Good morning! Not much has changed since my post last week about the possibility of a subtropical storm developing in the western Atlantic north of the Bahamas. The system still has not developed, yet is showing some signs it is undergoing some changes this morning. Remember, a subtropical storm is one that is like a tropical strom near the surface but a regular, cold-season area of low pressure aloft. All weekend long, the system was highly tilted (strongly wind sheared), with the deep thunderstorm activity located well to the east of an exposed center of circulation near the surface with limited cloud cover around it (really, it was just a little tiny area of spin northeast of the Bahamas!).
This morning, on the first image above, I've highlighted where the old surface low was from the weekend and where I think a new one is developing. Wind shear is decreasing in the western Atlantic near this low (Image 2 -- notice the large area I've highlighted in blue with weaker wind shear.. the cooler colors), and this is likely allowing a new area of low pressure to develop closer to where the thunderstorms area. More importantly, this is also allowing the center of circulation to potentially move away from the very dry air in place over much of the western Atlantic right now off the east coast of Florida (remember how dry it was for us this weekend; this was the reason why!.. you can see this in the third image, the brown/dusty color over the left hand side of the image). Not only did the wind shear keep things from developing over the weekend, but the dry air played an important role too -- deep moisture is key for tropical or subtropical development.
As I mentioned last week, though, water temperatures are really slightly too cool for tropical development (ideal is 80 degrees or higher to a depth of at least approx. 200 feet). As the low becomes better organized and kicks toward the west with a ridge of high pressure aloft, it will be key to watch how close it gets to the Gulf stream.. the ribbon of very warm water than runs up the east coast. This is highlighted in red on the 5th image.. lower right hand panel. If the low pressure areas near the ground and aloft become vertically stacked, in a weak area of wind shear, and end up over the Gulf stream (it will get close), we could have some pretty quick development along the east coast.
Now for the big question; where would this low (whether extratropical, subtropical, or tropical) go?
Not toward southwest Florida! There is a large area of high pressure located over the Ohio Valley now; this will initially drive the low westward toward the Carolinas. As that ow builds toward New England, the steering level wind will become northeasterly, pushing whatever develops back toward to the southwest... so along the southeast coast. However, this steering wind is weak, which would serve to more or less keep the low in place off the GA/SC coast. For us, we'd continue to be on the much drier side of this system and, while our chance of rain would increase somewhat at midweek (especially inland), we would never see any steady or heavy rain -- in fact, it's possible no area in the US will see heavy rain from this! Ultimately, the low should be kicked to the east out into the Atlantic again as an upper level disturbance moves through the jet stream at mid week (circled in Image 5).. with only some unsettled weather lingering behind it through the first half of Memorial Day weekend.
Elsewhere, there is still the possibility for some weak development in the Caribbean later this week, but it likely would become wrapped into the circulation of the low I've mentioned above and get pulled well east of Florida. Another model, the Canadian, implies the possibility of some additional development in the far western Caribbean, but not until the weekend.
Brian
