The Atlantic has been mighty quiet during the first two months of the 2009 Hurricane Season, in fairly stark contrast to the past few years. Tropical wave after wave have crossed over Africa, only to fall apart over the Atlantic. A number of these waves have subsequently redeveloped over the Eastern Pacific – Tropical Storm Enrique and Hurricane Felicia are the latest two named storms with T.D. 9-E becoming the latest tropical system to form. By way of comparison, the Atlantic has seen only one T.D. this year, and that was way back in late May. Note that this isn’t all that unusual: August is climatologically the month when tropical storms start to form with some regularity in the Atlantic, especially under El Nino conditions.
Well, two waves are transiting the Atlantic Ocean this week that are showing signs that the season may begin to see more activity. The National Hurricane Center (NHC) has begun monitoring a cluster of storms south of the Cape Verdes Islands: Invest-99. A satellite passed over this disturbance this morning and a closed circulation has almost formed. Wind gusts of 45mph were detected in a band of thunderstorms on the southwestern portion of the disturbance.
Invest-99 isn’t in the best environment for tropical storm development. Sea-surface temperatures are only moderately warm: 27-28°C, compared to the 28-31°C SSTs in the western Caribbean Sea. Additionally, there is a large amount of dry air to the disturbance’s north right now, which came off of Africa along with the disturbance. This air came from the Sahara and is very stable, very much unlike the moist, relatively unstable air from Equatorial Africa.
Invest-99 has a 30-50% chance of developing into a tropical depression in the next 48 hours. Most model solutions keep the disturbance near 15°N for the better part of the next three days, after which a slight curve toward the northwest is possible. That track would keep the storm well away from the Lesser Antilles in the 5-7 day range. I will point out that the lack of disturbances this year also means the skill of models should be taken with a large grain of salt. This season’s storm behavior has yet to be ascertained by even one storm near the tropics. Until then, climatology or chance likely provides almost as much skill in predicting future storm motion.
In the span of time since this morning, a second area of potential tropical formation has come to light: a band of storms near the Windward Islands. This westward moving disturbance is less defined than its cousin to the east. It is closer to warmer SSTs, but also closer to a pretty hostile vertical wind shear environment. As such, the NHC is giving it a <30% chance of development within the next 48 hours.