Special tropical weather analysis - April 21, 2011
A surface low pressure system located several hundred miles northeast of Puerto Rico (dubbed "Invest 91L" by the National Hurricane Center), has been showing signs of organization over the last couple days, but strong vertical wind shear has prohibited much in the way of any organized convection, and infrared satellite animations do not show much in the way of convective banding at this time, though the distant convection has been rather persistent over the last 24 hours, particularly given the hostile environmental conditions 91L has been dealing with.
Latest storm statistics
Wind: 40 mph, with higher gusts
Movement: NNW at 10 mph
Pressure: 1009 mb
Location: 25.4°N 63.1°W*
*My own estimate. Not official.
Figure 1. Latest infrared satellite image of Invest 91L, courtesy of NOAA's Satellite Service Division (SSD).
While a five day animation of the wind shear tendency from University of Wisconsin's CIMSS laboratory along with corroboration from high cloud motions observed on water vapor imagery supply ample evidence that upper-level shear has lessened over the system over the last 24 hours, perhaps considerably, 91L is still battling 30 kt of southwesterly vertical shear due to a persistent upper low centered near 26.4°N 67.2°W. It is worth noting, however, that the upper low is closer to 91L now than it was this time yesterday. Assuming the two can become vertically stacked (a scenario painted as possible some of the global models, particularly the GFS), shear will be practically nonexistent, as is always the case at the axis of a weather system. For now, however, the upper low continues to shear the system, which, when combined with intense subsidence in the middle troposphere, will essentially prevent any appreciable organization of the system.
If this system is to develop, it will have to be between now and tomorrow evening, as upper-level winds will quickly become hostile thereafter. The 18z GFS creates a small window of opportunity with which 91L can possibly develop, positing a narrow zone of relatively light vertical shear during the 18-24 hour time frame, especially for a subtropical hybrid system such as this. Perhaps not surprisingly, this seems to be why the GFS deepens it during this time.
Figure 2. 850-200 mb vertical wind shear forecast from the 18z run of the GFS.
While 91L is currently moving poleward on a path that would ultimately lead it safely out to sea due to a break in the Atlantic subtropical ridge caused by a shallow cold front over the western Atlantic waters, global models unanimously fill this feature. Consequentially, a dramatic slowdown and retrograding of the system is expected beyond 24 hours as the ridge rebuilds westward. By day five, the system is unanimously forecast to approach south Florida. This will be concurrent with the amplification of a very deep longwave trough that is forecast to approach the southeast United States at that time. Given the strength of the trough (a respectable circulation will extend all the way down to at least 850 mb), 91L appears poised to recurve shortly after reaching the peninsula on a path similar to 1984's Tropical Storm Isidore.
Figure 3. "Best track" of 1984's Tropical Storm Isidore. I picked this storm because I feel the future path of 91L will be roughly analogous to it.
Even if this low does not develop, it poses an ostensible danger to marine interests over the next day or two; the pressure gradient to the northeast of the low-level center suggests it is prudent to assume that gale force winds are still occurring to the north of the surface low, again within the convective showers.
All that said, I give 91L a 20% chance of development, slightly higher than the NHC, but still unlikely.