Tropical Depression 4 struggling in the Atlantic

Tropical Depression 4 formed last night after a major burst of convection, seen on the satellite image below.  This convection, while strong at the time, has weakened and the low level circulation has gradually weakened throughout this morning and early afternoon.   Some additional increase in strength is expected but Firsthand Weather does not anticipate that TD Four will become a Tropical Storm at this time.

Regional Conditions in the Tropics

Currently, dry air from an earlier Saharan Air Layer continues to be in place over the Atlantic as seen on the Water Vapor image above.  The entrainment of this dry air continues to be the biggest prohibiting factor preventing Tropical Depression 4 from increased development.  As Chris had forecast in his previous article, other conditions  are currently conducive for tropical development at this time, but wind shear is expected to increase as Tropical Depression 4 moves north of the Lesser Antillies.  This will cause the depression to weaken into a Post Tropical Cyclone and dissipate before reaching the coast of the United States.

Tropical Depression 4 Expected Track and Model Analysis

At this time, Tropical Depression 4 is moving West-Northwest around 21 mile per hour.  This track is expected to continue for then next couple of days as Tropical Depression 4 moves just north of the Lesser Antillies.  4 should then continue on that general heading until dissipation to the Northeast of the Bahamas.

Tropical depression 4


The latest GFS model, show below, does show some marginal strengthening in the short  term, and the forecast calls  for maximum sustained winds  to reach 35 miles per hour, but beyond the next day or 2, conditions become unfavorable and the Tropical depression quickly comes apart as shown in the second image.

As seen in this second image, the weakening system begins to dissipate while North of Puerto Rico and then completely dissipates in later images.




Possible Tropical Development in the Atlantic

Tropical systems are not what we typical look to talk about in forecasts for this time of year, but a low pressure system north of Puerto Rico and the Dominican Republic could become our first tropical system of the year over the coming days. This system would be the first March Tropical system since 1908 and would be named Arlene should this develop.  There is currently no risk to the Southeast United States.

Current Tropical Conditions

Currently, this system is not overly strong, but is generating a large area of Gale force winds over the open ocean waters and has an area of tropical thunderstorms associated with it.  Those areas of strong winds and heavy storms are not currently near the center of the system as we normally see with Tropical Cyclones. Sub-Tropical Systems, or systems that have mainly Tropical characteristics but maintain some non tropical characteristics, can have the worst conditions away from the center.  Sub-Tropical status is what we will be watching for over the next week as this system moves slowly toward Bermuda.

Sea Surface Temperatures

The map above is the current Sea Surface Temperatures for the Atlantic.  Water temperatures are averaging 80 degrees near the islands and remain in the lower 70s southeast of Bermuda.  While these temperatures only slightly above average for this time of year for the Caribbean and Atlantic waters, water temperatures near the coast of the Southeastern United States are running 5 or more degrees above average.

Model Analysis

For the ongoing forecast, model data suggests this system will slowly move north toward Bermuda over the next couple days.  This should leave the system in fairly good conditions to slowly develop, and data suggests that is what will occur when the wind shear in the area drops.  the shear will pick up again towards the middle of  next week so the timeframe for development could be short.


This is the expected location for the system by Monday morning.   Those in the Eastern Caribbean should anticipated occasionally gusty winds and scattered downpours.  Rough seas can be expected north of the big islands up towards Bermuda West to the Eastern Bahamas.

By Tuesday, models bring the system to the southeast of Bermuda as it continues to develop.  Bermuda should see rain Monday night into Tuesday and should expect heavy surf and high ways as the system passes.

By Wednesday and Thursday, the system should continue to be strengthening as it moves well out into the Atlantic.  There will be no further development in the tropics after this at this time.  Firsthand Weather will continue to monitor for any further developments in the region.


Robert Millette