Forecast for 92L: dissipation by Friday

A low pressure system about 700 miles east of the Lesser Antilles Islands, Invest 92L, was near tropical depression status early this morning, but is currently weakening. Infrared satellite loops show the disturbance's heavy thunderstorm activity has decreased markedly in the past few hours, with the cloud top temperatures warming noticeably, indicating that 92L's thunderstorms are no longer pushing as high into the atmosphere. Water vapor satellite loops show that the storm is surrounded on all sides by dry air, though there is a region of moister air in front of it that 92L will encounter on Thursday. Wind shear as diagnosed by the University of Wisconsin is near 20 knots, though the SHIPS model is diagnosing the shear at a higher 25 - 30 knots. This high shear is pushing 92L's heavy thunderstorm activity to the east side of the center of circulation, and the center will probably become exposed to view late this morning. Had 92L been able to maintain the heavy thunderstorm activity it had early this morning for 12 or so hours, it could have been classified as a tropical depression. However, classification as a TD requires persistent heavy thunderstorm activity, typically interpreted to mean 12 hours of consistent heavy thunderstorm activity, and 92L did not meet that criterion.


Figure 1. Morning satellite image of Invest 92L. Image credit: NOAA.

The forecast for 92L: dissipation
Wind shear is the main story in the forecast for 92L, as a band of very high wind shear of 20 - 50 knots lies to the northwest of the storm. The current expected track of 92L carries it into this band of high wind shear, and the SHIPS model (based on the GFS model) is predicting that the shear will remain in the 25 - 30 knot range through Friday. Other models predict higher shear levels. It is likely that the high shear, combined with the dry air surrounding the storm, will destroy 92L by Friday. The National Hurricane Center is giving 92L a low (10% chance) of developing into a tropical depression by Friday morning, and this is a reasonable forecast. It is likely that 92L will bring heavy rain showers and wind gusts up to 35 mph on Friday to the Lesser Antilles Islands. I don't expect 92L to be organized enough to cause flooding problems to any of the islands in its path. None of our reliable global computer models develop 92L into a tropical depression. The rest of the tropical Atlantic is quiet, and none of the reliable computer models is calling for tropical cyclone development in the Atlantic over the next seven days.

Is the formation of 92L a harbinger of an active hurricane season?
According to the Hurricane FAQ, Goldenberg (2000) found that during the period 1944 - 1999, formation of a named storm in the tropical Atlantic south of 22°N and east of 77°W during June and July was a harbinger of at least an average season, and in many cases an above average season. The formation of a storm in this region during June or July is one factor the NOAA and Colorado State University seasonal hurricane forecast teams have used in the past as a predictor for an active season in their early August forecasts. Now, 92L didn't make it to named storm status, though it was pretty close to being a tropical depression. However, the near-formation of 92L into a tropical depression, is, in my mind, a clear harbinger that we can expect a severe hurricane season this year. It's very rare to have a development like 92L in that portion of the tropical Atlantic this early in the season. The lower than average wind shear and higher than average SSTs that helped 92L get organized are more likely than not to carry over into the main portion of hurricane season, giving us a much more active hurricane season than normal.


Figure 2. Synthetic Aperture Radar (SAR) image of the oil spill taken at 7:51 am EDT Tuesday June 15, 2010, by the Canadian Radarsat-1 satellite, operated by MDA GeospatialServices of Richmond, Canada. A large amount of oil was present on the Florida Panhandle coast near Pensacola, and was headed east towards Panama City. Image credit: Center for Southeastern Tropical Advanced Remote Sensing, University of Miami Rosenstiel School of Marine and Atmospheric Science. SAR images have a resolution of 8 - 50 meters, and can be taken through clouds and precipitation.

Oil spill wind and ocean current forecast
Light and variable winds less than 10 knots will blow in the northern Gulf of Mexico for the next five days, according to the latest marine forecast from NOAA. The winds will tend to have a westerly component for the most part, which will maintain a slow (1/4 mph) eastward-moving surface ocean current that will transport oil eastwards along the Florida Panhandle coast, according to the latest ocean current forecast from NOAA's HYCOM model. These winds and currents may be capable of transporting oil east past Panama City, Florida to Cape San Blas by Monday. Oil will continue to threaten the coasts of Louisiana, Alabama, and Mississippi for the remainder of the week as well, according to the latest trajectory forecasts from NOAA and the State of Louisiana. The long range 8 - 16 day forecast from the GFS model indicates a typical summertime light wind regime, with winds mostly blowing out of the south or southeast. This wind regime will likely keep oil close to the coastal areas that have already seen oil impacts over the past two weeks.

NOAA has lauched a great new interactive mapping tool that allows one to overlay wind forecasts, ocean current forecasts, oil location, etc.

Oil spill resources
My post, What a hurricane would do the Deepwater Horizon oil spill
My post on the Southwest Florida "Forbidden Zone" where surface oil will rarely go
My post on what oil might do to a hurricane
NOAA has lauched a great new interactive mapping tool that allows one to overlay forecasts and oil location observations
Gulf Oil Blog from the UGA Department of Marine Sciences
Oil Spill Academic Task Force
University of South Florida Ocean Circulation Group oil spill forecasts
ROFFS Deepwater Horizon page
Synthetic Aperture Radar (SAR) imagery from the University of Miami

I'll have an update on Thursday.

Jeff Masters