Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.
By: KoritheMan , 12:39 AM GMT on October 21, 2012
An area of low pressure over the central Caribbean Sea is primarily associated with a tropical wave. While the associated shower activity has diminished this evening, this is most likely attributable to the ordinary diurnal convective minimum period, and the system as a whole appears to be organizing.
Figure 1. Latest infrared satellite image of Invest 99L. Image credit: NOAA
Surface pressures are falling in the area, which is a sign of a developing system. However, there is still some westerly shear over the system, as evidenced by high cloud motions derived from satellite pictures. This is associated with the northern periphery of an anticyclone to the south of Invest 99L. If the GFS is correct, this feature will collocate with 99L over the next day or so as the system as a whole moves off toward the west. Longer term, the models are unanimous in suggesting that a deep-layer trough emerging from the Great Lakes in about a week will be sufficient to recurve the system. More specifically, it appears that the trough that is currently off the eastern seaboard will join the large upper low over the north Atlantic southeast of Greenland, and the enhanced southwesterly flow with these two features will keep a residual weakness in place before the arrival of the Great Lakes trough; the latter feature then prevents a ridge from rebuilding in to the north. An alternate scenario, supported by some of the GFS ensembles, is that the system could come a little closer to south Florida than currently predicted. I consider this to be an outlier scenario for now.
The GFS 850-200 mb upper wind fields suggest that upper-level winds could once again become unfavorable in about four days, which should prevent 99L from rapidly intensifying as so many systems in the past have done in this area. While there is still some chance of a hurricane, I do not consider the idea of anything stronger than a Category 1 of much merit due to close proximity of the system to strong upper-level shear. The, GFS suggests that there will be a lot of westerly shear for 99L to contend with after it enters the western Atlantic. In theory, this sort of speed maximum could cause more divergence than shear, depending on its relative proximity to 99L; this could suggest some reintensification over that portion of the basin, but it is simply impossible to predict the specifics of such an event right now.
Environmental conditions appear favorable for a tropical depression to form over the next day or two.
Regardless of development, heavy rainfall capable of causing flash flooding and mudslides are likely over portions of Hispaniola, Jamaica, the Cayman Islands, and Cuba over the next several days as the low moves off toward the west, then north.
Probability of development in 48 hours: 60%
A tropical wave interacting with an upper low over the central Atlantic about 1000 miles east-northeast of the northern Leeward Islands has become a little better organized over the last several hours.
Figure 3. Latest infrared satellite image of Invest 90L. Image credit: NOAA
Convection remains minimal at this time, but upper-level winds are forecast to become more favorable for development over the next couple of days, which should allow for some development. The system is currently in close proximity to an upper low, which is orbiting just to the south of the circulation center associated with the tropical wave. How much interaction occurs between the two features will determine whether or not any development that occurs from this wave is tropical or subtropical. Right now, given forecasts from the global models as well as current trends, I feel that the upper low will steadily distance itself from 90L as the wave axis moves west-northwestward. While this is is the expected evolution, things can change, and my best guess is that the upper low will act to keep some degree of vertical shear over the disturbance regardless.
Recurvature should begin at around roughly 72 hours as the deep-layer trough off the United States east coast moves eastward.
Probability of development in 48 hours: 30%
Gulf of Tehuantepec disturbance
An area of disturbed weather has formed in the far eastern Pacific several hundred miles south of the Gulf of Tehuantepec. Upper-level winds appear favorable for gradual development of this area as it moves generally westward. Long range forecasts from the global models suggest another possible threat to Baja at longer ranges, but it is too early to be confident of this.
Probability of development in 48 hours: 20%
The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.
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