Tropical weather analysis - July 19, 2010

Invest 97L

A strong tropical wave located just to the north of Puerto Rico was designated Invest 97L by the National Hurricane Center early this morning. This wave is generating a significant amount of deep convection this evening. However, 97L is also experiencing 20 kt of westerly vertical shear, associated with a large and persistent upper-level low, the axis of which is centered near 28N 68N, based on animation of water vapor imagery. As far as the center estimate for 97L goes, it is pretty difficult to locate, but my best estimate is 19N 65W. Surface pressures are not significantly falling, and there are no signs of a surface circulation.

Overall, despite the fact that convection has been persisting near the estimated center position this evening, the aforementioned factors should limit any development over the next day or so, and I expect that this current convective burst will soon be sheared off to the east due to strong upper-level winds. In the meantime, locally heavy rainfall and strong gusty winds will continue overspreading portions of the Leeward Islands, Puerto Rico, the Virgin Islands, and Hispaniola over the next day or so.

97L appears to be moving toward the W at the moment. However, this should be short-lived, as a weakness in the low-levels is becoming apparent in the subtropical ridge to the north. This weakness is even more pronounced at 500 mb. This should begin to impart a WNW component of motion within the next 6-12 hours. I find it unlikely that 97L will run into the Greater Antilles, at least in the near-term. However, by around 60 hours or so, it could be approaching eastern Cuba. This is the solution unanimously provided by the computer models, and is what I'll go with for the time being. This will really depend on how quickly the aforementioned upper low fades, and the subsequent rebuilding of the ridge.

A weaker ridge may allow it to avoid Cuba entirely, while a stronger one would take it farther south, across eastern Cuba. I'll continue monitoring the evolution of the upper low/ridge over the next couple of days. It should be noted that as the system approaches Cuba or the Bahamas on day two or three, upper-level shear will still be strong, but falling.

In any case, after moving across Cuba, the system should emerge into the eastern Gulf of Mexico sometime on day four. After that, the models foresee a large weakness developing in the subtropical ridge, which would tend to impart a poleward component of motion after the system enters the Gulf. However, I would like to see some more consistency with regards to this before I jump on the poleward bandwagon, since not only has the anomalously strong ridging across the Gulf of Mexico been difficult to break or bend this year (ala, Alex and Tropical Depression Two), the GFS was foreseeing a stronger ridge at 12z than it was at 18z.

Residents all along the Gulf Coast should closely monitor the progress of this system over the next several days. I do think that this system has a decent chance of ultimately becoming a tropical cyclone, though this might not happen until after it enters the Gulf. I want residents to be aware of this situation, as the GFS continues to build an anticyclone across the Gulf at four days and beyond, which would greatly favor intensification or development.

A reconnaissance aircraft is scheduled to investigate this system tomorrow, if necessary.

Caribbean disturbance

Another tropical wave, this one located across the western Caribbean Sea, is generating a large area of disorganized showers and thunderstorms across the western Caribbean Sea. Upper-level winds are forecast to remain favorable for development as this system moves generally westward. In fact, CIMSS analysis indicates that an upper-level anticyclone may be forming, and the 18z GFS builds this anticyclone over the next several days. Though the NHC has officially ceased mentioning the disturbance in their Tropical Weather Outlook, I will continue to monitor it, as conditions appear rather favorable for development over the next several days. The biggest inhibiting factor I can see is land interaction; this system will run into Central America, Belize, or the Yucatan Peninsula in about three days. Hence, if it's going to develop, it has to do so quickly.

Regardless of development, locally heavy rainfall will continue affecting portions of Jamaica, the Cayman Islands, and western Cuba over the next couple of days.