Tomas
Tropical Storm Tomas is slowly barreling toward western Haiti whilst gradually intensifying. As of the latest NHC intermediate (8:00 PM EDT) advisory, here is the latest information on Tomas:
Wind: 60 mph, with higher gusts
Movement: NNE at 8 mph
Pressure: 993 mb
Location: 16.9°N 75.9°W
Category: Tropical storm
Analysis of shortwave and infrared satellite loops indicate that Tomas is gradually becoming better organized as it approaches western Haiti. Deep convection has increased substantially in both areal coverage and intensity, and now completely covers the circulation center. Additionally, a large, loosely-defined convective band containing heavy precipitation stretches southward to northward for several hundred miles from Panama to Haiti, and then into Tomas' center.
Upper-level outflow is good to the northeast, fair to the east, and poor elsewhere, especially to the south and southwest. The poor outflow in these areas is being caused by 20 kt of southwesterly vertical wind shear associated with an unseasonably strong deep-layer trough and associated cold front currently moving across the Gulf of Mexico and western Atlantic. The latest run of the SHIPS model, however, depicts considerably shear of Tomas, only 13 kt. This is a rather sharp contrast to the aforementioned CIMSS observation. Given high-cloud motions observed on satellite imagery, I am inclined to go with the SHIPS model's assessment of the vertical shear.
The SHIPS model, along with the GFS, the latter of which has its observations based upon output from the SHIPS, forecast an upper-wind environment that is at least marginally conducive to continued gradual intensification of the tropical cyclone before it reaches the coast of eastern Cuba or western Haiti in about 18-24 hours.
Vertical shear is forecast to increase substantially by 30-36 hours, however, as Tomas loses its upper-level support and the upper flow becomes zonal. At that point, there are two scenarios, as depicted in the global model fields:
1. Tomas continues generally northward to north-northeastward under the influence of the aforementioned deep-layer trough, and quickly becomes extratropical as it enters an environment of strong baroclinicity.
2. Tomas instead moves generally eastward, remaining more distant to the trough. While this would prevent the cyclone from becoming extratropical, vertical shear would still be prohibitive enough to allow for the storm to weaken and ultimately dissipate.
The NHC is splitting the difference between the two scenarios, as a sort of compromise. Either way, Tomas will ultimately meet its demise over the western or central Atlantic Ocean over the next several days. Indeed, given the strong vertical shear profiles forecast by both the SHIPS and the GFS, Tomas may not survive throughout the entire forecast period (five days). Rapid weakening is certainly a distinct possibility once Tomas loses its upper-level support and moves into the aforementioned belt of westerly to southwesterly vertical shear.
I anticipate that Tomas will become a minimal hurricane with 75 mph winds over the next 24 hours, provided the center remains offshore. Any deviation to the right of the current NHC forecast track will bring the center of southwestern Haiti.
It is unlikely that Tomas will become anything stronger than an 80 mph hurricane due to an only marginally favorable upper-level wind environment andinteraction with the mountainous terrain of eastern Cuba and western Haiti.
The intensity of Tomas is wholly irrelevant however, as either way, the storm is likely to produce very heavy rains across eastern Cuba and western Haiti through at least Saturday. Like Jeff Masters, I suspect that the heaviest rains associated with Tomas will be found along the southwest and northwest Haitian peninsulas, since the cyclone appears to have moved farther west than what the NHC was predicting this time yesterday. I forecast that anywhere from 5 - 8 inches will fall over these peninsulas, with locally higher amounts. Lesser, but still heavy amounts, in excess of 3 - 6 inches, with locally higher amounts, are possible farther east across the country, both along coastal and inland areas. This includes the capital city of Port-au-Prince, where residents are still living in tents subsequent to the January 12 earthquake, and are also battling a cholera epidemic. Finally, Jamaica can anticipate 1 - 3 inches of rain, with locally higher amounts, particularly along the eastern end of the island, which has the greatest exposure to Tomas' heavy precipitation.
Rainfall in excess of 2 - 4 inches with locally higher amounts is also possible across the Dominican Republic, especially the western portion of the country.
As far as track is concerned, based upon various steering analysis as well as computer model forecasts, I pretty much concur with the NHC's current forecast track, taking the cyclone center across extreme eastern Cuba in about 18 hours, followed by a generally NNE motion with an eventual turn to the NE, then E. However, it is possible that Tomas could come a little more westward and impact more of Cuba and less of Haiti, based upon current CIMSS steering analyses.
However, my own forecast track is a bit west of the NHC's at days three through five, based on various steering data I've analyzed, as it appears that ridging could be a bit stronger than anticipated during that time.
It no longer appears likely that Tomas will stall in the vicinity of Hispaniola. The trough appears to be strong enough to deflect it out to sea, which is great news.
Elsewhere
Elsewhere in the tropics, several of the global models are calling for another tropical depression to form in the eastern or central Caribbean in about 5 - 7 days. The eventual track of this system is uncertain, but right now, there are two possibilities that I see:
1. This system runs into Central America.
2. This system becomes another system that threatens Hispaniola and the Greater Antilles.
It is impossible to choose which scenario is more likely at this juncture. The catalyst for this system could be a perturbation in an unusually active ITCZ, currently near 13N 60W.
We will have to watch this situation closely, as upper-level winds are forecast to begin improving across the central Caribbean during this time.
Thursday, November 4, 2010
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