Tropical Storm Update

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TD 14 likely to become Maria; new Gulf of Mexico system brewing

Tropical Depression Fourteen formed yesterday from a strong tropical wave that moved off the coast of Africa early this week, and is headed west-northwest towards an encounter with the Northern Lesser Antilles Islands. Satellite loops show a large a steadily organizing system with plenty of heavy thunderstorm activity, good upper-level outflow to the north and west, and some respectable low-level spiral bands beginning to form. TD 14 is probably a tropical storm now, and is very likely to be named Tropical Storm Maria later today. Water vapor satellite images show that 95L is embedded in a very moist environment. Ocean temperatures are near 28.5°C, which is 2°C above the 26.5°C threshold usually needed to sustain a tropical storm. With wind shear predicted to remain low to moderate the next five days, the atmosphere expected to stay moist, and ocean temperatures predicted to gradually warm, TD 14 should generally show a strengthening trend. Curiously, most of the intensity forecast models show little strengthening of TD 14, so NHC is keeping their intensity forecast lower than is typical for a storm in these conditions at this time of year.
The track forecasts for TD 14 from the various models agree that the storm will affect the Northern Lesser Antilles Islands, though there are some differences in forward speed, resulting in some uncertainty whether the storm will arrive at the islands as early as Friday night, or as late as Saturday afternoon. After it passes the Lesser Antilles, TD 14 has the usual amount of high uncertainty in its 5 – 7 day track forecast. The models are split on how strong the steering influence a trough of low pressure along the U.S. East Coast will have. The ECMWF and UKMET models prefer a more southerly track for TD 14 through the Bahamas towards the U.S. East Coast, while the GFS, NOGAPS, and HWRF models predict a more northwesterly track, with a potential threat to Bermuda. It’s too early to guess which track the models will eventually converge on. Climatology favors a track that would miss land, with Dr. Bob Hart’s track history pages suggesting TD 14 has a 22% chance of hitting Canada, 19% chance of hitting Bermuda, and an 11% chance of hitting North Carolina.


Figure 1. Morning satellite image of TD 14.

Gulf of Mexico disturbance 96L
A cold front swept into the Gulf of Mexico off the coast of Texas behind Tropical Storm Lee on Monday, and has stalled out along a line from Tampa, Florida to Mexico’s Bay of Campeche in the southern Gulf of Mexico. Heavy thunderstorms have begun to build along the tail end of this front in the Bay of Campeche, and this disturbance has been designated Invest 96Lby NHC. Latest visible satellite loops do not show that 96L has a closed surface circulation yet, but buoy and surface observations along the coast of Mexico suggest that there may be a large-scale counter-clockwise circulation present over the Bay of Campeche. Sustained winds at Buoy 42055, about 100 miles to the northwest of the suspected center of 96L, were northeast at 27 mph at 6:50 am CDT this morning. Water vapor satellite loops show that here is a large area of very dry air from Texas to the north of 96L, and this dry air may interfere with 96L’s development. A hurricane hunter mission is scheduled for this afternoon into 96L.

Most of the computer models develop 96L into a tropical depression in the next 1 – 2 days, and these same models did very well at anticipating the formation of Tropical Storm Lee in the Gulf of Mexico last week. Given the moderate wind shear, warm waters, and presence of an old cold front to serve as a nucleus for development, I give 96L an 80% chance of becoming a tropical depression over the next tow days, a bit higher than the 60% probability NHC is going with. Steering currents are weak in the Bay of Campeche, making for a lot of uncertainty in where 96L might go. The only model predicting a U.S. landfall is the ECMWF, which predicts 96L might hit between Louisiana and the Florida Panhandle on Monday. A more popular solution is for the storm to meander in the Bay of Campeche for many days, and eventually make landfall on the coast of Mexico between Veracruz and Tampico. None of the models is hinting at a track towards Texas, and the intense dome of high pressure associated with their record drought and heat wave will tend to discourage any tropical cyclones from making a Texas landfall over the coming seven days.

Hurricane Katia 
Hurricane Katia continues to the northwest as a Category 1 hurricane with 90 mph winds. Latest satellite loops show that dry air has eaten into the southwest side of the storm. The computer models continue to agree that a low pressure system over the Eastern U.S. associated with the remnants of Tropical Storm Lee will turn Katia to the north. As the storm moves northwards past North Carolina, Katia will get caught up in west-to-east moving winds associated with the jet stream, and taken northeastwards out to sea. No land areas are in Katia’s cone of uncertainty, and Katia’s outer rainbands should remain just offshore from North Carolina, New England, and the Canadian Maritime provinces at the point of closest approach. Bermuda may see a few rain showers from Katia, but the storm will not cause hazardous weather there. The main impact of Katia will be a multi-day period of high surf leading to beach erosion and dangerous rip currents. The East Coast is lucky that Tropical Storm Lee came along, since Lee helped to create the steering pattern that will keep Katia from hitting the U.S.


Figure 2. GOES-13 image of Hurricane Katia and the remains of Tropical Storm Lee, taken at 11:45 am EDT Tuesday September 6, 2011. At the time, Katia was a Category 3 hurricane with 120 mph winds. Image credit: NOAA Visualization Laboratory.