A westerly wind burst is expected to occur in the Eastern Pacific this weekend. Westerly wind bursts are often associated with the development of El Niño, but they can also enhance cyclonic turning at the low latitudes. Most models agree that an area of low pressure is likely to form in the Southwestern Caribbean Sea in 48-72 hours, with the low likely moving inland into Central America by hour 96. Tropical cyclone development at this latitude is fairly rare because usually there is not enough of the Coriolis Effect to allow cyclonic turning, in addition to the area’s proximity to land. However, the westerly wind burst could promote cyclonic turning within the area of low pressure expected to form, and there is at least a low chance of a weak tropical cyclone forming in the Southwestern Caribbean this weekend. Wind shear is expected to be low to moderate, and the system’s low latitude means that convective activity is nearly guaranteed with this system. Sea surface temperatures in the region are a favorable 29°C (84.2°F) – warm enough to support a significant tropical cyclone. The GFS model also predicts favorable upward motion in the area this weekend, which should help enhance convective activity.

The 06z Operational GFS developed a somewhat compact well-defined low near the Costa Rica-Nicaragua border, with the system coming close to tropical depression status before landfall on Sunday. The 06z GEFS ensembles were slightly more optimistic with the development idea, with several ensembles showing a well-defined low developing and coming close to, if not reaching, tropical depression status. In contrast, the 06z GFS-Parallel model developed a minimal Tropical Storm Bret in the Southwest Caribbean, while the 00z ECMWF does not even develop an area of low pressure in the Caribbean. The 00z CMC run developed a weak area of low pressure, but did not show development of a tropical cyclone. Alternatively, the low pressure area could cross over into the East Pacific and develop in that basin in several days. It also could move inland into central America and track northwestward, either dissipating inland or reaching the Northwest Caribbean or Bay of Campeche.

It should be noted that Southwest Caribbean tropical development is rare for the month of June. According to a graphic posted by Dr. Phil Klotzbach of Colorado State University, only two tropical cyclones of tropical storm strength or stronger have ever formed in this region in June.  Southwest Caribbean development is more common late in the season, such as last November when Hurricane Otto formed and made landfall in southern Nicaragua on Thanksgiving Day as a category 3 hurricane.

Since Southwest Caribbean tropical development is not favored historically for this time of year, I only give a 10 percent chance of a tropical depression or tropical storm forming in the area during the next ten days. I do give a 30 percent chance of a tropical cyclone forming in the East Pacific basin during the next ten days, however. The most likely outcome, in my opinion, is that an area of low pressure forms in the Southwest Caribbean but is unable to develop into a tropical cyclone there because of its proximity to land. The National Hurricane Center has not mentioned the system in its tropical weather outlook as of this morning. I will be back for another blog post tomorrow.