Western Caribbean disturbance 97L likely to become a tropical depression

An area of low pressure with a surface circulation has developed in the Southwestern Caribbean, off the coast of Costa Rica. This disturbance, designated Invest 97L by the National Hurricane Center this morning, appears likely to develop into a tropical depression over the next two days. Satellite loops clearly show that 97L has a surface circulation, and low-level spiral bands have begun to develop. There is not very much heavy thunderstorm activity, though it is steadily increasing. An ASCAT pass from 10pm EST last night showed top winds of about 30 mph in the heaviest thunderstorms.


Figure 1. Morning satellite image of Invest 97L.

The disturbance is currently under moderate wind shear, 10 - 15 knots, and shear is expected to remain in the moderate range for the next five days over the Western Caribbean. Sea Surface Temperatures (SSTs) are 29°C and the Tropical Cyclone Heat Potential is about 40 kJ/cm^2, which is plenty of energy for a hurricane to form. There is dry air over the northern Caribbean, but this is too far north to slow down development over the next three days. The main limiting factor for development will be the slow movement of the 97L, which will allow it to stir up cold water from the depths, cooling surface SSTs.

The forecast for 97L
Steering currents are weak in the Southwest Caribbean, and 97L will move little over the next three days. If the disturbance intensifies into a tropical storm in the next three days, as seems likely, 97L will tap into moisture from the Pacific Ocean. This moisture will flow over Costa Rica, western Panama, and southern Nicaragua into 97L's circulation, bringing 4 - 8 inches of rain to these areas Wednesday through Friday. There is a high potential for life-threatening flash flooding and mudslides in these regions. By Friday, steering currents are expected to pull 97L north or northwest, along the coast of Nicaragua or inland over northeastern Nicaragua and Honduras. By Monday, 97L may pass over Western Cuba and enter the Gulf of Mexico, as predicted by the ECMWF model; an alternate solution, provided by the GFS model, keeps the storm farther south, pushing it into Belize and Mexico's Yucatan Peninsula. With the storm just beginning to organize and steering currents weak, it is impossible to rate the likelihood of these scenarios. NHC is currently giving 97L a medium (30 - 50% chance) of developing into a tropical depression by Thursday. I rate 97L's chances at high (greater than 50%). An Air Force Hurricane Hunter mission is scheduled to investigate 97L on Wednesday afternoon.

Typhoon Mirinae kills at least 40 in Vietnam
Typhoon Mirinae is no more, but has left in its wake serious flooding in the Philippines and Vietnam. Mirinae hit Vietnam yesterday as a Category 1 typhoon, dumping rains responsible for at least 40 deaths, with 11 people missing. Vietnam is still recovering from the effects of Typhoon Ketsana last month, which left 163 dead and $801 million in damage.

In the Philippines, residents are cleaning up the mess left by Mirinae, which hit Luzon Island on Saturday as a borderline Category 1 or 2 typhoon with 95 - 100 mph winds. Mirinae killed at least sixteen people in the Philippines. Rainfall amounts from the typhoon in the Philippines were less than six inches, but that was enough to create substantial flooding, due to the saturated soils left by two previous typhoons.

Jeff Masters