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!