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Erika Floods Dominica; Major Uncertainties on Potential U.S. Impact

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Heavy rains and strong gusty winds are sweeping through much of the northern Lesser Antilles Islands today as Tropical Storm Erika heads west at 16 mph. An Air Force hurricane hunter aircraft was in the storm Thursday morning, and at 8 am EDT found Erika's top surface winds were 50 mph, with a central pressure nearly unchanged at 1004 mb. The aircraft also found the center had jogged to the south by about 20 miles between 5 am and 8 am, a position which put the center of circulation closer to Erika's heaviest thunderstorms. According to the Antigua Met Service, Canefield Airport on Dominica recorded 8.86" (225 mm) of rain Wednesday night from Erika, and heavy flooding has been observed on the island. The Guadaloupe Airport has recorded 1.18" of rain today as of 9 am EDT, with a peak wind gust of 36 mph. A Personal Weather Station (PWS) on St. Thomas in the Virgin Islands at higher elevation recorded a wind gust of 47 mph Thursday morning. Erika's tropical storm-force winds were on the east side of the storm, so the strongest winds of the storm will not occur in the Virgin Islands until Thursday afternoon. Satellite loops on Thursday morning showed that Erika continued to be disorganized in the face of dry air and wind shear. There is not much heavy thunderstorm activity on the storm's north side, where there was dry air from the Saharan Air Layer, and Erika had only a modest area of heavy thunderstorms on its east side. These thunderstorms did not change much in intensity or areal coverage on Thursday morning. Wind shear due to upper-level winds out of the west was a high 20 knots, and this shear was driving dry air on the northwest side of Erika into its core, disrupting the storm. Sea surface temperatures (SSTs) were favorable for development, though—near 28.5°C (83°F).

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3091

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We can see good upper level outflow for Erika (the high cloud streaks are typical of that), including two distinct outflow channels (from NE into the Atlantic and from SW across the Caribbean). This raises the potential for strong low level convergence to take place, which means the system has an opportunity to lower the surface pressures and intensify the wind field.

If this happens during the next 24 hours, it will defy the recent NHC projections, which have showed no increase in intensity during that time due to fairly strong wind shear and some relatively dry air getting in on the act. The trouble is, the storm's large size, with very expansive convection, appears to have modified the surrounding environment enough to shield the core of the system from these detrimental features at least for the time being. Impressively, this has occurred during the evening hours, when tropical cyclone convection is typically at its weakest (due to the reduced sea/air temperature contrast I think).

 

A stronger than expected storm may track more NW than projected by the NHC, which raises the risk of it avoiding much interference from the high terrain of Hispanola for example, which would be bad news for the Bahamas, as conditions look to be very favourable for rapid intensification of a well structured storm in that region. The storm needs to be as disoraganised as possible if a hurricane is to be avoided.

 

The storm may then take one of a wide range of paths - anything from crossing southern Florida to turning north and tracking up along the East U.S. Coast looks possible. There's even an increasingly favoured scenario in which the storm gets trapped beneath a ridge of high pressure while east of the U.S., wandering about for a few days or perhaps performing a 'left turn' and moving NW into the U.S. landmass, in the style of Hurricane Sandy from 2012. You heard it here first...!

 

Seriously though, that's just one of countless possibilities so please don't take that as anything more than speculation  :unsure2:  :ninja:

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Erika Nears Puerto Rico with High Wind, Heavy Rain

 

Still poorly organized--but already deadly, and growing in size--Tropical Storm Erika is likely to cause problems in Puerto Rico on Thursday night into Friday. At least four people were killed by mudslides in the wake of Erika’s passage over Dominica. The capital of Roseau, on the island’s southwest coast, was hard-hit with major river and street flooding. According to weather.com, Canefield Airport on Dominica received 12.64†of rain between 2:00 am and 2:00 pm EDT Thursday, as the island was struck by an intense blow-up of thunderstorms (convection) on the south edge of Erika’s ill-defined circulation. In a similar fashion, the typical nighttime intensification of convection could bring torrential rain to Puerto Rico and the Virgin Islands late Thursday night and Friday. In a local statement issued at 5:32 pm AST (4:32 pm EDT) on Thursday, the National Weather Service in San Juan called for widespread 4-8†totals across Puerto Rico and nearby islands, with up to 12†possible. Although the islands are in desperate need of rain to assauge an intense drought, Erika may deliver too much of a good thing.

 

http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=3092

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Erika appears to have a problem with multiple circulations at the moment, all attempting to exploit the decent outflow and intensify. This can't actually happen as the centres interfere with one another.

 

Unfortunately, this messy situation may actually result in Erika hitting Hispanola head-on, which would likely bring devastating rains as the huge moisture field collides with the high mountains over the central part of the island.

 

Yet we don't want to see the cyclone get very organised and move N of Hispanola, as that gives rise to the 'Bahamas Bomb' scenario.

 

Really, there's no real way to 'win' with this cyclone. The official NHC track is one of the least troublesome scenarios yet still threatens a lot of severe conditions, as the cyclone grazes Hispanola without strengthening and then just about manages to organise and reach hurricane status over the Bahamas,  after which it moves very, very close to Florida:

 

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Fair to say the rains with this scenario could be a major issue for Florida.

 

The one possible silver lining is the major reduction or total removal of drought conditions in Puerto Rico over the next 24 hours. Heavily countered by the flash flooding risks though!

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