The evolution of a complex pattern this week will create an opportunity for a hybrid low to develop off the SE U.S. coast, and at the very least a continued wet pattern from Cuba, Florida, and the Bahamas up to the Carolinas. An upper low moving across the southern U.S. over the next few days will be splitting off from the westerlies and becoming stuck near the SE U.S. coastline by Friday and into this weekend. An old frontal boundary has already stalled out east of Florida and extending down towards the Yucatan Peninsula, sparking showers across the entire area. This trough is tapping into a stream of tropical moisture originating in the eastern Pacific, where Tropical Storm Aletta recently formed and another monsoonal low is developing south of Mexico. As the upper low stalls this weekend, a hybrid, subtropical low may form underneath of it as it gets trapped south of a blocking ridge over New England for a few days.
The models are currently divided fairly evenly on their support for this event, with the UKMET and CMC leading the way and showing a tight low on their most recent runs, but the ECMWF and GFS are more timid with their solutions. The key difference seems to lie in the positioning of the upper low. The UKMET and CMC take the upper low closer to the coastline or out over the water, allowing the low shear environment to envelope the surface low and facilitate warm-core development. The ECMWF and GFS, on the other hand, keep the upper low inland and do not allow warm-core processes to intensify the surface low, though the ECMWF joins the UKMET and CMC in showing a weakening system as it moves inland, suggesting at least a partially warm-core low that would weaken over land. It will be interesting to see if the GFS is too far inland with the upper low because of an overdone forecast for tropical development in the western Caribbean, which would increase the resistance from the upper ridge southeast of Florida, thereby blocking the upper low from moving out over the Gulf Stream.
Despite the overbearing GFS, a festering low may start developing in the western Caribbean this weekend as well, and into next week may at least make the region look interesting after the southeast hybrid low has left the scene. With the MJO pulse currently reaching its maximum amplitude over the eastern Pacific and Caribbean, a system of interest cannot be ruled out, especially with the aforementioned classic early-season development pattern of low heights and old frontal boundaries south of the United States. This kind of a setup consisting of one threat to the north of the subtropical jetstream, and then a second threat to the south of the jetstream later, of more truly tropical origin, is a common progression of things in the pre-season or early hurricane season. We don't always get development from either, but it sets up at least the opportunity.
Overall, at this time I don't expect a particularly significant system to develop off the SE U.S. coast, however, the pattern favors some kind of mischief to take place as all of this tropical moisture made available by the MJO interacts with old surface troughs lying around in the region and the cut-off upper low that will be stalling nearby. Regardless, I expect the pattern will turn even wetter for the Carolinas this weekend as at least a surface trough laden with tropical moisture becomes blocked by the New England high and makes a move for the coast as it gets pulled underneath the upper low. Florida and the Bahamas may also get in on some more rain. Later next week, Florida and the Bahamas may get yet another shot of tropical rains as something gets drawn out of the western Caribbean, possibly a weak development threat.