Tropical wave has potential to develop into tropical depression

A tropical wave over the mid-Atlantic near 12N 42W, about 1300 miles east of Barbados, has the potential to develop into a tropical depression. However, the long-term survival of this system is questionable. The wave has a well defined surface circulation, and is currently under about 10-15 knots of wind shear, due to upper level winds from the east. As the wave moves westward over the next 36 hours, it should encounter an area of lower wind shear where these upper level winds will relax, and I believe a tropical depression will probably develop by Tuesday night. However, as the wave continues westward at 15-20 mph, it is forecast to encounter strong upper-level westerly winds associated with the counter-clockwise flow of air around an upper-level low pressure system northeast of Puerto Rico (Figure 1). This strong wind shear should weaken or destroy the wave. This is shown well by the GFDL model, which intensifies the wave into a weak tropical storm by Wednesday, then dissipates it later in the week. None of the other computer models develop the wave at all.


Figure 1. Water Vapor satellite image from 8:45am EDT August 7. Note the substantial dry air (brown colors) to the northwest of the tropical wave we're watching, and the upper level low to the northwest of the wave that is expected to bring strong westerly wind shear later this week.

Intense thunderstorm activity associated with the wave is mostly on the west side of the circulation, due to the strong upper-level winds from the east pushing all the convection over to that side. There is a substantial amount of dry air to the northwest of the wave, as seen in the water vapor image from this morning (Figure 1.) This dry air has resulted in a decrease in the intensity and areal coverage of the wave's thunderstorm activity this morning. If we do get the expected decrease in wind shear later today and tomorrow, this should allow the thunderstorm activity to build back in over the circulation center.


Figure 2. Preliminary model tracks for the mid-Atlantic tropical wave.

For today, it does not appear that this tropical wave is going to be a major threat. I'll be back with an update on Tuesday. Elsewhere in the tropical Atlantic, there are no threat areas to talk about.

Jeff Masters