Western Caribbean disturbance 98L a threat to develop; Otto weakening

An area of disturbed weather (98L) is in the Western Caribbean, a few hundred miles east of the coast of Nicaragua. The disturbance has a modest and slowly increasing amount of intense thunderstorms, and is showing some spin. The disturbance is drifting northwest at less than 5 mph, and is under a moderate 15 knots of wind shear. Dry air in the Western Caribbean has been interfering with development, but 98L is now generating enough thunderstorms that the environment in the Western Caribbean is moistening, which will support further development. Pressures at Colombia's San Andres Island near the center of 98L are not falling, and satellite imagery show disturbed weather only over a very modest portion of the Caribbean, so any development today will be slow to occur.


Figure 1. Morning satellite image of 98L.

Forecast for 98L
98L is likely to bring heavy rains to northeastern Honduras and Nicaragua on Sunday, and possibly the Cayman Islands and Jamaica as well. The latest SHIPS model forecast calls for wind shear to stay in the moderate range, 10 - 20 knots, through Tuesday. This should allow 98L to reach tropical depression status early next week, though NHC is only calling for a 20% chance of 98L becoming a tropical depression by Monday morning. The GFDL and HWRF models are the most aggressive developing 98L. These models predict 98L will intensify into Tropical Storm Paula by Monday, move northwest and then north, and pass through the Cayman Islands on Monday night and Tuesday morning as a tropical storm. Paula would then hit western or central Cuba as a hurricane on Tuesday or Wednesday, brush the Florida Keys, then accelerate northeastward through the western Bahamas on Wednesday or Thursday. This is probably too aggressive of a forecast, given 98L's current small size and lack of organization. The UKMET model also develops 98L, but keeps the storm in the Western Caribbean over the next seven days. The NOGAPS model keeps 98L weak and predicts a more west-northwesterly motion into Mexico's Yucatan Peninsula on Wednesday. The GFS and ECMWF models do not develop 98L. One argument against the development of 98L would be that the phase of the Madden-Julian Oscillation is currently promoting sinking, stable air over the Western Caribbean, which tends to make the atmosphere dryer and more stable. However, I think that 98L will spend enough time in the Western Caribbean to overcome the relatively stable, dry air, and become a tropical depression or tropical storm by Tuesday. The likelihood of the storm hitting Cuba versus moving more to the west-northwest and hitting Mexico's Yucatan Peninsula is difficult to call at this point.

Otto weakening, pulling away from the the islands
The deluge has finally ended for Puerto Rico, the Virgin Islands, and the northern Lesser Antilles Islands from Hurricane Otto. This is welcome news in St. Thomas in the U.S. Virgin Islands, where Otto and its precursor storminess dumped 15.25" of rain over the past eight days. On average, St.Thomas expects to receive just 1.44" of rain during the first eight days of October. Satellite imagery shows that Otto is beginning to suffer the ill-effects of high wind shear of 20 - 30 knots. The cloud pattern has become distorted and non-symmetric, with the clouds on the southwest side of the storm being eaten away by the strong upper-level winds from the southwest creating the shear. Otto will continue to deteriorate due to increasing wind shear until the storm transitions into an extratropical storm on Monday.


Figure 2. Satellite image of Otto taken by NASA's MODIS instrument on their Terra satellite at 11:05 am EDT October 8, 2010. Image credit: NASA.


A challenge to the validity of the world extreme heat record of 136.4°F (58°C) at Al Aziza, Libya
One of the "sacred cows" of world weather extremes has been the widely reported "hottest temperature ever recorded on earth", a reading of 58°C (136.4°F) reported from Al Azizia, Libya on Sept. 13, 1922. In a remarkable piece of research, our featured Weather Extremes blogger Christopher C. Burt concludes: the temperature observations at Al Azizia prior to 1927 (when the site and instruments were changed) are obviously invalid. The shelter housing the thermometer was most likely over exposed and measuring heat radiating of off the black-tarred concrete of the terrace on which it was placed.

Has Mr. Burt slain one of meteorology's most sacred cows? You be the judge. Check out the full story at his blog.

I'll have an update Sunday by 2pm EDT.

Jeff Masters