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Wet and windy subtropical storm possible for Southeast U.S.; Mexico eyes TD-10E

By: Dr. Jeff Masters, 3:15 PM GMT on October 06, 2011

A large low pressure system with heavy rain is expected to develop over Cuba and South Florida on Saturday. The counter-clockwise flow around this low will bring strong winds and heavy rains to much of the Florida coast on Saturday, and these conditions will spread northwards to Georgia by Sunday and South Carolina by Monday. The storm may evolve into a subtropical storm that gets a name by Monday or Tuesday, but the potential location of such a storm is still murky. The extended forecast discussion from NOAA's Hydrometeorological Prediction Center favors a more westerly location, in the eastern Gulf of Mexico, as predicted by the ECMWF model. The GFS model, which puts the storm's center east of Florida, is pushing the weather system that will spawn the subtropical storm too fast to the east. In any case, the exact center location of the storm will not matter that much, since this will be a large, diffuse system that will bring strong winds and heavy rains to a large area of the Southeast U.S. coast, regardless of the exact center location. Portions of the coastal waters from Southeast Louisiana to the Florida Panhandle, as well as from Northeast Florida to South Carolina, are likely to experience sustained winds of 35 - 45 mph Monday and Tuesday. Since the storm is going to get its start as a cold-cored upper-level low pressure system with some dry air aloft, it will probably start out subtropical, with a large band of heavy rain developing well north of the center. Subtropical storms cannot intensify quickly, due to their lack of an organized inner core, and I'm not concerned at present about this storm potentially becoming a hurricane.

Figure 1. Rainfall forecast for the 5-day period ending at 8 am EDT Tuesday, October 11, 2011. The storm system affecting Florida this weekend is expected to bring up to 7 inches of rain along the coast. Heavy rains associated with a strong trough of low pressure are also expected to dump 4 - 6 inches of rain over drought-stricken areas of Texas, Oklahoma, and Kansas. Image credit: NOAA/HPC.

Philippe becomes a hurricane
After 12 days and 49 advisories, Philippe has finally intensified into hurricane, becoming the fifth hurricane of the 2011 Atlantic hurricane season. The fifth hurricane normally arrives on October 7, so this is a very average season for hurricanes, despite the fact it is already the 7th busiest season since record keeping began in 1851 for number of tropical storms--sixteen. Satellite loops show Philippe is a small hurricane with just a hint of an eye. A wide band of clouds to Philippe's northwest is associated with the trough of low pressure that has recurved Philippe to the northeast. By Friday, the trough will bring very high wind shear of 30 - 50 knots, which should cause rapid weakening. Philippe will not trouble any land areas.

Figure 2. Morning satellite image of Philippe. The band of clouds to the northwest of Philippe is associated with a cold front that has recurved Philippe to the northeast.

A double threat to Mexico's Eastern Pacific coast
In the Eastern Pacific off the coast of Mexico, two new tropical cyclones have formed. The one of greatest concern is Tropical Depression 10-E. TD 10-E is currently headed west-northwest, parallel to the coast, but will turn north and then northeast over the weekend as a strong trough of low pressure dives southward over northern Mexico. The computer models have a fairly wide spread for the track of TD-10E, with the region of coast centered on Puerto Vallarta between Manzanillo and Tuxpan at greatest risk of a strike. TD 10-E is under moderate shear of 10 - 20 knots, and shear is predicted to stay in the low to moderate range for the next five days. Ocean temperatures are warm, 28 - 29°C, but the warm waters do not extend to great depth, limiting TD-10E's potential for rapid intensification. Nonetheless, the GFDL model predicts TD-10E will intensify into a major Category 3 hurricane before landfall on Monday on the Mexican coast, and the HWRF model brings the storm to Category 2 strength. The official NHC forecast is less aggressive, bringing TD-10E to Category 1 strength, but this is conservative, and I put the odds at 30% that the storm will be a Cat 2 or stronger at landfall. One possible impediment to development may be TD-10E's close proximity to Tropical Storm Irwin to the west. Upper-level outflow from Irwin could weaken TD-10E, and the two storms may compete for the same moisture. Regardless of TD-10E's strength at landfall, the storm will bring very heavy rains to the Mexican coast capable of causing dangerous flash floods and mudslides, beginning on Monday.

Once TD-10E has made landfall, Mexico may need to concern itself with Tropical Storm Irwin, which is gathering strength farther to the west. Irwin is also moving to the west-northwest, and will also be turned north and then northeast towards the coast of Mexico this weekend by the same trough of low pressure expected to affect TD 10-E. The longer range computer forecast models show Irwin could make landfall on the Mexican coast late next week.

European heat wave: hottest October temperatures on record for the UK
The British Isles have been basking in an unprecedented October heat wave this week, which has brought the warmest temperature ever measured in the UK for the month of October. On Oct. 1, a reading of 29.9°C (85.8°F) was recorded at Gravesend, Kent, beating the previous UK October record of 29.4°C (84.9°F) at Cambridgeshire on Oct. 1, 1985. Wales also broke their warmest temperature for October with a 28.2°C (82.7°F) at Hawarden, Flintshire. Edinburgh, Scotland reached 24.7°C (76.4°F) for Scotland's warmest temperature in at least 50 years. Thanks go to wunderground's weather historian Christopher C. Burt. His latest post is on record hurricanes of the past in the Pacific Ocean.

Jeff Masters


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