TropicalAnalystwx13's WunderBlog

Unforeseen circumstance: Tropical Storm Alvin weakening, dissipation likely soon

By: TropicalAnalystwx13, 3:05 AM GMT on May 17, 2013

Tropical Storm Alvin, the first named storm of the 2013 Pacific hurricane season, developed well south of the coastline of Mexico last night as a result of an area of low pressure that formed several days prior, west of Guatemala. At the time of formation, Alvin was a well-developed tropical cyclone, with fantastic banding and upper-level outflow. The same cannot be said tonight, unfortunately. Strong westerly wind shear on the order of 20 to 25 knots, not foreseen by the SHIPS nor the global models, abruptly halted the cyclone's intensification process last night, and the center became exposed on conventional satellite imagery this morning. Throughout the day, Alvin has continued to degrade, with weakening and shabby convection. The vigorous low-level center seen yesterday evening is no longer visible; in fact, an ASCAT pass from this evening revealed an open circulation associated with Alvin as it has become part of the Intertropical Convergence Zone once again. For continuity purposes, the National Hurricane Center continues to write advisories on the storm, though I suspect these will cease in the morning. The latest information on the storm is as follows: maximum sustained winds of 40 mph, minimum barometric pressure of 1006 millibars, movement towards the west-northwest at 14 mph.


Figure 1. MODIS image of Tropical Storm Alvin at the time of classification.


Figure 2. Enhanced visible satellite image of Tropical Storm Alvin this afternoon. At the time, maximum sustained winds were estimated at 45 mph, with stronger gusts.

Forecast for Alvin
The forecast for Tropical Storm Alvin really is not complicated at this point. The storm has become embedded within the Intertropical Convergence Zone once again and likely no longer contains a closed circulation. If no evidence of a closed circulation is found within the next 12 hours, it is very likely the National Hurricane Center will discontinue advisories on the cyclone, as this is one of the core criterion of a tropical system. No re-intensification is likely due to continued, strong westerly wind shear. The track forecast is also simple. Alvin, or what's left of it, is expected to continue west-northwest for the next two days or so, steered around the southwestern periphery of a large subtropical ridge over Mexico. By 72 hours, the global models advertise a weakness over the Southwest United States, created by an approaching trough. All model solutions show the low-level and mid-level centers of Alvin splitting at this point, with the former continuing westward, caught in the east-to-west flow of the Pacific, and the latter being drawn north into the trough. Both are expected to dissipate by the end of the forecast period. Congratulations to the CMC and ECMWF, both of which never showed appreciable intensification of this cyclone.

...MAX WINDS...

INIT 17/0300Z 35 KT 40 MPH
12H 17/1200Z 30 KT 35 MPH...POST-TROP/REMNT LOW
24H 18/0000Z 25 KT 30 MPH...POS-TROP/REMNT LOW
36H 18/1200Z 20 KT 25 MPH...POST-TROP/REMNT LOW
48H 19/0000Z 20 KT 25 MPH...POST-TROP/REMNT LOW
72H 20/0000Z 15 KT 20 MPH...POST-TROP/REMNT LOW
96H 21/0000Z 15 KT 20 MPH...POST-TROP/REMNT LOW
120H 22/0000Z...DISSIPATED

I'll have a new blog on the potential for development in the eastern Pacific and Atlantic tomorrow,

TropicalAnalystwx13

Tropical Cyclones of 2013 Tropical Weather

Updated: 3:25 AM GMT on May 17, 2013

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ALVIN! The first named storm of the 2013 Pacific hurricane season develops on cue

By: TropicalAnalystwx13, 10:13 PM GMT on May 15, 2013

Not dissimilar to the 2012 Pacific hurricane season, which featured both Tropical Storm Aletta and Hurricane Bud in May, we are once again starting the hurricane season right on cue. What was once Invest 90E intensified into Tropical Depression One-E early this morning and further to Tropical Storm Alvin as of the latest update. The most recent information from the National Hurricane Center states maximum sustained winds have increased to 40 mph, and the minimum barometric pressure has fallen to 1005 millibars. Alvin is tracking west-northwest at 13 mph. Visible satellite loops reveal a well-developed and intensifying tropical cyclone, with increasing spiral banding in all semicircles and bursting convection near the low-level center. Despite a bit of dry air around the tropical cyclone, Alvin remains further within a well-developed moisture bubble and is tracking within a moisture belt which should help to aid development and prevent many dry air intrusions. The latest satellite intensity estimates from SAB and UW-CIMSS-ADT remain at T2.0/30 knots and T2.5/35 knots, respectively. A recent 1612 UTC ASCAT pass revealed a closed circulation with 35 mph winds in the northern semicircle. Using a blend of the above, I would not have upgraded the system quite yet, but it makes little difference when considering the cyclone likely would have met the criteria of a tropical storm by the next update time.


Figure 1. MODIS image of Alvin taken earlier this afternoon. At the time, it was a tropical depression with winds of 35 mph.

Model forecasts
Before getting into specific model forecasts for Alvin, I would like to take the time to commend the CMC model for nailing the development of the tropical cyclone well in advance; over a week out, specifically. The GFS model only caught on about four days ago, and the ECMWF model does not yet acknowledge Alvin as a system. One of the main fall-backs of the European model is its tendency to "miss" tropical cyclogenesis, however.

There continues to be huge discrepancy in the forecast track and intensity forecast of Tropical Storm Alvin. The 12z run of the GFS had a fairly decent initialization, with a pressure of 1007 millibars, two closed isobars, and winds of roughly 30 knots. By 24 hours out, the system is at, or near, hurricane intensity with a minimum barometric pressure of 988 millibars. By late Friday evening, Alvin attains its peak intensity with a minimum barometric pressure of 983 millibars. Its track solution has not changed since yesterday evening, with the tropical cyclone headed west-northwest for 3-4 days. After that point, the cyclone turns to the north and eventually north-northeast while slowly weakening over cooler waters and a more stable environment.

The CMC remains the outlier. Not only is the model the weakest, with Alvin only peaking as a mid-grade tropical storm, but it is also one of the only models to show a west-northwest track and eventually a westward turn as the cyclone gets "stuck" under the southeastern periphery of the large ridge of high pressure to its northwest, over the northeastern Pacific. Unlike the GFS, the CMC does not amplify an upcoming trough over the West Coast; therefore, the eastern side of the ridge is not eroded, and no weakness for the cyclone to escape is made. This should be discounted for the time being given the strong model support towards the other solution.

On a brief note again, the HWRF and GFDL remain very bullish with the intensity of Alvin. The 12z HWRF showed intensification into a minimal hurricane by 24 hours, a Category 2 hurricane by 36 hours, and a peak intensity of 93.9 knots/965.1 millibars by 54 hours. The 12z GFDL showed intensification into a minimal hurricane by 30 hours, a high-end Category 2 hurricane by 54 hours, and a peak intensity equivalent to a major hurricane by 66 hours, at 97.1 knots/959.1 millibars. Both models show the northward turn of the cyclone, both they are too quick and too strong with the weakness in all likelihood. It should be stated that the GFDL and HWRF have a long history of being extremely inaccurate.

Forecast for Alvin
The intensity forecast for Alvin needs to be adjusted higher than the one from yesterday. The idea that Alvin will track through the Pacific moisture belt and has developed a tight moisture bubble means that dry air intrusions may not be as big of an issue as was thought yesterday. The SHIPS indicates lower than 15 knots of wind shear through 96 hours, and Alvin is forecast to track over sea surface temperatures of at least 28C through that time. Relative humidity values are forecast to continue in the low to mid-70s, sufficient for at least steady intensification of the cyclone. Rapid intensification would be a possible if it were not for low ocean heat content values, at 45 kJcm^-2 and decreasing. My forecast is, as aforementioned, higher than yesterday's and shows Alvin attaining hurricane intensity in 36 hours, with a peak of 80 knots thereafter. By 96 hours, increased wind shear from the trough forecast to be over the West Coast, decreasing sea surface temperatures, and lowering relative humidity values should all lead to steady weakening of the cyclone. Dissipation should occur shortly after the end of the forecast period.

Despite the CMC solution, a majority of the dynamical, statistical, and global models show a continued west-northwest track across the open eastern Pacific through 96 hours, a result of a small subtropical ridge over western Mexico. After 96 hours, a deepening trough—aided by a jet streak forecast to round the base of it—should erode both the eastern periphery of the large ridge over the northern Pacific and the western periphery of the subtropical ridge, creating a weakness over Baja California. For this reason, a turn towards the north is expected at that time; thereafter, the system is forecast to track north-northeastward as the eastern periphery of the ridge builds west of the cyclone and the trough moves through the Colorado Rockies.

...MAX WINDS...

INIT 15/2200Z 35 KT 40 MPH
12H 16/0600Z 40 KT 45 MPH
24H 16/1800Z 55 KT 65 MPH
36H 17/0600Z 65 KT 75 MPH
48H 17/1800Z 70 KT 80 MPH
72H 18/1800Z 80 KT 90 MPH
96H 19/1800Z 60 KT 70 MPH
120H 20/1800Z 40 KT 45 MPH

I'll have a new blog tomorrow,

TropicalAnalystwx13

Tropical Cyclones of 2013 Tropical Weather

Updated: 10:20 PM GMT on May 15, 2013

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Invest 90E likely to become Alvin; long-range development possible in both basins

By: TropicalAnalystwx13, 2:13 AM GMT on May 15, 2013

The Pacific hurricane season is jumping the gun once again this year, with an area of disturbed weather—dubbed Invest 90E, the first invest of the season—positioned several hundred miles south-southwest of Acapulco, Mexico. Satellite loops reveal the system remains relatively disorganized this evening, with weak and overall unconsolidated convection (it is in the process of organizing, however). In addition, Invest 90E remains a part of the monsoon trough and Intertropical Convergence Zone (ITCZ). When tropical disturbances are attached to these features, it becomes very hard for them to become tropical cyclones because of substantial convergence on the southern half of the center which prevents the counter-clockwise wind flow needed for classification. It's not unheard of for cyclones to develop while still attached, but this is the exception, not the rule. In their first Tropical Weather Outlook of the season, albeit a Special—out of normal occurrence time–one, the National Hurricane Center gave Invest 90E a Medium chance, 30%, of becoming a tropical cyclone during the next 48 hours. I think it's more likely than not to become a tropical cyclone by that time, and I assess the changes higher, near 60%.


Figure 1. MODIS image of Invest 90E, the first tropical disturbance of the year in the eastern Pacific.

Model Forecasts
Model consensus has been, and continues to be, quite bullish regarding the development of Invest 90E. And the development of this feature has been quite well forecast long in advance as well, with the CMC model, who received a major upgrade a few months ago, forecasting the development of this storm over a week in advance. The ECMWF continues to be the only reliable model to not show any development from this feature; one of the major drawbacks of the model itself is the ability to pick up on tropical cyclogensis, however, so this should be discounted.

The most recent run of the GFS, the 18z, begins its run with Invest 90E as a weak and hardly distinguishable 1012 millibar low. Within 24 hours, however, the pressure has fallen to 1005 millibars. By early Thursday morning, the disturbance has intensified into a minimal tropical storm, at which point it would receive the name "Alvin". Exactly 24 hours later, the system attains its peak intensity as a Category 1 hurricane with a minimum barometric pressure of 980 millibars. A westward-moving trough over California creates a weakness in the expansive ridge of high pressure to the cyclone's northwest and creates a pathway for "Alvin" to move west-northwest, north, and eventually northeastward. This is very similar to the 12z run which showed the cyclone being pulled northward.

The CMC is quite a bit different than the GFS. The first difference is that the intensity of the storm is greatly reduced. With a minimum pressure of 998 millibars, the model is likely showing a weak to mid-grade tropical storm. As far as the track forecast is concerned, the CMC does indeed show the cyclone tracking west-northwest initially. However, it depicts a much weaker trough over the West Coast and instead of breaking down the eastern periphery of the ridge and allowing 90E to be pulled northward towards the coastline, the model shows "Alvin" being caught under the ridge and stalls the cyclone out.

On a brief note, the HWRF shows 90E peaking as a 91 knot Category 2 hurricane with a minimum barometric pressure of 968 millibars, while the GFDL shows the storm peaking as a 44 knot, 1008 millibar tropical storm. Both of these models are horrible performers prior to actual development, and even then, still did not produce accurate results most of the time. They hardly require a mention.

Forecast for 90E
The intensity forecast for 90E isn't the easiest. The disturbance is embedded within an environment very favorable for development, with less than 10 kntos of wind shear and sea surface temperatures greater than 29 °C. However, relative humidity values–simply a measure of the amount of moisture in the air—generally are forecast to hover near 70-75%. These values are sufficient for slow development, but not anything too rapid like the SHIPS was suggesting yesterday evening (95% chance of 25-kt RI). Given the above factors, and help from the model guidance, I feel 90E should attain tropical depression status sometime during the day Thursday; intensification into Tropical Storm Alvin may...and should...occur by the following morning. Within 24 hours, it should attain its peak intensity as a mid-grade tropical storm before slow weakening occurs as the cyclone enters cooler waters and a more stable environment. It's worth noting that the storm could become slightly stronger than forecast, especially if it makes huge strides in organizing tomorrow.

Despite the CMC solution, a majority of the statistical and dynamical models indicate that 90E should take a general west-northwest track through 96 hours. This is further evidenced by the forecast of a jet streak in association with the trough at that time. A jet stream rounding the base of an upper-level trough helps to "dig" it farther south, such as the CMC indicates. As the trough forecast to be over the West Coast erodes the eastern periphery of the high situated over the northeastern Pacific, the system is expected to turn north-northwest and eventually to the north. Whatever becomes of this, it is not expected to impact any landmasses directly. The cyclone may pose the risk of increased rip currents and waves along the southeast-facing beaches of Baja California Sur and Mexico.

...MAX WINDS...

INIT 15/0200Z 25 KT 30 MPH (BASED OFF 18Z ATCF UPDATE)...INVEST
12H 15/1200Z 30 KT 35 MPH...INVEST
24H 16/0000Z 30 KT 35 MPH...INVEST
36H 16/1200Z 30 KT 35 MPH
48H 17/0000Z 40 KT 45 MPH
72H 18/0000Z 55 KT 65 MPH
96H 19/0000Z 50 KT 60 MPH
120H 20/0000Z 40 KT 45 MPH

Long-range development possible
Invest 90E is not the only thing in town to watch. As a strong upward pulse of the MJO moves across the Maritime Continent and quickly into Octants 8 and 1, the Western Hemisphere, both the Atlantic and eastern Pacific have the possibility to see increased tropical activity. Indeed, both the CMC and GFS show a potent tropical storm or minimal hurricane forming southwest of Guatemala and the coastline of Mexico by the end of next weekend; it's atypical to see such model consensus so far out in time. The former model indicates the development of two tropical cyclones, with the first moving into Nicaragua as a tropical depression or tropical storm. On the Atlantic side, development is also possible, though this would be later in time...most likely early June. For a few days the GFS indicated the formation of quite a potent tropical storm in the western Caribbean that either tracked northwestward and curved into Florida or that tracked northeastward and eventually moved out to sea after being picked up by a trough; the model has since that time dropped any development. However, it does still show increased precipitation and below-average pressures. With high pressure to the north over the East Coast, convergence—the subsequent surge and piling of air south of the high—would be forced into the region. What does this all mean? We have an above-average chance of seeing "Andrea" during the first week of what is expected to be an active and dangerous hurricane season.


Figure 2. 18z GFS ensembles 7-day accumulated precipitation anomalies across the western Atlantic and eastern Pacific in week 2. Image credit to Levi Cowan of Tropical Tidbits.


Figure 3. 18z GFS ensembles 7-day MSLP (mean sea level pressure) anomalies across the western Atlantic and eastern Pacific in week 2. Image credit to Levi Cowan of Tropical Tidbits.

Hurricane season forecast
I released my final outlook on the 2013 Atlantic hurricane season Sunday evening. Here is the summary:

"It feels like the 2012 Atlantic hurricane season just ended, and yet it's already time to start talking about the upcoming one. In summary, the 2013 Atlantic hurricane season is expected to draw out the almost continuous streak of Atlantic hurricane seasons above-average since 1995. A total of 16–19 named storms, 8–12 hurricanes, and 3–5 major hurricanes are expected, with an Accumulated Cyclone Energy (ACE) index of >150 units. Due to the positive Atlantic tripole keeping sea surface temperatures down relative to normal in the higher latitudes of the [sub]tropics, we may not see as many storms as the 2010 to 2012 period—namely due to fewer mid-latitude, subtropical, and short-lived cyclone—but this year instead will feature multiple long-tracked and strong hurricanes in the climatologically favorable region of the Atlantic, the Main Development Region. Primary factors in predicting seasonal activity include the state of the El Niño-Southern Oscillation and the Pacific Decadal Oscillation. In proximity to an active season, a dangerous year for landmasses within the Atlantic, specifically the United States, is expected. In fact, I expect at least 4 hurricane landfalls on U.S. coastline, of which at least 2 may be major—a Category 3 or higher on the Saffir–Simpson hurricane wind scale. This is a year where anybody could be impacted, and it's crucial to know what to do if a storm threatens your home. It only takes one."

If you would like to see the factors that went into making the forecast, or just want to read the blog, click here.

I'll have a new blog out tomorrow,

TropicalAnalystwx13

Tropical Cyclones of 2013 Tropical Weather

Updated: 2:15 AM GMT on May 15, 2013

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2013 Atlantic hurricane season: busy and dangerous

By: TropicalAnalystwx13, 12:09 AM GMT on May 13, 2013

It feels like the 2012 Atlantic hurricane season just ended, and yet it's already time to start talking about the upcoming one. In summary, the 2013 Atlantic hurricane season is expected to draw out the almost continuous streak of Atlantic hurricane seasons above-average since 1995. A total of 18–21 named storms, 8–12 hurricanes, and 3–5 major hurricanes are expected, with an Accumulated Cyclone Energy (ACE) index of >150 units. Due to the positive Atlantic tripole keeping sea surface temperatures down relative to normal in the higher latitudes of the [sub]tropics, we may not see as many storms as the 2010 to 2012 period—namely due to fewer mid-latitude, subtropical, and short-lived cyclone—but this year instead will feature multiple long-tracked and strong hurricanes in the climatologically favorable region of the Atlantic, the Main Development Region. Primary factors in predicting seasonal activity include the state of the El Niño-Southern Oscillation and the Pacific Decadal Oscillation. In proximity to an active season, a dangerous year for landmasses within the Atlantic, specifically the United States, is expected. In fact, I expect at least 4 hurricane landfalls on U.S. coastline, of which at least 2 may be major—a Category 3 or higher on the Saffir–Simpson hurricane wind scale. This is a year where anybody could be impacted, and it's crucial to know what to do if a storm threatens your home. It only takes one.

Little has changed since my previous outlook for the season. I have always felt it is best to start on a larger scale and work one's way down, so we'll start with the global sea surface temperature anomaly map to evaluate the Pacific.


Figure 1. Global sea surface temperature anomalies compared to 1982–2010 climatology. Image credit to Levi Cowan of Tropical Tidbits.

As was the case in my early March outlook, a few factors remain undecided as far as their impact on the hurricane season is concerned. The latest update from the Climate Prediction Center shows the ENSO is in its neutral phase, meaning sea surface temperature anomalies over a three-month period lie between -0.5 °C and 0.5 °C. As a refresher of the importance and classification of ENSO, I have included an excerpt from my previous blog.

"One of the most significant factors one must take into consideration when forecasting a hurricane season is the state of the ENSO, or El Niño-Southern Oscillation. Simply put, the ENSO is a quasi-periodic climate pattern observed across the equatorial Pacific. Within this pattern lie three phases: a warm stage, a cool stage, and a neutral phase. As the name would suggest, a warm phase, or El Niño, is characterized by anomalous sea surface temperatures at or above 0.5°C across the equatorial Pacific for at least three months. During the cool phase, or La Niña, anomalous sea surface temperatures are -0.5C or below for three consecutive months; the neutral phase, known as Neutral, represents the range between the two. It should be noted, however, that Neutral phases are typically short-lived and do not have as much of an influence on global patterns.

As aforementioned, all three phases of the ENSO have a significant influence on a particular hurricane season; we'll focus on the Atlantic for this post. During an El Niño, above-average sea surface temperatures across the eastern Pacific promotes the growth of convection--shower and thunderstorm activity--across the basin. Outflow as a result of this disturbed weather enhances vertical wind shear across the western Atlantic, therefore making it more difficult for tropical cyclones to develop. Oppositely, a La Niña promotes cooler-than-average sea surface temperatures in the eastern Pacific, focusing convergence in the Atlantic and allowing the development of anticyclones thanks to rising motion across that basin."

Knowing what we do now, it's a good idea to check out what the models say about the ENSO for this season.


Figure 2. Mid-April 2013 plume of model predictions for the ENSO.

As seen above, a majority of the dynamical and statistical models for SSTs in the Nino 3.4 region call for Neutral conditions, which generally has a positive effect on Atlantic activity. In fact, only 3 show a borderline El Nino by the end of 2013, while over half show sea surface temperature anomalies remaining below 0.0C. Predicting the ENSO is always the hardest when dealing with the Neutral phase and especially when it's Spring, as short-term changes can significantly influence the anomalies. I remain confident in ENSO neutral conditions throughout the equatorial Pacific through at least the peak of the season. In the short term, we are likely to see warming as an upwelling MJO pulse moves across the region. This should not be significant as far as the rest of the year is confirmed. Of note is the ECMWF forecast of a warm-biased Neutral or El Nino by the peak of the season; this should be thrown out, however, because of the warm bias the model tends to have during the pre-season.

Switching back to the global SST map, however, one can see that the PDO remains in a predominantly negative phase as observed by the patch of much above-average SSTs south of Alaska with a horseshoe-shaped ring of cool water surrounding it. It should be noted that, despite recent warming off the West Coast of the United States, this has little bearing on the eventual phase later on; in fact, the recent cooling of the region south of Alaska and warming waters around that is a result of a large and elongated, though weak, area of low pressure across the region. The forecast is for these lows to move eastward and be replaced with high pressure once again, meaning the PDO should take on a configuration much more similar to the negative phase. During a negative PDO during the summer, high pressure across the Pacific focuses convergence, the piling and subsequent rising of air, into the Atlantic. This provides greater lift and ultimately more convective activity across the basin.


Figure 3. Atlantic sea surface temperature anomalies compared to 1982–2010 climatology. Image credit to Levi Cowan of Tropical Tidbits.

The Atlantic remains well above-average as far as sea surface temperatures are concerned, and remains on par with years that ended up as above-average seasons once they were done. One region in particular—the MDR—has been the warm spot of the basin, with sea surface temperature anomalies averaging out at nearly +0.5 °C. One reason the MDR has been able to sustain above-average SSTs is due to a powerfully negative North Atlantic Oscillation (NAO) during the late winter and early spring. A negative NAO promotes a weaker and more spread out Bermuda ridge; this reduces the low-level trade winds underneath it, across the deep tropics, and promotes a lesser degree of evaporative cooling.

During the past month, the NAO has been in a mainly positive phase; this typically promotes a stronger-than-average Bermuda high and thus above-average trade winds across the deep tropics. However, because the dipole—separation of positive and negative values—has shifted farther north relative to normal, the trade winds have shifted farther north relative to normal, and the MDR has not been significantly effected. A warm MDR not only provides more fuel for tropical cyclones, but if it's warmer relative to other basins, can help focus the Madden–Julian Oscillation (MJO) in the region. There are two phases of the MJO: positive and negative. During the positive phase, increased lift and moisture makes it easier for tropical cyclones to form, while the opposite is true for the downward phase.

Another interesting region in the Atlantic is the mid-latitudes/subtropics, where sea surface temperatures remain below-average. Similar to what kept the MDR warm, the negative NAO has also cooled the aforementioned region through persistent troughing and low pressure. Cooler waters in the subtropics promotes sinking which, once reaching the surface, spreads out and promotes convergence farther south, across the deep tropics. Farther north, warmer waters exist in the higher latitudes due to the influence of blocking high pressure over Greenland. This series of warm water over the deep tropics, cool water in the subtropics, and warmer water higher up is known as the positive Atlantic tripole.

Analogue years

Certain years throughout history show a similar global atmospheric and oceanic pattern similar to 2013. Those years are known as analogues and give important insight as to where the majority of the storms will form, how many will form, and where they will track. Searching for years with a similar sea surface temperature distribution and neutral ENSO, I have used several years to build my forecast, listed in order from most similar to least similar: 2005, 1996, 2004, 2008, 1952, and 1966. The averages of these seasons gives 15 named storms, 9 hurricanes, and 5 major hurricanes. Personally, I feel this year should be most similar to 2004 (15 tropical storms) and 2008 (16 tropical storms) as far as seasonal activity is concerned.


Figure 4. Atlantic SST anomalies year-to-date.


Figure 5. Atlantic SST anomalies from January to May from my analogue package.

Landfall Prospects

Forecasting seasonal activity, while still uncertain in neutral ENSO years primarily, has become much easier over the past decades with an increase in understanding and technology. However, what still presents new troubles to us every season is the track distribution of tropical cyclones during that year. One of the best places in the pre-season to start is to examine the analogues (which I mentioned just above). If we take a look at the track distribution in 1952, we notice that many tropical cyclones affected land in the western Atlantic Ocean, despite only two officially making landfall in the USA. In 1966, two hurricanes struck Florida: Alma in June and, for all intents and purposes, Inez (the storm did not officially make landfall, the eye passed about 10 miles offshore). Regardless of these two, the Caribbean Islands were severely impacted this season. In 1996, the East Coast of the United States was hard hit, particularly the state of North Carolina, where two hurricanes made landfall. As was the case with the previous two years, a majority of the tracks were focused in the western Atlantic.

The two hurricane seasons many remember most are the seasons of 2004 and 2005. The former was a horrible, horrible year for the United States as a whole, but in particular for the state of Florida, with four hurricane landfalls. The latter, the mammoth of a hurricane season, had a track distribution pretty much everywhere. The United States coastline was, to add emphasis, "shredded" during the year. The impacts of these two years in particular were due mainly to an expansive ridge of high pressure over eastern Canada and the northeastern United States, with a trough over the central United States.


Figure 6. 500mb geopotential height anomalies from January 1 to May 10, 2013.

If we take a look at the mean pattern since the start of the year (pictured above), one thing in particular sticks out: a very large and intense area of high pressure positioned southeast of Alaska. Ridging is also noticed over Greenland, which is a big culprit in the cold and ever-lasting winter the United States has endured thus far in 2013.


Figure 7. 500mb geopotential height anomalies from April 1 to May 10, 2013.

In the short term, the pattern appears even more dangerous (picture above), with the ridge continuing southeast of Alaska, troughing over the central United States, and ridging just off the Northeast. This mean setup is particularly worrisome because a tropical cyclone tracking westward would be caught under the southern periphery of the ridge over Canada and the United States, bringing it westward into the East Coast. Storms farther south and west, specifically in the Caribbean, would be drawn into the central and eastern Gulf Coast due to the presence of the trough over the Plains. And the forecast from several models—the UKMET and CFS in particular—forecast a continued dangerous setup. Both models indicate the development of an elongated area of high pressure stretching from Newfoundland, into central Canada, and down into Montana (banana-shaped). This again, helps tropical cyclones get caught underneath it, leading to U.S. landfalls.

In general, because of the forecast—and already observed—ridge of high pressure across Canada (and southeast of Alaska; extended ridge eastward messes up wavelength frequency...trough normally over west is pushed over central USA, ridge over central USA is pushed over East) and negative PDO, which favors multiple East Coast strikes, coupled with what appears to be a new phase of hurricane landfalls in this region, I am calling for multiple impacts on the U.S. coastline...specifically the East Coast (repetitive, I know). I believe we will see at least 4 hurricane landfalls on our coastline, with two or more of these being major hurricanes. It has now been 7 years since a Category 3 or higher strength hurricane hit the coastline (Wilma in '05 was our last). I wish I could say this streak will continue, but the odds are stacked against it. Regardless of the forecast, be prepared any season because it only takes one storm—major hurricane or not—to change your life forever. If one threatens your area, at least you'll be ready.


Figure 8. My landfall probability map [of at least one hurricane landfall] for the Atlantic for the 2013 Atlantic hurricane season. Not pictured: Bahamas: high risk (60%+); Lesser Antilles: high risk (north), moderate risk (30-50%, south); Newfoundland, Bermuda, Azores, Cape Verde Islands: moderate risk.

TropicalAnalystwx13

Tropical Weather Tropical Cyclones of 2013

Updated: 9:18 PM GMT on May 24, 2013

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About TropicalAnalystwx13

Teenager. Weather aficionado. Soccer fan. Realist. Posts subject to sarcasm. Goal: National Hurricane Center.