TropicalAnalystwx13's WunderBlog

Henriette strengthening, Gil hanging on; watching the Caribbean in mid-August

By: TropicalAnalystwx13, 12:04 AM GMT on August 06, 2013

The wave of tropical cyclones in the East Pacific basin has continued since my last blog entry, specifically with the development of Henriette. This system is a result of the invest previously dubbed Invest 90E; initially, no development of this feature was expected given its proximity to at-the-time Hurricane Gil. Due mainly to the change in the upper-air pattern for Gil, however, the system dramatically weakened, subsequently reducing the amount of outflow produced and wind shear over 90E. Shower and thunderstorm activity increased and organized accordingly, and the eighth tropical depression of the season was declared on August 3. The following day, it was upgraded to Henriette. Since that time, the system has continued to strengthen. As of the latest advisory from the National Hurricane Center, Henriette was located within 25 nautical miles of 12.8N 129W, about 1,435 miles west-southwest of the southern tip of Baja California. Maximum sustained winds were up to 65 mph, the minimum barometric pressure was down to 996 millibars, and the cyclone was moving west-northwestward at 9 mph. Visible satellite loops show a much improved structure compared to 24 hours, with spiral banding to the southwest, a central dense overcast, albeit irregular, and decent outflow to the east. In addition, a 1414z microwave pass revealed the development of a large eye underneath the convective mass. Satellite intensity estimates are a unanimous T3.5/55 kt from SAB, TAFB, and UW-CIMSS ADT.


Figure 1. Visible satellite image of Tropical Storm Henriette.

Forecast for Henriette
Tropical Storm Henriette is in a favorable environment for continued intensification, characterized by vertical wind shear less than 15 knots, sea surface temperatures greater than 28°C, and mid-level relative humidity values over 70%. The greatest deterrent for strengthening appears to be a lack of thetha-e, or vertical instability. This has been a common negative factor thus far for the 2013 Pacific hurricane season, evidenced by a lack of Category 2 or stronger hurricanes. Nonetheless, a majority of the statistical model guidance foresees intensification for another 48-60 hours, and I have little reason to disagree. Although the SHIPS is giving a 3/10 chance for 25 kt rapid intensification over the next 24 hours, the cyclone's large size may prevent anything other than steady intensification. My forecast follows a blend of the SHIPS and LGEM models, which show a peak of 75 and 70 knots, respectively. By 72 hours out, Henriette should cross the 26°C isotherm and enter a region of very stable air which should induce weakening. Steady weakening is expected throughout the remainder of the forecast period, though it appears Henriette should remain a tropical storm.

Henriette is currently moving west-northwest at 9 mph, as aforementioned. This motion is a result of a weakness created in the mid-level height field due to a shortwave trough positioned west of California. The statistical model guidance has come in much better agreement compared to previous days, with a continued track in the same direction for the next 72 hours. Thereafter, the trough is expected lift out and the subtropical ridge currently northeast of the system should build westward, forcing Henriette back towards the west. By the end of the period, when the cyclone is forecast to be a low-to mid-grade tropical storm, it should become embedded within the quick easterly trade wind flow that usually dominates the East Pacific. Through the 120 hour forecast period, Henriette is not expected to pose a threat to land. Thereafter, however, the model guidance suggests that the system could pass close to the Hawaiian Islands, perhaps as a tropical cyclone still. This will be something to monitor over the coming days.

FORECAST POSITIONS AND MAX WINDS

INIT 05/2100Z 12.8N 129.0W 55 KT 65 MPH
12H 06/0600Z 13.7N 129.9W 60 KT 70 MPH
24H 06/1800Z 14.9N 131.4W 65 KT 75 MPH
36H 07/0600Z 15.9N 133.0W 75 KT 85 MPH
48H 07/1800Z 16.8N 134.9W 70 KT 80 MPH
72H 08/1800Z 17.4N 138.4W 60 KT 70 MPH
96H 09/1800Z 17.4N 142.5W 50 KT 60 MPH
120H 10/1800Z 17.3N 147.1W 40 KT 45 MPH

Gil remains a tropical cyclone
There have been a lot of changes with tropical cyclone Gil since my blog entry. At the time, the system was an intensifying Category 1 hurricane that was forecast to become a Category 2. Instead, due to changes in the surrounding environment, Gil rapidly weakened to a tropical storm and eventually a tropical depression. The main issue seemed to an unexpected intensification in westerly wind shear atop the cyclone for which I have no explanation. However, it should be noted that a suppressed convectively-coupled kevlin wave (CCKW) was passing over the region at the time that Gil suddenly weakened; in fact, this suppressed CCKW was 3 standard deviations above the mean. Such a feature promotes dry, sinking air, limiting convective activity. Regardless, Gil remains a tropical cyclone this evening. As of the latest advisory from the National Hurricane Center, Gil was located within 25 nautical miles of 13.2N 139.4W, about 1,130 miles east-southeast of Hilo, Hawaii. Maximum sustained winds were up from 30 mph to 35 mph, the minimum barometric pressure was down to 1006 millibars from 1007 millibars, and the cyclone was moving west-southwest at 9 mph. Visible satellite imagery shows that Gil is a disorganized tropical cyclone, with transient convection located mainly east of an otherwise exposed low-level center of circulation. Latest satellite intensity estimates were T1.5/25 kt from SAB, T2.0/30 kt from TAFB, and T2.5/35 kt from UW-CIMSS ADT.


Figure 2. Visible satellite imagery of Tropical Depression Gil.

Forecast for Gil
The intensity forecast for Tropical Depression Gil is complicated. While a majority of the statistical model guidance flatlines the intensity through 120 hours, our two best models, the SHIPS and LGEM, show gradual intensification back to a moderate tropical storm. And this isn't totally implausible...the cyclone is located within a region of low vertical wind shear, less than 10 knots, sea surface temperatures of 27°C, and mid-level relative humidity values near 60%. While the final of the three factors does not typically favor intensification, the warm ocean temperatures underneath may offset this deterrent. The small size of Gil makes it very susceptible to subtle changes in the surrounding environment. Given that none of the global models intensify the cyclone either, I am not going to forecast intensification, but note that it is a possibility. On the other hand, despite the fact that neither the global or statistical models show dissipation over the next 48 hours, I note that this is also a possibility. My forecast, for now, shows degeneration in 72 hours. In the meantime, the depression is tracking west-southwest due to the anticyclonic flow around a mid-level region of high pressure southwest of Hawaii. A turn generally towards the west is expected over the coming days as Gil becomes embedded within the quick easterly trade wind flow. The cyclone is not expected to be a threat to land.

...FORECAST POSITIONS AND MAX WINDS...

INIT 05/2100Z 13.2N 139.4W 30 KT 35 MPH
12H 06/0600Z 12.8N 140.4W 30 KT 35 MPH
24H 06/1800Z 12.5N 141.8W 30 KT 35 MPH
36H 07/0600Z 12.6N 143.2W 30 KT 35 MPH
48H 07/1800Z 12.7N 144.3W 30 KT 35 MPH
72H 08/1800Z 12.8N 147.8W 25 KT 30 MPH...POST-TROP/REMNT LOW
96H 09/1800Z 12.9N 150.8W 25 KT 30 MPH...POST-TROP/REMNT LOW
120H 10/1800Z 13.2N 154.9W 20 KT 25 MPH...POST-TROP/REMNT LOW

Watching the Atlantic
The Atlantic remains quiet for the first week of August, though there are a few indications that this may not last very long. The first area to watch for potential mischief is the eastern Gulf of Mexico. At this time, a stationary front stretches south southeast of the Carolinas to Mississippi farther northwest into Idaho. This front is expected to drift southward as an expansive region of high pressure builds over the Northeast. The tail-end of this front should be positioned southwest of Florida by 120 hours out. Over warm sea surface temperatures, and within a region of forced convergence due to higher heights farther north, it is not out of the question that a brief tropical cyclone could spin-up. Given the pattern, any system would track westward towards southern Texas or the northern coastline of Mexico.

The second area of interest comes in a little over a week out. The GFS has been consistently showing a vigorous mid-level tropical wave axis tracking westward across the Atlantic and entering the western Caribbean in 10 days. Though it doesn't show development, it shows a very favorable environment for development. The TUTT, Tropical Upper Tropospheric Trough, is forecast to retrograde westward while simultaneously weakening. Such a setup allows for upper-level ridging to developed east of the trough, which subsequently reduces wind shear and provides a favorable outflow pattern for intensification. Development of this wave should also be supported by a weak upward pulse of the MJO, enhancing precipitation and convergence. While no models foresee development, I believe it is a distinct possibility.

I'll have a new blog tomorrow,

TropicalAnalystwx13

Tropical Cyclones of 2013 Tropical Weather

Updated: 3:36 AM GMT on March 10, 2014

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East Pacific basin lit up with activity; watching the Atlantic basin

By: TropicalAnalystwx13, 5:17 AM GMT on August 02, 2013

Gil, the seventh named storm and fifth hurricane of the 2013 Pacific hurricane season, continues its west-northwest track across the eastern Pacific basin this morning. As of the latest advisory from the National Hurricane Center, Gil was positioned within 20 nautical miles of 14.4°N 126.4°W, about 1230 miles west-southwest of the southern tip of Baja California. Maximum sustained winds were up to 85 mph, the minimum barometric pressure was down to 985 millibars, and the system was tracking in the aforementioned direction at 14 mph. Despite the increase in intensity, Gil has changed very little in organization since this time yesterday. The cyclone continues to display a central dense overcast, though convection in the eastern semicircle of the storm seems to have been eroded somewhat by stable air. Outflow is fair in all four quadrants, except to the north and south where it is excellent. A series of microwave passes reveal that the inner core of Gil has changed a bit over the subsequent 24 hours, most notably a substantial increase in the size of the storm's eye. This feature has been visible intermittently throughout the day, but was recently obscured by a large burst of convection directly atop the center. The satellite intensity estimate from UW-CIMSS-ADT continues to be conservative with the strength of Gil, currently reading T3.4/53 kt. The 00z intensity estimates from SAB and TAFB were T4.5/77 kt and T5.0/90 kt, respectively. A blend of these data would typically give an intensity of 85 knots; however, given the cyclone's ragged appearance on satellite imagery, the NHC has opted to be conservative for the time being. As evidenced by an afternoon ASCAT pass, Gil is a very small system. Tropical storm-force winds extend outward up to 60 miles from the center, and hurricane-force winds extend outward up to 25 miles.


Figure 1. Infrared satellite imagery of Hurricane Gil.

Forecast for Gil
The intensity forecast for Gil remains a bit complicated. The rate of intensification observed after formation and into yesterday indeed slowed as stated it may. Despite seemingly favorable atmospheric conditions, the storm has failed to intensify steadily as originally predicted. Wind shear as analyzed by SHIPS is a whopping 1 knot and sea surface temperatures are near 28°C. However, mid-level relative humidity values have dropped to the upper 50s (%). It is possible that the system is occasionally ingesting dry air, which would be supported by a lack of convection in one quadrant of the storm. Fortunately for Gil, it is expected to pass through a region where values rise to the mid-60s over the next 72-96 hours. All things considered, the intensity forecast has been lowered from yesterday's prediction but continues to show steady intensification over the next 48 hours at least. A significant shift in the amount of time Gil spends in a favorable wind shear environment has occurred during the past few model suites today; as a result, both the SHIPS and LGEM show the system becoming a Category 2 hurricane at peak. I have no reason to disagree. Between 48 and 72 hours, the hurricane is expected to cross the sub-26°C isotherm and steadily weaken instead. My forecast follows a steady weakening trend throughout the forecast period, though it should be noted that a few reliable models weaken the storm much slower than depicted.

Gil is moving towards the west-northwest at 14 mph. There has been little change to the forecast track of the system, and there hardly ever is when dealing with East Pacific tropical cyclones during the hurricane season. The storm is being steered in the aforementioned direction as a result of a deep-layer ridge to the systems north-northeast. This motion is expected to continue over the next three days as the pattern remains locked in place. By days 4 and 5, Gil is expected to turn towards the west-southwest as the system becomes embedded within the traditional east-to-west low-level trade wind flow across the East Pacific. This is supported by all reliable global model guidance. Due to this motion, Gil is not expected to be a threat to the Hawaiian islands like its predecessor, and should remain hundreds of miles from any landmasses throughout the entire forecast period.

...FORECAST POSITIONS/MAX WINDS...

INIT 02/0300Z 14.2N 121.8W 75 KT 85 MPH
12H 02/1200Z 14.7N 123.4W 75 KT 85 MPH
24H 03/0000Z 15.0N 125.4W 80 KT 90 MPH
36H 03/1200Z 15.3N 127.1W 85 KT 100 MPH
48H 04/0000Z 15.6N 128.9W 80 KT 90 MPH
72H 05/0000Z 15.8N 132.3W 65 KT 75 MPH
96H 06/0000Z 15.7N 135.5W 55 KT 65 MPH
120H 07/0000Z 15.6N 138.6W 40 KT 45 MPH

Invests 90E and 91E of no concern
The East Pacific is a hotbed of activity lately as a result of the upward pulse of the MJO. In addition to Hurricane Gil we find invests 90E and 91E.

As of the 0z ATCF update, Invest 90E was located at 12.7°N 116.8°W. Maximum sustained winds were 30 mph and the minimum barometric pressure was 1009 millibars. This disturbance has been distinguishable for days, forming within the Intertropical Convergence Zone shortly after Gil was initially classified as a tropical depression. Satellite loops reveal 90E isn't particularly well organized. A vigorous low-level circulation is observable but the disturbance lacks deep, widespread, and organized shower and thunderstorm activity. Wind shear over the disturbance as analyzed by the SHIPS was less than 10 knots, the disturbance was in a region of decent atmospheric moisture, and sea surface temperatures were just below 29°C. All of these factors favor at least slow development of the system. However, due to its proximity to Hurricane Gil, conditions may become less favorable in the long term as wind shear in the form of outflow impedes on the system. 90E is expected to cross the 26°C isotherm in roughly 96 hours, at which time any development will cease. The National Hurricane Center is currently giving 90E a medium chance, 40%, of developing over the next 48 hours and a medium chance, 50%, of developing over the next 48 hours. I agree with these percentages. Regardless of development, 90E is not a threat to land.


Figure 2. Infrared satellite imagery of Invest 90E.

On the other side of Hurricane Gil lies recently-declared Invest 91E. As of the latest ATCF update, the disturbance was located at 12.0°N 136.1°W. Maximum sustained winds were 30 mph and the minimum barometric pressure was 1009 millibars. Dissimilar to 90E, the environment is marginal at best for development. Wind shear is less than 10 knots and sea surface temperatures currently lie at or above 28°C, but the former is expected to increase and the latter are expected to decrease rapidly over subsequent days. The disturbance is within a region of dry, stable air; mid-level relative humidity values were analyzed in the lower 50s. The system has a 48-hour window before conditions become even more unfavorable for development. The NHC is currently giving 91E a low chance, 10%, of becoming a tropical cyclone over the next 48 hours and a low chance, 20%, of becoming a tropical cyclone over the next 120 hours. I put these chances at Near 0% and 10%, respectively. Regardless of development, 91E is not a threat to land.



Watching the Atlantic
The Atlantic as a whole this evening remains pretty quiet. The Gulf of Mexico is dominated by difluence and moderate wind shear, the Caribbean is dominated by high wind shear, the subtropical Atlantic is dominated by stable, sinking air, and the central and eastern Atlantic continue to be engulfed in a large and potent region of Saharan Air Layer. However, in this small area southeast of Florida, in the Bahamas, lies what is once again Invest 91L, a.k.a the remnants of never-ending Dorian. Over the past few days, the remains of the system have been interacting with an upper-level low north of the Greater Antilles. Just recently, however, has this low backed southwest into the northwestern Caribbean, allowing for a relaxation in wind shear. As evidenced by radar out of Miami, Florida and satellite imagery, 91L has a decent amount of vorticity. Surface pressures have been falling near Andros Island over the past few hours as well. Convection had decreased in coverage and intensity until most recently, when a respectable burst of shower and thunderstorm has occurred over the suspected center. This may just be a result of diurnal maximum - when instability is the greatest as a result of the maximized temperature difference between the atmosphere and surface - however...which would not be unexpected given the system's performance since degeneration.

Invest 91L is located in a region of weak steering. A ridge lies to the system's northeast and a stalled-out frontal boundary attached to a trough lies over the central Gulf of Mexico. A majority of the model guidance indicates a slow west-northwest to northwest motion over the next day or so, before accelerating northeast out to sea as whatever the system becomes is absorbed into the aforementioned trough. Wind shear is low, sea surface temperatures are warm, and the environment is moist. One glaring problem is a lack of convergence - air piling up and rising at the surface - however. Surface convergence is required to establish a well-defined surface low which is in return able to sustain organized shower and thunderstorm activity. This is the exact same issue Ex-Dorian had several days ago near the northeast Caribbean Sea. Development chances are low unless this is established. The NHC is currently giving 91L a 30% chance of developing in both the 2-day and 5-day periods, and I agree with these chances.

We are likely to see a break in meaningful disturbances over the next few days after 91L. There are indications that we may have to watch the Caribbean in the day 10-15 period, however. The MJO is currently located in the East Pacific, and assuming it remains uniform as forecast by the global models, should enter the basin in a little over a week. In addition, the TUTT - an upper-level trough that produces strong wind shear - is expected to retrograde westward, providing a favorable upper-air environment across the west and southwest Caribbean Sea. As a result, development chances are likely to be increased. Both the 12z and 18z GFS runs developed a tropical cyclone in the western Caribbean Sea by day 15, though the latter run did not show nearly as much development as the former run due to prolonged interaction with Central America. The CMC is indicating an area of low pressure in the southwest Caribbean at the end of the period; it will be interesting to see if subsequent runs show further development.

CSU's updated hurricane forecast
As a reminder, William Gray and Phil Klotzbach of Colorado State University will be releasing their updated forecast for seasonal activity in the Atlantic during the 2013 hurricane season. In several news articles, Phil has expressed concerns about a slightly drier-than-average environment across the East Atlantic. In addition, concerns have been expressed about cool sea surface temperatures - which have rebounded in a big way over the past two weeks - across the East Atlantic, which may act to hamper the intensity of storms. I expect CSU will lower their numbers by one to two storms in each category (tropical storm, hurricane, major hurricane). Their previous forecast called for 18 named storms, 9 hurricanes, and 4 major hurricanes.

I may have a new blog tomorrow,

TropicalAnalystwx13

Tropical Weather

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Gil rapidly intensifies into a hurricane; no areas of interest in the Atlantic basin

By: TropicalAnalystwx13, 3:53 AM GMT on August 01, 2013

Gil, the seventh named storm of the thus-active 2013 Pacific hurricane season, has been rapidly intensify over the subsequent 24 hours and is now the season's fifth hurricane. As of the latest advisory from the National Hurricane Center, Gil was located within 20 nautical miles of 14.2N 121.8W, or about 845 miles southwest of the southern tip of Baja California. Maximum sustained winds were up to 80 mph, the minimum barometric pressure was down to 990 millibars, and the system was tracking towards the west-northwest at 12 mph. A combination of the remainder of visible satellite images available and nighttime infrared satellite imagery reveals that Gil is a well-organized hurricane. Spiral bands extend out in all four quadrants and a central dense overcast is noted. A very small eye -- one may call it a pinhole -- was visible earlier this evening, but a recent burst of convection has obscured it. As a result, the satellite intensity estimate from UW-CIMSS-ADT is quite low, at T3.5/55 kt. The recent values from SAB and TAFB were T4.0/65 kt and T4.5/77 kt, respectively.


Figure 1. The final visible satellite image of the day depicting Hurricane Gil.

Forecast for Gil
The intensity forecast for Gil is slightly complicated. The tropical cyclone has been rapidly intensifying since formation, evidenced by a 35 mph increase in winds during a 24 hour interval. However, a recent microwave scan and obscurity of the eye of the cyclone suggest that the rate of intensification has slowed at least for the moment. Gil is located atop sea surface temperatures in excess of 28°C and in an environment of mid-level relative humidity values near 60%. In addition, anticyclonic flow aloft is creating vertical wind shear values less than 10 knots over the hurricane. Maps from the University of Wisconsin indicate the presence of an upper-level low over the southwest coastline of Mexico; this feature may be enhancing the poleward outflow channel and helping the system to develop quicker. A majority of the statistical intensity guidance suggests that Gil will attain a peak intensity of 80 knots in 12 to 24 hours. However, given the environment the storm is within, I have chosen to forecast a peak slightly higher, at 90 knots. Though the intensity guidance levels the intensity off between 24 and 36 hours, there is a chance it could get a little stronger. By 48 hours, Gil will be entering a reigon of higher vertical wind shear, less atmospheric moisture, and cooler sea surface temperatures. The system should steadily weaken to a tropical storm by 96 hours and rapidly weaken by 120 hours.

Gil is currently being steered west-northwest by southeasterly flow around a subtropical ridge to the cyclone's northwest. This general motion is expected to continue for the next three to four days. After this time, global models indicate that an approaching shortwave over the far north Pacific will weaken the western periphery of the aforementioned high pressure region. This should cause Gil to steadily slow its forward motion and perhaps turn towards the northwest for a brief period of time. However, as the cyclone begins to weaken to a low-end tropical storm and eventually a tropical depression, Gil should be steered by the predominant low-level easterlies across the Pacific Ocean, forcing it back towards the west or west-southwest. Regardless of the eventual track, Gil is not expected to impact the Hawaiian Islands such as its predecessor did. On a side note, if Gil makes it to 140W, it will enter the Central Pacific Hurricane Center's area of responsibility where advisories may shortly be written on the system. It's quite unusual to see a storm in the far East Pacific make such a journey, but certainly not unheard of.

...FORECAST POSITIONS/MAX WINDS...

INIT 01/0300Z 14.2N 121.8W 70 KT 80 MPH
12H 01/1200Z 14.7N 123.4W 80 KT 90 MPH
24H 02/0000Z 15.0N 125.4W 90 KT 105 MPH
36H 02/1200Z 15.3N 127.1W 90 KT 105 MPH
48H 03/0000Z 15.6N 128.9W 80 KT 90 MPH
72H 04/0000Z 15.8N 132.3W 70 KT 80 MPH
96H 05/0000Z 15.7N 135.5W 60 KT 70 MPH
120H 06/0000Z 15.6N 138.6W 45 KT 50 MPH

Atlantic quiet
There are no areas of immediate interest in the Atlantic basin this afternoon. The remnants of what was once Tropical Storm Dorian are now interacting with an upper-level low near the Bahamas, producing disorganized shower and thunderstorm activity. Upper-level winds and atmospheric moisture are both unfavorable for any redevelopment of the system, and the National Hurricane Center has dropped this area from their Tropical Weather Outlook. On another side note, the NHC will be implementing their new 5-day Tropical Weather Outlook...in text form...at 8 am EDT tomorrow morning. The graphical product is still being created and may or may not be available by the end of this season. Elsewhere, a tropical wave is emerging off the coast of Africa. However, due to an impressive Saharan Air Layer (SAL) outbreak, development of this feature is not expected. The CMC ensembles indicate the potential for mischief west of the Cape Verde Islands in 10 days or so, and this will be watched accordingly.

I will have a new blog tomorrow,

TropicalAnalystwx13

Tropical Cyclones of 2013 Tropical Weather

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About TropicalAnalystwx13

16-yr-old weather aficionado, with primary focus on tropical cyclones. High school and college student, working towards the National Hurricane Center.

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