African disturbance 92L could develop into tropical depression

A strong tropical wave (Invest 92L) moved off the coast of Africa last night, south of the Cape Verde Islands. This low has the potential to develop into a tropical depression late this week as it moves westwards at 15-20 mph. The low is under about 10 knots of wind shear, which is favorable for development. Sea Surface Temperatures are about 28°C, which is about 2-3°C above average for this date, and well above the 26°C threshold for tropical storm formation. There is not much African dust or dry air near the storm, but so far the low has not been able to generate much heavy thunderstorm activity. The low has plenty of spin, and has developed some broad, curved bands that are a sign of organization. I can't really find any negatives for development, except for climatology--there has never been a tropical depression that has formed east of 34° W longitude in the first half of July (see the first image I posted in yesterday's July Atlantic hurricane outlook). NHC is giving a low chance (less than 20%) that 92L will develop into a tropical depression by Thursday afternoon. Given the system's current disorganization, that's a reasonable forecast. However, all the models are developing this system into a tropical depression by late this week, and I think that the odds rise to a 50% chance of a tropical depression by Saturday. Wind shear is expected to remain low, and the waters will stay warm (above 26°C) for the next 2-3 days of 92L's life.


Figure 1. Latest satellite image of Invest 92L.

2008 African dust forecast
According to an experimental dust forecast by Amato Evan of the University of Wisconsin's Cooperative Institute for Meteorological Satellite Studies, 2008 should have fairly normal levels of African dust activity, similar to what was seen in 2007. This is a change from 2004, 2005, and 2006, which had fairly low levels of dust activity. This lack of dust resulted in 0.2°C less cooling of the eastern Atlantic Sea Surface Temperatures in 2004, 2005, and 2006 compared to 2007. High levels of dust block sunlight, leading to cooling of the sea surface.

Jeff Masters