of data, though.


Figure 2. Double trouble for Hawaii: True-color VIIRS image of Hurricane Iselle (left) and Tropical Storm Julio (right) approaching Hawaii, taken between 3 - 6 pm EDT August 5, 2014. At the time, Iselle was a Category 2 hurricane with 110 mph winds, and Julio had 65 mph winds. Image credit: NOAA Visualization Lab.

I'll have a new post Wednesday morning.

Jeff Masters

Hurricane Climate Change

Updated: 4:25 PM GMT on October 14, 2014

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Hurricane Iselle a Threat to Hawaii; Latvia's 100°F an All-Time Record

By: JeffMasters, 3:45 PM GMT on August 05, 2014

Hurricane Iselle began a gradual weakening process overnight, falling from a Category 4 storm with 140 mph winds to Category 3 with 125 mph winds at 11 am EDT Tuesday. Ocean temperatures beneath the storm are about 26°C, which is marginal for maintaining a hurricane, and plots of Maximum Potential Intensity from the Center for Ocean-Land-Atmosphere Studies show that the Iselle should only be able to maintain Category 2 strength with these ocean temperatures and the current atmospheric background conditions. Iselle is headed westwards at 9 mph towards Hawaii, and will begin affecting the Hawaiian Islands Thursday night. Satellite images show an Iselle is an impressive storm with a large eye and intense eyewall clouds with very cold cloud tops, but the storm is no longer symmetric, due to wind shear and dry air eating away at its southwest side. The relative lack of spiral bands and large, thick eyewall qualify Iselle to be a rare breed of hurricanes known as "annular". Annular hurricanes are a subset of intense tropical cyclones that are significantly stronger, maintain their peak intensities longer, and weaken more slowly than average tropical cyclones. Only 4% of all hurricanes are annular hurricanes.


Figure 1. True-color MODIS image of Hurricane Iselle from 19:40 UTC (3:40 pm EDT) August 4, 2014. At the time, Iselle was a Category 4 hurricane with 140 mph winds. Iselle was showing an annual structure--a lack of spiral bands and large, thick eyewall. Image credit: NASA.

Forecast for Iselle
Wind shear is expected to stay moderate for the next four days, and ocean temperatures will remain near 26°C. However, the atmosphere surrounding Iselle will begin to dry considerably beginning on Tuesday afternoon, which should force steady weakening until the storm reaches the Hawaiian Islands on Thursday night. Due to is annular structure, Iselle will likely weaken more slowly than a typical hurricane, and it could still be a strong tropical storm capable of generating dangerous heavy rains when it reaches the islands. The Tuesday morning 06Z run of the HWRF model predicted that Iselle would dump widespread rains of 4 - 8" over the islands, with some isolated areas of 8 - 16". The 11 am EDT Wind Probability Forecast from NHC gave Hilo on the Big Island a 50% chance of experiencing tropical storm-force winds of 39+ mph, and a 4% chance of hurricane-force winds. These odds were 31% and 1%, respectively, for Honolulu. The Tuesday morning runs of our four top intensity models were in poor agreement, predicting Iselle would arrive in the islands with maximum sustained winds ranging from 45 mph to 90 mph. Historically, no hurricane approaching from the east has ever affected the islands, and I expect Iselle will weaken below hurricane strength before reaching the islands. The NOAA Hurricane Hunters' jet is scheduled to fly a dropsonde mission on Tuesday evening out of Honolulu, and an Air Force hurricane hunter aircraft is scheduled to fly a low-level mission into the hurricane early Wednesday morning. The data from these flights should help make better forecasts for Iselle beginning early Wednesday morning.

After Iselle comes Julio
After Iselle finishes its close encounter with the Hawaiian Islands late this week, the islands need be concerned with yet another tropical cyclone: Tropical Storm Julio, which had top winds of 60 mph at 11 am EDT on Tuesday. Satellite loops show that Julio is headed westwards towards Hawaii on a path very similar to Iselle's, and the storm should be able to take advantage of light to moderate wind shear and warm ocean temperatures to become a hurricane by Wednesday. The Tuesday morning 06Z run of HWRF model predicted that Julio, like Iselle, would be capable of dumping widespread rains of 4 - 8" in the vicinity of the islands, with some isolated areas of 8 - 16". The model had Julio's heavy rain swath barely missing the islands. However, given the large errors present in 5+ day hurricane forecasts, it is quite possible that Julio's heaviest rains will hit the islands, potentially falling on soils already saturated by Hurricane Iselle, resulting in extremely dangerous and destructive flooding. The latest 12Z (8 am EDT) Tuesday runs of our top hurricane track models (the GFS, European, HWRF, and GFDL) all show Julio missing the Hawaiian Islands to the north in their 5-day forecasts, so I am cautiously optimistic that Hawaii can avoid a devastating one-two punch from Iselle and then Julio.


Figure 2. Tracks of all tropical cyclones (tropical depressions, tropical storms, and hurricanes) to pass within 100 miles of the Hawaiian Islands, 1949 - 2013. Hurricanes approaching from the east typically fall apart before they reach Hawaii due to the cool waters and dry air that lie to the east of the islands. Only one named storm approaching from the east has hit the islands since 1949, an unnamed 1958 tropical storm that hit the Big Island. Hurricanes approaching from the south represent the biggest danger to the islands, due to the warmer waters and more unstable air present to the south. The only two major hurricanes to have affected the islands since 1949, Hurricane Iniki of 1992 and Hurricane Dot of 1959, both came from the south. Image credit: NOAA/CSC.

Hawaii's hurricane history
On average, between four and five tropical cyclones are observed in the Central Pacific every year. This number has ranged from zero, most recently as 1979, to as many as eleven in 1992 and 1994. August is the peak month, followed by July, then September. Tropical storms and hurricanes are rare in the Hawaiian Islands. Since 1949, the Hawaiian Islands have received a direct hit from just two hurricanes--Dot in 1959, and Iniki in 1992. Both hit the island of Kauai. Only one tropical storm has hit the islands since 1949--an unnamed 1958 storm that hit the Big Island. A brief summary of the three most significant hurricanes to affect Hawaii in modern times:

September 1992: Hurricane Iniki was the strongest, deadliest, and most damaging hurricane to affect Hawaii since records began. It hit the island of Kauai as a Category 4 on September 11, killing six and causing $2 billion in damage.

November 1982: Hurricane Iwa was one of Hawaii's most damaging hurricanes. Although it was only a Category 1 storm, it passed just miles west of Kauai, moving at a speed of nearly 50 miles per hour (80 km/h). Iwa killed one person and did $250 million in damage, making it the second most damaging hurricane to ever hit Hawaii. All the islands reported some surf damage along their southwest facing shores, and wind damage was widespread on Kauai.

August 1959: Hurricane Dot entered the Central Pacific as a Category 4 hurricane just south of Hawaii, but weakened to a Category 1 storm before making landfall on Kauai. Dot brought sustained winds of 81 mph with gusts to 103 mph to Kilauea Light. Damage was in excess of $6 million. No Dot-related deaths were recorded.


Figure 3. True-color MODIS image of Hurricane Bertha at 1:50 pm EDT August 4, 2014. At the time, Bertha was a Category 1 storm with top winds of 80 mph. Image credit: NASA EARTHDATA.

Bertha headed out to sea
In the Atlantic, Tropical Storm Bertha is on its way to the North Atlantic graveyard for the tropical cyclones--the cold waters south of Canada. Bertha was able to hold onto hurricane status for 18 hours on Monday and Tuesday morning, but higher wind shear has taken its toll on the storm, reducing it to a 60 mph tropical storm as of 11 am EDT Tuesday. Visible satellite loops on Tuesday morning showed the typical signature of a weak tropical storm struggling with wind shear--a low level center that was exposed to view with only a small area of heavy thunderstorms that was limited to one side of the circulation. High wind shear and very cool waters of 20°C will convert Bertha into a powerful extratropical storm on Wednesday, and its remnants could bring some heavy rain showers and tropical storm-force winds gusts to Southeast Newfoundland on Thursday. Along with Hurricane Arthur, Hurricane Bertha gives us two Atlantic hurricanes so far this year, matching the total number of hurricanes during the entire 2013 Atlantic hurricane season. The second (and final) hurricane of the 2013 season (Ingrid) did not arrive until September 14. On average, the second hurricane of the Atlantic season arrives on August 28. The last time the first two named storms in the Atlantic became hurricanes was in 1983, when Alicia, Barry and Chantal all became hurricanes (kudos to TWC's Stu Ostro for this stat.)


Figure 4. Typhoon Halong as photographed and tweeted by astronaut Reid Wiseman at 09:35 UTC August 5, 2014. At the time, Halong was a Category 2 storm with 105 mph winds.

Weakening Typhoon Halong headed towards Japan
In the Western Pacific, Typhoon Halong, formerly a mighty Category 5 super typhoon with 160 mph winds on Sunday, had weakened significantly to a Category 2 storm with 100 mph winds at 11 am EDT Tuesday morning. Satellite loops show that Halong no longer has an eye, though a new eyewall is trying to build. Halong is expected to affect Southern Japan as a Category 1 typhoon late this week.

Latvia sets a new national heat record
On Monday, August 4, 2014, for the second consecutive day, the nation of Latvia recorded its hottest temperature in its recorded history. The mercury hit 100.0°F (37.8°C) at Ventspili (also spelled Ventspils), the first 100°F reading ever recorded in the Baltic countries (Latvia, Lithuania, and Estonia.) Latvia's previous all-time heat record of 98°F (36.7°C) was set just the day before, on August 3, 2014. Prior to that, the hottest temperature ever recorded in Latvia was 97.5°F (36.4°C) in August 1943 at Daugavpils. Wunderground's weather historian Christopher C. Burt has more details in his latest blog post.

According to weather records researcher Maximiliano Herrera, one other nation has seen an all-time national heat record in recent weeks--Iran, where the mercury rose to 127.4°F (53.0°C) at Gotvand on July 17, 2014, tying the record set at Delhoran, Iran in July 2011. I hear you saying, yes, but it was a dry heat. True enough. In fact, the air was so dry over Iran during the July 17, 2014 heat wave that in Delhoran, where the temperature topped out at 125°F (51.5°C), an astonishingly low relative humidity of 0.8% was recorded. There was a difference of 131°F (73°C) between the temperature and dew point.

Jeff Masters

Hurricane Heat

Updated: 7:57 PM GMT on August 05, 2014

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Hurricane Iselle Headed Towards Hawaii; Bertha Becomes a Hurricane

By: JeffMasters, 2:10 PM GMT on August 04, 2014

Hurricane Iselle continued to intensify overnight, reaching Category 4 strength with 140 mph winds at 11 am EDT on Monday. Iselle is likely at peak intensity, since ocean temperatures beneath the storm are now 26°C, which is marginal for maintaining a hurricane. Interestingly, plots of Maximum Potential Intensity from the Center for Ocean-Land-Atmosphere Studies show that the Iselle should only be able to maintain Category 2 strength with these ocean temperatures and the current atmospheric background conditions, so the storm is definitely over-achieving. Iselle is headed westwards at 10 mph towards Hawaii, and could affect the Hawaiian Islands as a tropical storm by Thursday night. Satellite images show an impressive storm with a large eye, good symmetry, and plenty of upper-level outflow. The relative lack of spiral bands and large, thick eyewall qualify Iselle to be a rare breed of hurricanes known as "annular". Annular hurricanes are a subset of intense tropical cyclones that are significantly stronger, maintain their peak intensities longer, and weaken more slowly than average tropical cyclones. The latest SHIPS model output indicates that Iselle has passed the initial screening step to be considered an annular hurricane, and the model's "Annular Hurricane Index" shows a high level of annularity for the hurricane. Only 4% of all hurricanes are annular hurricanes. The most recent annular hurricane in the Eastern Pacific that I am aware of was Category 4 Hurricane Kenneth of November 2011.


Figure 1. True-color MODIS image of Hurricane Iselle from approximately 6 pm EDT August 3, 2014. At the time, Iselle was a Category 3 hurricane with 115 mph winds. Iselle was showing an annual structure--a lack of spiral bands and large, thick eyewall. Image credit: NASA.


Figure 2. Category 4 Hurricane Kenneth of November 22, 2011--the most recent annular hurricane to appear in the Eastern Pacific.

Forecast for Iselle
Wind shear is expected to stay light to moderate for the next four days, and ocean temperatures will remain near 26°C. However, the atmosphere surrounding Iselle will begin to dry considerably beginning on Tuesday, which should induce a steady weakening trend Tuesday through Thursday. By the time Iselle reaches the Hawaiian Islands on Thursday night, rapid weakening may be occurring, but Iselle could still be a strong tropical storm, capable of generating dangerous heavy rains. Hurricanes approaching from the east typically fall apart before they reach Hawaii, though, due to the cool waters and dry air that lie to the east of the islands. It is hurricanes approaching from the south that represent the biggest danger to the islands, due to the warmer waters and more unstable air present to the south. The only two major hurricanes to have affected the islands since 1949, Hurricane Iniki of 1992 and Hurricane Dot of 1959, both came from the south.

The NOAA Hurricane Hunters' jet is scheduled to fly a dropsonde mission on Tuesday evening out of Honolulu, and an Air Force hurricane hunter aircraft is scheduled to fly a low-level mission into the hurricane early Wednesday morning.

After Iselle comes Julio
After Iselle finishes its close encounter with the Hawaiian Islands late this week, the islands need be concerned with yet another tropical cyclone: Tropical Storm Julio, which formed in the Eastern Pacific south of Baja Mexico this morning. Satellite loops show that Julio is headed westwards towards Hawaii on a path very similar to Iselle's, and the storm should be able to take advantage of moderate wind shear and warm ocean temperatures to become a hurricane by Tuesday. Long range forecasts from the GFS and European models have been consistently predicting that Julio will pass very close to Hawaii on Sunday night and be stronger than Iselle. It's been a very active hurricane season in the Eastern Pacific, which has seen 10 named storms, 4 hurricanes, and 3 intense hurricanes so far in 2014. On average, we expect to see 6 named storms, 3 hurricanes, and 1 intense hurricane by August 4 in the Eastern Pacific.


Figure 3. Latest satellite image of Bertha.

Bertha a hurricane
In the Atlantic, Hurricane Bertha took advantage of decreasing wind shear and a moister atmosphere to intensify into a Category 1 hurricane with 80 mph winds as of 11 am EDT Monday. Visible satellite loops on Monday morning showed a Central Dense Overcast (CDO) of high cirrus clouds over Berth's core, which is a typical feature of intensifying tropical storms about to reach hurricane strength. However, Bertha's satellite presentation was probably the lamest I've even seen for a hurricane, with only a small, misshapen area of heavy thunderstorms, and little in the way of spiral bands. Bertha is headed northwards, and will pass midway between the U.S. East Coast and Bermuda. After a short stint as a hurricane later today and on Tuesday, high wind shear and very cool waters of 20°C will convert Bertha into a powerful extratropical storm on Wednesday, halting the intensification process. Bertha will not be a threat to any more land areas, though its remnants could bring some heavy rain showers and tropical storm-force winds gusts to Southeast Newfoundland on Thursday. Along with Hurricane Arthur, Hurricane Bertha gives us two Atlantic hurricanes so far this year, matching the total number of hurricanes during the entire 2013 Atlantic hurricane season. The second (and final) hurricane of the 2013 season (Ingrid) did not arrive until September 14. On average, the second hurricane of the Atlantic season arrives on August 28. The last time the first two named storms in the Atlantic became hurricanes was in 1983, when Alicia, Barry and Chantal all became hurricanes (kudos to TWC's Stu Ostro for this stat.)

Weakening Typhoon Halong headed towards Japan
In the Western Pacific, Typhoon Halong, formerly a mighty Category 5 super typhoon with 160 mph winds, has weakened significantly to a Category 3 storm with 120 mph winds on Monday morning. Satellite loops show that Halong's eye is no longer distinct the eyewall has collapsed, but the typhoon still has a large area of very intense eyewall thunderstorms. Halong is expected to affect Southern Japan as a Category 1 typhoon late this week.


Figure 4. The NOAA P-3 Orion hurricane hunter aircraft, N42RF and N43RF. Image credit: Alan Goldstein/Terry Schricker.

A dangerous flight through Hurricane Hugo, remembered 25 years later on The Weather Channel
Twenty five years ago, on September 15, 1989, the fifteen members of the crew of NOAA hurricane hunter aircraft N42RF very nearly became Hurricane Hugo's first victims. Expecting to encounter a powerful yet manageable Category 3 hurricane east of the Lesser Antilles Islands, the plane instead hit extreme turbulence in an intensifying Category 5 storm, and very nearly did not make it out. I was the Flight Meteorologist on that mission, and photographed the wild events of that unforgettable flight. My remarkable story of that flight into Hurricane Hugo is a must-read for all who follow these great storms. On Monday, August 4, 2014, The Weather Channel will be showing a 3-minute piece on that flight, which will be airing at 3:40 pm, 4:40 pm, and times later in the day. I flew to Tampa in June to help film the piece, which interviews myself and two members of that mission who still work for the NOAA Hurricane Hunters--Hurricane Project Manager Jim McFadden, and the Science and Engineering Division chief Alan Goldstein. The piece will also play again on the actual 25th anniversary of the flight, on September 15, 2014.

Jeff Masters

Hurricane

Updated: 12:52 PM GMT on August 05, 2014

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Bertha Headed Out to Sea; Iselle a Major Hurricane; Halong Hits Category 5

By: JeffMasters, 5:00 PM GMT on August 03, 2014

Tropical Storm Bertha was cruising northwest out of the Southeast Bahama Islands at 20 mph on Sunday morning, and was spreading gusty winds and heavy rain showers across the Southeast Bahamas and Turks and Caicos Islands. A personal weather station at Cockburn Harbour in the Caicos Islands reported a wind gust of 34 mph and 0.48" of rain as of noon Sunday. Rainfall amounts of 1 - 3" on Saturday and Sunday were common over Puerto Rico from Tropical Storm Bertha, and rainfall amounts of 6 - 8 inches over the center of the island brought several rivers close to flood stage. However, there were no reports of major damage or flooding from the storm, and Bertha's rains were welcome in an area that has suffered significant drought this year. Visible satellite loops on Sunday morning showed that Bertha was growing slightly more organized, with increased heavy thunderstorm activity and more low-level spiral bands, despite high wind shear of 25 knots. The atmosphere surrounding Bertha is now quite a bit more moist, and this extra moisture is likely allowing the storm to grow more organized. On Monday and Tuesday, Bertha is expected to encounter lower wind shear and a moister atmosphere, and these conditions may allow the storm to briefly intensify into a Category 1 hurricane. Bertha will not be a threat to any more land areas.


Figure 1. Rainfall amounts over Puerto Rico and the Virgin Islands from Tropical Storm Bertha. Image credit: National Weather Service.

Hurricane Iselle becomes 3rd major hurricane of 2014 in the Eastern Pacific
In the Eastern Pacific, Hurricane Iselle put on a bout of rapid intensification that brought the storm to Category 3 strength with 115 mph winds at 11 am EDT on Sunday. Iselle is headed westwards towards Hawaii, and could affect the Hawaiian Islands as a tropical storm by Thursday night. Satellite images show that Iselle has developed impressive symmetry and a large eye. Wind shear is expected to stay light to moderate for the next five days, but ocean temperatures will slowly cool to 26°C and the atmosphere will dry significant by Wednesday. Iselle is likely to slowly weaken beginning on Monday, but will probably remain a hurricane through Wednesday. The NOAA Hurricane Hunters' jet is scheduled to fly a dropsonde mission on Tuesday evening out of Honolulu, and an Air Force hurricane hunter aircraft is scheduled to fly a low-level mission into the hurricane early Wednesday morning. It's been a very active hurricane season in the Eastern Pacific, which has seen 9 named storms, 4 hurricanes, and 3 intense hurricanes so far in 2014. On average, we expect to see 6 named storms, 3 hurricanes, and 1 intense hurricane by August 3 in the Eastern Pacific.


Figure 2. Latest satellite image of Iselle in the Eastern Pacific.

Super Typhoon Halong headed towards Japan
In the Western Pacific, Super Typhoon Halong put on an impressive burst of rapid intensification on Saturday, topping out as a mighty Category 5 super typhoon with 160 mph winds between 2 pm EDT Saturday - 2 am EDT Sunday. Halong weakened slightly to a Category 4 super typhoon with 150 mph winds at 8 am EDT on Sunday, due to an eyewall replacement cycle. Satellite loops show that Halong's eye has grown less distinct, but the typhoon still has a large area of very intense eyewall thunderstorms. The storm is expected to turn northwards today, and will encounter higher wind shear and cooler waters over the next few days, which will likely weaken the storm to Category 2 strength before is reaches Japan's Ryukyu Islands on Thursday.


Figure 3. Super Typhoon Halong at 08:32 UTC Sunday, August 3, 2014. At the time, Halong was at peak strength: a Category 5 storm with 160 mph winds. Image credit: NOAA/NESDIS.

Jeff Masters

Hurricane

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Bertha Spreading Rain to Puerto Rico and Virgin Islands, but is not Intensifying

By: JeffMasters, 2:06 PM GMT on August 02, 2014

Tropical Storm Bertha was racing west-northwest across the Northeast Caribbean at 21 mph on Saturday morning, spreading gusty winds and heavy rain showers across the Virgin Islands, Puerto Rico, and islands of the Northern Lesser Antilles. A personal weather station at an elevation of 325 feet on the east end of St. Croix in the Virgin Islands recorded sustained winds of 47 mph, gusting to 56 mph on Saturday morning, along with 0.5" of rain. However, these winds were likely amplified by the surrounding terrain, and Bertha has not been generating sustained tropical storm-force winds (39+ mph) at any other land stations. A few peak wind gusts and rainfall amounts from the passage of Bertha through the islands:

Dominica: 43 mph wind gust, 1.38" of rain
Guadaloupe: 43 mph wind gust, 0.38" of rain

Visible satellite loops on Saturday morning showed that although Bertha was more organized than on Friday, the storm had only a limited amount of heavy thunderstorms. The misshapen structure of the storm suggested that dry air and wind shear continued to be a problem for it, and Bertha's lack of organization was also apparent on Puerto Rico radar, where very little low-level spiral banding was apparent. Wind shear due to strong upper-level winds out of the southwest was a high 20 knots on Saturday morning. These winds were driving dry air to the west of Bertha into the circulation, limiting heavy thunderstorm formation. An Air Force C-130 hurricane hunter aircraft is scheduled to investigate Bertha Saturday evening.


Figure 1. Radar image of Tropical Storm Bertha at 9:52 am EDT Saturday August 2, 2014, from the San Juan, Puerto RIco radar.

Forecast for Bertha
Moderate wind shear of 10 - 20 knots is expected to affect Bertha through Monday morning, according to the 12 UTC Saturday forecast from the SHIPS model. With the atmosphere around Bertha quite dry, the shear will be able to drive dry air into Bertha's circulation, keeping any intensification slow. Passage over the rough terrain of the eastern Dominican Republic Saturday night and Sunday will disrupt the storm, and it is possible that Bertha will be downgraded to a tropical wave on Sunday. However, Bertha will still be capable of dumping heavy rains on the Southeast Bahamas on Sunday and Monday, as the storm turns north in response to a strong trough of low pressure over the Eastern United States. This trough should be strong enough to recurve Bertha to the northeast without the storm hitting the mainland U.S. coast. Wind shear will be lower and the atmosphere will be moister as the storm heads northwards, potentially allowing Bertha to intensify into a Category 1 hurricane over the open ocean.


Figure 2. Latest satellite image of Iselle in the Eastern Pacific.

Hawaii keeping an eye on Hurricane Iselle
In the Eastern Pacific, Category 1 Hurricane Iselle is headed westwards towards Hawaii, and could affect Hawaiin Islands by Thursday night. Satellite images show that Iselle has developed an eye, and the hurricane has a large area of heavy thunderstorms that are improving in organization. With wind shear a moderate 10 - 15 knots and SSTs near 28°C, Iselle is likely to remain a hurricane over the weekend. By Tuesday, the storm will encounter higher wind shear, drier air, and cooler waters of 26°C, which will induce weakening. The GFS and European models predict that Iselle will pass close to Hawaii Thursday night and Friday, but the storm may be close to dissipation by then. It's been a very active hurricane season in the Eastern Pacific, which has seen 9 named storms, 4 hurricanes, and 2 intense hurricanes so far in 2014. On average, we expect to see 6 named storms, 3 hurricanes, and 1 intense hurricane by August 1 in the Eastern Pacific.


Figure 3. True-color MODIS image of Typhoon Halong from approximately 02 UTC August 2, 2014. At the time, Halong was a Category 3 storm with top winds of 115 mph. Image credit: NASA.

Typhoon Halong a threat to Japan
In the Western Pacific, Typhoon Halong has intensified into a dangerous Category 4 storm with 135 mph winds. Satellite loops show an impressively organized storm with a large eye surrounded by eyewall thunderstorms with very cold cloud tops. The storm is expected to head northwards and affect Japan's Ryukyu Islands on Wednesday night.

Jeff Masters

Hurricane

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Tropical Storm Bertha Hits the Lesser Antilles

By: JeffMasters, 8:19 PM GMT on August 01, 2014

The center of Tropical Storm Bertha was passing between the Lesser Antilles islands of Martinique and Dominica near 4 pm EDT on Friday, but has brought little in the way of strong winds or heavy rain to the Lesser Antilles Islands so far. As of 4 pm, Martinique had received 0.35" of rain, with top winds of 22 mph. Dominica had a wind gust of 43 mph at 4 pm EDT, and had picked up 0.08" of rain. The storm's top winds of 50 mph were located about 100 - 150 miles east-northeast of the center, and this portion of the storm will affect the northernmost Leeward Islands Friday night as Bertha speeds west-northwest at 22 mph. Visible satellite loops on Friday afternoon showed that although Bertha's surface circulation was exposed to view due to wind shear, the storm was growing more organized. Bertha had a modest area of heavy thunderstorms on the east side of the circulation, but heavy thunderstorms were beginning to fire up near the center of circulation. Martinique radar also showed increased organization, with more spiral bands forming and growing more intense near the center. This modest increase in organization may be due to the fact that wind shear due to strong upper-level winds out of the west had dropped by 5 knots since Friday morning, and was a moderate 15 knots on Friday afternoon. These winds were still driving dry air to the west of Bertha into the circulation, limiting heavy thunderstorms on the west side of the storm. An Air Force C-130 hurricane hunter aircraft was investigating Bertha early Friday afternoon, and found that the storm's central pressure had fallen 3 mb since Friday morning, to 1007 mb. Top surface winds measured by the plane were about 50 mph.


Figure 1. Radar image of Tropical Storm Bertha as the center passed between Martinique and Dominica at 4 pm EDT August 1, 2014. Image credit: Meteo France.


Figure 2. True-color MODIS image of Tropical Storm Bertha from approximately 1:15 pm EDT August 1, 2014. At the time, Bertha had top winds of 50 mph. A long, near-surface arc-shaped cloud racing away from Bertha to the north and west is seen. These arc clouds are the sign of a tropical cyclone struggling with dry air, and form when dry air at mid-levels is ingested into a tropical cyclone's thunderstorms, creating strong downdrafts that rob the storm of moisture. When the downdrafts hit the surface, the air spreads out in an arc-shaped pattern, creating clouds at the edge of the outflow boundary. The low level center of Bertha was fully exposed to view, the sign of a tropical storm struggling with wind shear. Image credit: NASA.

Forecast for Bertha
Moderate wind shear of 15 knots is expected to affect Bertha through Saturday morning, according to the 18 UTC Friday forecast from the SHIPS model. With the atmosphere around Bertha quite dry, the shear will be able to drive dry air into Bertha's circulation, keeping intensification slow through Saturday morning. Given that Bertha was beginning to fire up heavy thunderstorms on the west side of its circulation Friday afternoon, in defiance of the shear and dry air, makes it more likely that the storm can have sustained winds as strong as 65 mph when it makes its closest approach to the Virgin Islands and Puerto Rico on Saturday. Wind shear is forecast to fall to the light range, 5 - 10 knots, by Saturday afternoon. When Bertha leaves Puerto Rico and heads towards the Southeast Bahamas on Saturday night, wind shear is expected to remain low. However, passage over the rough terrain of Puerto Rico and the eastern Dominican Republic may disrupt the storm. The 12Z Friday runs of the GFS, European, and GFDL models showed Bertha's core tracking over the eastern Dominican Republic Saturday night, and this track would significantly weaken the storm. Bertha will still be capable of dumping heavy rains on the Southeast Bahamas on Sunday and Monday, though, as the storm turns north in response to a strong trough of low pressure over the Eastern United States. This trough should be strong enough to recurve Bertha to the northeast without the storm hitting the mainland U.S. coast.


Figure 3. Latest satellite image of Iselle in the Eastern Pacific.

Hawaii keeping an eye on Tropical Storm Iselle
In the Eastern Pacific, Tropical Storm Iselle is close to hurricane strength, and could affect Hawaii late next week. Satellite images show that Iselle has developed a large and ragged eye, and Iselle has a large area of heavy thunderstorms that are improving in organization. With wind shear a moderate 10 - 15 knots and SSTs near 28°C, Iselle is likely become a hurricane today and maintain hurricane status over the weekend. Early next week, the storm will encounter cooler waters of 26°C, which are marginal for maintaining a hurricane. The GFS and European models predict that Iselle will pass close to Hawaii next Friday, but the storm should be weakening and may be close to dissipation by then. It's been a very active hurricane season in the Eastern Pacific, which has seen 9 named storms, 3 hurricanes, and 2 intense hurricanes so far in 2014. On average, we expect to see 6 named storms, 3 hurricanes, and 1 intense hurricane by August 1 in the Eastern Pacific.

Tropical Storm Halong a threat to Japan
In the Western Pacific, Tropical Storm Halong is intensifying, and satellite strength estimates put Halong at Category 1 typhoon strength on Friday afternoon. The storm is expected to head northwards and affect Japan's Ryukyu Islands on Wednesday.

Jeff Masters

Hurricane

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Tropical Storm Bertha Nearing Lesser Antilles Islands

By: JeffMasters, 1:28 PM GMT on August 01, 2014

Tropical Storm Warnings are flying Friday morning in most of the Lesser Antilles Islands as Tropical Storm Bertha, with top winds of 45 mph, cruises west-northwest at 20 mph towards the islands. Bertha was born last night at 03 UTC August 1, when tropical disturbance 93L was finally able to produce enough heavy thunderstorms to be classified as a tropical storm. The formation of Bertha on August 1 was right on schedule, according to climatology from 1966 - 2009: the Atlantic's 2nd named storm has historically formed on August 1 during this period. Visible satellite loops on Friday morning showed that Bertha was not well-organized; the surface circulation was partially exposed to view, with a modest area of heavy thunderstorms confined to the east side of the circulation. The storm was fighting moderate to high wind shear of 15 - 20 knots, due to strong upper-level winds out of the west. These winds were driving dry air to the west of Bertha into the circulation, keeping heavy thunderstorms from forming on the west side. An Air Force C-130 hurricane hunter aircraft was investigating Bertha Friday morning, and found the storm's central pressure was a rather high 1010 mb at 11:48 UTC (7:48 am EDT). Top surface winds measured by the plane were about 45 mph, so Bertha is showing no signs of intensifying. Barbados radar on Friday morning showed that Bertha was generating some respectable rain bands, which will bring heavy rains to the Lesser Antilles beginning late Friday morning. Water vapor satellite loops and the Saharan Air Layer analysis showed that a large amount of dry air lay to the north and west of Bertha. Ocean temperatures were about 28°C, which is 2°C warmer than the typical 26°C threshold for development.


Figure 1. Latest satellite image of Bertha.


Figure 2. Short-term drought conditions in the Caribbean as of July 2014, as measured by the one-month Standardized Precipitation Index (SPI). Bertha's rains will cause welcome drought relief in some areas, though may also cause flash flooding. Image credit: NOAA's Global Drought Portal.

Forecast for Bertha
Moderate to high wind shear of 15 - 20 knots is expected to affect Bertha through Saturday morning, according to the 12 UTC Friday forecast from the SHIPS model. With the atmosphere around Bertha quite dry, the storm will have to work hard to insulate itself from disruptive dry air incursions, and only slow intensification is likely through Saturday morning. I don't see Bertha being stronger than a 55 mph tropical storm during this period. By Saturday afternoon, wind shear is forecast to fall to the moderate range, 10 - 15 knots, and this may allow Bertha to intensify as it moves over Puerto Rico. Passage over the high mountains of Puerto Rico may disrupt the storm some, counteracting the decrease in wind shear. If the system takes a more southwesterly track over the eastern Dominican Republic like the European model is suggesting, this would also disrupt Bertha. I don't see Bertha being stronger than a 60 mph tropical storm as it affects Puerto RIco and the Dominican Republic. Two of our best intensity models, the GFDL and HWRF, predicted in their 06Z Friday runs that Bertha would have winds of about 40 mph as it passed over Puerto Rico on Saturday.

The GFS and European models continue to agree on the long-range fate of Bertha. The storm is expected to clip the southeastern Bahamas and Turks and Caicos Islands on Sunday, then turn north in response to a strong trough of low pressure over the Eastern United States. This trough should be strong enough to recurve Bertha to the northeast without the storm hitting the mainland U.S. coast.

I'll have a new post this afternoon.

Jeff Masters

Hurricane

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About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

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