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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.


Wet Start to June Likely for W. Caribbean Region – Potential for Tropical Development

   Posted by Levi at 7:12pm on May 25, 2013

The weather pattern is likely to turn very wet by the end of the month and into the first week of June for the western Caribbean and greater Antilles. The GFS and ECMWF are finally in agreement on the timing of large-scale upward motion associated with the MJO moving over the Caribbean during the next 5-15 days. We have already been discussing this event during the past few weeks, as the twin tropical cyclones in the Indian Ocean during the first week of May yielded a clue that the MJO would likely end up in our area of the world by the end of the month. The GFS was originally too fast with this event, bringing upward motion over the Caribbean by May 23rd. The correction for the fast bias of the GFS seems to be on its way to verifying, as the GFS and ECMWF ensembles now lower pressures and increase precipitation in the western Caribbean during the target period of the final days of May and the first week of June, as anticipated since early May. Now that these two models agree on the general evolution of the pattern, confidence is high in an intrusion of the central American monsoon circulation into the western Caribbean in several days.

The overall pattern over North America is also likely to facilitate the development of low pressure in the western Caribbean during the first week of June. The image below depicts the average MSLP anomaly forecast for the first week of June from the GFS ensemble mean. As large-scale upward motion invades the Caribbean, anomalously high pressure is forecasted to simultaneously build over the eastern United States, a response to a positive arctic oscillation and negative PDO-type pattern. This setup with low pressure in the Caribbean and high pressure to the north means that air is forced to converge (pile up) and rise in the Caribbean, further strengthening the low pressure area. This will likely result in a large area of heavy precipitation developing from central America through the greater Antilles and even into the Bahamas and southern Florida. Such a pattern is also conducive for the formation of a tropical storm, though such an event cannot be forecasted with confidence this far in advance. More details will become available once the monsoon circulation materializes and we are able to physically track it.

Overall, the wet pattern for the western Caribbean region that has been advertised over the last few weeks for the end of May and the beginning of June is coming into focus on the models, and is now within the 10-day forecast period. The GFS operational is now showing a 1004 mb low north of Honduras in 8 days. While details of the possible tropical storm formation cannot yet be known, the potential that we have been talking about still exists, and is now moving up the timeline on the model forecasts. Whether or not tropical formation occurs, the medium-long range forecast is confident in a very wet pattern invading the western Caribbean and greater Antilles islands by May 29th, likely lasting through the first week of June. Once this event begins and the monsoon circulation develops, we will be able to properly assess the potential for tropical development.

We shall see what happens!


Eastern Pacific Hurricane Season off to Fast Start – Will the Atlantic be Next?

   Posted by Levi at 7:50pm on May 16, 2013

Just a video again today. Once we have real storms to track in the Atlantic I will resume writing text discussions alongside the videos.


Hints at a Potential Early Start to the EPAC or Atlantic Hurricane Season This Year

   Posted by Levi at 11:07pm on May 7, 2013