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We have an unusually active day in the Atlantic for late October, with three areas of interest that are being monitored for development. A couple of cold-core subtropical lows are being painted orange and red by the NHC for potential development, and we'll see if they try to eek out a name from one of them.
The main feature is Invest 91L, a system of low pressure northeast of French Guiana that I have been tracking for several days. This is what I mentioned earlier this week would try to become Shary as it came westward and would make a run at the Caribbean. Satellite imagery this morning shows that it is definitely attempting to develop, as vigorous mid-level rotation has opened up in the last few hours. It is unclear how much of this rotation is manifesting itself at the surface, but there is a clear turning of the surface winds apparent in visible loops. This has the look of a developing tropical cyclone, and in my opinion should have more than the yellow paint that the NHC has on it right now.
As has been warned about for a long while now, this system will move into the eastern and central Caribbean trying to punch westward, but will likely be stopped in its tracks by a monster trough digging into the eastern United States in 6-8 days. This will try to tug whatever's in the Caribbean northeastward, and if we have Shary sitting down there at this time then the northern Caribbean islands will have to be watching closely. It is possible that if 91L remains weak that it will not get caught by the trough and will continue westward after stalling for a while, but conditions are favorable for 91L to develop, and I find it more likely that the system will slowly try to find its way out of the Caribbean to the north or northeast, perhaps taking several days to do so, but eventually getting out.
Overall, we have a developing tropical system north of South America that will be threatening Trinidad, Tobago, and the southern Antilles Islands in 36-48 hours, and moving on from there to potentially threaten the northern Caribbean islands next week.
We shall see what happens!
Invest 91L Visible/IR2 Satellite (click image for loop):
Invest 91L Track Models:
Caribbean/East Pacific Visible/IR2 Satellite (click image for loop):
Central Atlantic Visible/IR2 Satellite (click image for loop):
Atlantic Tropical Surface Analysis:
200mb Vertical Velocity (green areas represent upward motion associated with the MJO):