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May 2013

Thursday Evening Atlantic Tropical Outlook

   Posted by Levi at 1:25am on May 31, 2013

The pattern remains active and wet across the western Caribbean Sea, Bay of Campeche, and much of central America. The region continues to be under the influence of large-scale upward motion associated with the convective peak of the MJO wave, which is forecasted to continue as the GFS and ECMWF evolve the MJO index into phases 1 and 2 during the next 10 days. It is under this regime that Hurricane Barbara formed in the EPAC and quickly moved ashore over Mexico. Her remnant circulation now resides over the Bay of Campeche, and is no imminent threat for tropical development, though her remnants may serve as a focus point for the development of the monsoonal gyre over central America in a few days.

Over the last few days, it has become clear that the GFS and GEFS overestimated the strength of the large-scale ascent in this region of the world. Although the MJO is at a favorable position for upward motion, the subsident portion of a Kelvin wave is moving into the eastern tropical Pacific, which may largely counter-balance the MJO. However, both the GFS and CMC ensemble means indicate that the greatest region of sinking associated with this Kelvin Wave will set up very near the equator just west of Columbia, eventually moving into northern South America next week. This setup is supported by the recent cooling of the Nino 3 ENSO region, and would result in an enhanced poleward low-level flow, supporting the monsoonal southwesterlies into central America. This, in combination with the enhanced phase of the MJO acting at higher latitudes, along with a mid-latitude trough passing through the Tennessee valley in 3 days, will lead to a northward displacement of the monsoon trough.

A convergent belt of trade wind flow has set up in the southwestern Caribbean in response to the already enhanced monsoonal pattern, and current model guidance shows this continuing well into next week, with weak perturbations running southeast to northwest out of the ITCZ through this zone. These waves, along with the large-scale forcing at work, will act to facilitate the development of a monsoonal gyre sometime early next week, likely in the vicinity of the Yucatan Peninsula based on the latest model guidance. The operational GFS and CMC have periodically portrayed a weak tropical cyclone developing within this gyre and lifting northward into the Gulf of Mexico, though the CMC is characteristically way too fast and too strong. The ECMWF has recently tended towards this solution, with the most recent 12z run today depicting a tropical storm moving into New Orleans at Day 10. With the expected exception of the CMC, the global models have come into reasonable agreement during the last day or two on the timing and placement of the disturbance which could spawn such a storm mid-late next week, but with the time frame still being 6-8 days, and the time of year being early, there cannot be any degree of confidence attributed to tropical development at this time. However, in light of the rather favorable setup for an active monsoon, and the very low pressures being forecasted over the Gulf of Mexico by the GFS and ECMWF ensemble means after Day 6, the possibility for an early Atlantic tropical cyclone will have to be watched carefully if forecast trends continue during the coming days.

Regardless of whether tropical development becomes a significant concern, the pattern will remain a wet one for large portions of central America, the western Caribbean, the western Greater Antilles, and the Bahamas as large-scale upward motion and a convergent trade wind flow enhance convection in the region through next week. Should there be an attempt at tropical development, associated heavy rainfall will likely spread northward into the eastern Gulf of Mexico and southeastern U.S., as the mid-latitude flow is expected to be far enough south to eventually catch any disturbance that moves north of 20°N or so. However, this would occur in 6+ days from now, and cannot be predicted with specificity until/if a tropical disturbance forms.