Two unimpressive Atlantic disturbances to watch

A low pressure system (99L) that tried to organize into a tropical depression Sunday in the Western Caribbean moved inland Sunday evening over the Yucatan Peninsula before being able to do so. The system is headed west-northwest at about 10-15 mph, and should emerge over the Gulf of Mexico tonight. This morning's QuikSCAT pass showed top winds of about 25 mph to the northwest of 99L in the Gulf of Mexico. Satellite loops show only limited heavy thunderstorm activity at present. Wind shear is about 10-15 knots, and is expected to remain 10-20 knots through Tuesday afternoon. This may allow 99L to organize into a tropical depression by Tuesday night. A trough of low pressure is expected to pull 99L northwards by Tuesday night, and a landfall in Texas on Wednesday is a likely scenario. Since the storm may be moving almost parallel to the coast, it is difficult to say where in Texas landfall might occur. The trough will also bring increased levels of wind shear to 99L Tuesday night as it approaches Texas, and there is also some very dry air over the Gulf of Mexico for the storm to contend with. These factors should keep 99L from getting any stronger than a 45 mph tropical storm. I don't expect the system will become a tropical depression, and NHC does not think highly enough of 99L to bother putting the Hurricane Hunters on call to fly the storm on Tuesday. The primary threat from 99L will be heavy rain.

On Sunday afternoon, 99L gave us a lesson on why it is difficult to predict the track and intensity of a system trying to get organized into a tropical depression. The storm initially attempted to form in the extreme southwestern Caribbean (Figure 1). However, strong upper level winds from the northeast were creating about 15 knots of wind shear here. When a new burst of heavy thunderstorms developed underneath an upper level anticyclone with very low wind shear (Figure 2), the center of 99L reformed there, jumping over 100 miles to the north-northwest.


Figure 1. Satellite image of 99L Sunday afternoon, showing the old center in the southwest corner of the Caribbean Sea, which strong wind shear due to upper-level northeasterly winds had exposed to view. A new center formed to the north-northwest of the old center. Image credit: NASA/MSFC.


Figure 2. Wind shear map of 99L Sunday afternoon, showing that wind shear was 15 knots over the old center in the southwest corner of the Caribbean Sea. The new center developed to the north-northwest of the old center, in a region where wind shear was only 5 knots. Image credit: University of Wisconsin CIMSS group.

Tropical wave approaching Nicaragua
A tropical wave (98L) in the southern Caribbean will move ashore over Nicaragua and Honduras later today. This morning's QuikSCAT pass showed no circulation and top winds of only 25 mph. The system is under about 25 knots of wind shear, and no development is expected before the system makes landfall. The center will remain over land the next two days, spreading heavy rains across Nicaragua and Honduras today, then into Mexico, Guatemala, and Belize on Tuesday.

I'll have an update Tuesday morning, and present my hurricane season outlook for the remainder of October.

Jeff Masters