Tropical weather analysis - July 20, 2010

Invest 97L

Invest 97L has become better organized today, with infrared satellite animations depicting deep convection developing and persisting near the estimated surface center (don't confuse this with a surface low, as there isn't one just yet), which, based on animation of shortwave infrared satellite imagery is located near 19N 68W. Surface observations from Puerto Rico show no evidence of a closed surface circulation in associated with the system, with no reports of westerly winds along the southern flank of the circulation. In addition, analysis of observations from the National Data Buoy Center indicate that, other than some slight pressure falls noted across Puerto Rico this afternoon, surrounding pressures really aren't all that low.

This is another strong indication that 97L has not yet developed a surface low pressure area. Until this happens, and the center subsequently closes off, tropical cyclogenesis cannot occur.

97L is still experiencing strong vertical shear, in excess of 15 to 20 kt, due to a large upper-level low centered a couple hundred miles north of the disturbance. The current motion of 97L is very difficult to ascertain, due to deep convection obscuring the center, along with the overall lack of a well-defined surface center. My best estimate is very slowly to the WNW. The aforementioned upper low responsible for the shear, centered near 25N 70W, was revealed rather well from 350 to 150 mb. Analysis of the 18z GFS 300 mb vorticity suggests that this upper low will continue moving generally westward away from 97L, which should allow 97L to begin to consolidate more thoroughly by tomorrow.

Thereafter, the GFS develops a weak anticyclone aloft over the system, which would favor intensification. I am not expecting 97L to become a tropical depression until tomorrow afternoon at the earliest, with the most likely time being Thursday morning.

My forecast track has been shifted northward from yesterday. I expect that 97L will continue moving WNW over the next three to four days, making landfall across south Florida in around 60-72 hours. Late on day three, the system should emerge from Florida into the eastern Gulf of Mexico. Upper-level winds don't appear as favorable there as they will be in the western Atlantic, but are still more than adequate to support some steady intensification of the system until landfall occurs.

The ultimate landfall location of 97L along the Gulf Coast will depend upon the strength and orientation of the westward extension of the subtropical ridge. Some models keep this weaker, which would place the northeastern Gulf Coast at higher risk, while some keep it stronger, which would put the northern Gulf Coast in the bullseye.

An alternate scenario, as depicted by the 12z CMC, is for the system to run into Hispaniola in the next 12 hours, before bending back toward the WNW. Current 850 mb steering analysis from CIMSS somewhat supports this, but the ridge to the north has also been weakening in recent hours, so I am not expecting 97L to traverse Hispaniola.

Residents all along the Florida peninsula, as well as the Gulf Coast, particularly from Morgan City to Tallahassee, should carefully monitor the progress of this system over the next several days. Regardless of development, locally heavy rainfall and gusty winds in excess of tropical storm force will continue impacting portions of Puerto Rico, the Virgin Islands, and Hispaniola over the next couple of days. These rains could cause flash flooding and mudslides across mountainous regions of these countries.

Elsewhere

An area of disturbed weather, associated with a tropical wave, continues in the western Caribbean Sea. The system is very poorly-defined, with very little evidence of low- to mid-level vorticity. Upper-level winds are expected to become unfavorable for development as the system moves generally westward. While I am not expecting development from this area, it could produce locally heavy rainfall across portions of the Cayman Islands and the Yucatan Peninsula over the next day or so.

The ECMWF and GFS continue to suggest the possibility of our first Cape Verde storm developing on July 25. Given the abundant moisture currently across eastern Africa (as well as the entire African continent), this is a believable forecast. Upper-level winds will be favorable for anything that does happen to form here. These models have been indicating this possibility for over a day now, so it certainly bears watching.