Tropical weather analysis - June 19, 2010

Began writing at approximately 7:31 eastern

Invest 92L continues to stubbornly generate deep convection, in spite of hostile upper-level winds, analyzed at 20 kt per University of Wisconsin's CIMSS. Based upon recent buoy observations, surface observations from Puerto Rico, doppler radar animation from San Juan, along with shortwave infrared satellite loops, I have fixed a center at 17N 67W.

92L has finally been able to generate some convection near the center this evening, probably because the shear is only 20 kt, compared to the much more oppressive 30 to 50 kt shear it has been dealing with over the last several days. However, animation of water vapor imagery indicates that dry air lies just to the west of the system, and will continue to be ingested into the system thanks to the aforementioned upper-level westerlies. Interestingly, in a sharp contrast compared to recent days, 92L's vorticity only extends upward to 700 mb.

This further suggests that, in spite of convection finally being closer to the center this evening, 92L will not organize into a tropical depression in the near-term. The main impact from this system in the near-term will be continued locally heavy rains across portions of the northern Leeward Islands, the Virgin Islands, Puerto Rico (particularly the southern half), and across Hispaniola. Already, doppler radar estimates from San Juan indicate a large swath of 2 to 4 inches of rain from the southwestern end of the island eastward, with some localized amounts in excess of 5 to 6 inches.

I would expect an additional 1 to 2 inches across these areas, with locally higher amounts possible across mountainous areas.

92L is currently moving W based on satellite animations. I expect this general motion to continue for about the next 12 hours or so, after which point a steady WNW movement should continue, even as the system moves into the southern or southeastern Gulf of Mexico by day four. At this point, the models sharply diverge, with the CMC being farthest south, taking the system westward into the Yucatan Peninsula by day four.

It's very difficult to pinpoint this far out just which portion of the Gulf Coast ultimately gets affected by this system. I know I normally forecast a definite path for an invest throughout the forecast period, but the uncertainty is much too great at this point to do that. If I was forced to do so though, I would say the western Gulf Coast is most at risk from this system. However, this is highly uncertain.

The models do agree on one thing, however, and that's the fact that 92L will make landfall across southwestern Hispaniola in about 24 to 30 hours. Afterward, I expect a continued WNW motion toward the western Caribbean to south of central Cuba. At this point in the period is around to 48 to 54 hours. Upper-level shear should still be unfavorable at this point, but will be gradually lessening. More importantly, 92L will be traversing across some of the hottest water in the basin. These waters extend to great depth, and are easily enough to generate a major hurricane.

Not that I expect 92L to achieve that feat, of course. Far from it, in fact. These warm waters, combined with the lowering (but still prohibitive) vertical shear, should allow the system to generate vigorous convection, and a flash flood event is possible across portions of Cuba during this time, particularly across mountainous areas.

When 92L enters the Gulf in four days, shear should be favorable enough to allow steady intensification of 92L, assuming that its structure is not too disrupted by passage across Hispaniola and Cuba. I will continue to give 92L a 20% chance of eventually developing into a tropical cyclone in the Gulf of Mexico, though this will need to be increased if its structure remains intact after passage across the islands, or if it misses them altogether.

Elsewhere

Elsewhere in the tropics, the NOGAPS is developing a tropical depression in the central Caribbean Sea by late next week. Upper-level winds will be near zero in this region, so we need to monitor this very carefully, should this transpire. Given the timeframe, it is possible that this system originates from the tropical wave I've been mentioning that was forecast to move across the Windward Islands in less than a week. None of the other models are developing this area, but the GFS indicates a large area of concentrated moisture across that region, so we should carefully watch this, as the potential for mischief is certainly there.

There is also a rather nice looking low pressure system currently just to the north of Panama. Steering currents are weak in this area at the moment, but the models foresee a slow westward drift into Central America. However, there is a well-defined anticyclone centered over this system in the upper-levels, so should this system deviate from the current westward projection, it will need to be carefully monitored for the potential formation of a tropical depression. Locally heavy rainfall, particularly across mountainous areas, will be possible across portions of Panama, Costa Rica, Nicaragua, and Honduras over the next couple of days.