I thought about blogging yesterday about the possibility of a subtropical storm forming next week off of the east coast, but wanted to give it another few model runs first. Those new model runs have come in and continue to suggest that a large area of low pressure will develop in the upper levels of the atmosphere between the Carolinas, Bermuda and the Bahamas over the weekend and get trapped by a ridge of high pressure building to its west and north. Some of our models are indicating chances of rain here through the weekend and early next week; if you've been watching us this morning, you've probably noticed I'm going against those models right now because, generally speaking, low pressure to our east aloft (and at the surface) favors drier weather in southwest Florida.
Now, onto the possibilities of a sub-tropical storm -- one that has tropical characteristics in the low levels of the atmosphere but is much more like a regular, cold-season area of low pressure aloft. At this point, the sea temperatures are marginally supportive of a tropical system over parts of the Atlantic off the southeast coast (80 degrees or higher; not many reports of this).. this is one the feature that leads me to believe we may get close to a subtropical storm, but not quite there. That said, wind shear is expected to relax along the southeast coast and this would favor the development of such a storm.
When looking at whether a storm or tropical, sub-tropical, or extra-tropical, we look at changes in temperature in the atmosphere. Frontal (or extra-tropical) storms feature significant wind shear and big changes in temperature both vertically and horizontally. In the upper levels, next week's system should resemble a frontal system. In the low levels, though, temperatures will be more uniform around the expected center of low pressure (according to current modeling), indicative of a more sub-tropical or tropical system. I've attached one of our forecast models, the European -- in the top right hand panel, circle in blue, is the potential area of low pressure. It will be one to watch this weekend and early next week but, at this point, I'm thinking that water temperatures are probably a little too cool still in this area to allow for a truly sub-tropical system to develop.
Farther south, several models have hinted for days at the possibility of some weak development in the far southern Caribbean next week. Unlikely, but we'll watch it. And, in any event, any development well to our south would have to cross a belt of intense wind shear located through the southern Gulf and northern Caribbean, where it would be shredded apart. But, the general idea is things are starting to look a little more summer-like as we approach the start of the 2010 hurricane season.
Brian
