Tropical weather analysis - July 22, 2010

Tropical Depression Three

A vigorous tropical wave located across the Turks and Caicos Islands, previously labeled Invest 97L by the National Hurricane Center, has organized into Tropical Depression Three.

Visible satellite animations confirm what the National Hurricane Center has said, a closed surface circulation is clearly evident. The partially exposed center of the newly developed depression isn't particularly difficult to locate, and the NHC puts it at 21.9N 75.0W. In addition, the maximum sustained winds are estimated to be near 35 mph. A reconnaissance aircraft will investigate the cyclone later today and give us a better assessment of the intensity of the system. The NHC has the motion estimated at WNW/15 mph, and satellite loops agree quite well with this.

The cyclone is still in its formative stages, and as a result, is not well organized. However, it is steadily improving as the upper flow slowly abates with the westward retreat of the persistent upper-level low that has been vehemently shearing the system (and still is, to a degree) over the last several days. Indeed, analysis of vorticity data from University of Wisconsin CIMSS suggests that the low- and mid-level centers have become more symmetrically aligned in the vertical, further confirming a gradual reduction in the vertical wind shear. Visible satellite imagery indicates that the center is located near the western edge of the deep convective mass, and the imagery also reveals less of a shearing pattern than it did just two hours ago.

Additionally, high arc clouds are streaming toward the center, rather than away from it as was previously the case. This indicates that the overall circulation is slowly becoming better defined.

Water vapor animations, used in conjunction with visible satellite loops, strongly suggest that the aforementioned upper low is moving faster than TD3. In fact, it may soon move far enough away to ventilate it, thereby generating a poleward upper-level outflow channel. Should this occur, it will assist in quicker intensification prior to the system reaching south Florida or the Florida Keys in about 18 hours or so. The depression is currently traversing very warm Sea Surface Temperatures (SSTs) of 29-30°C, well above the minimum threshold of 26C typically needed for tropical cyclogenesis.

Additionally, the cyclone is poised to traverse an area of increased oceanic heat content across the central Bahamas within the next six hours or so, which will assist in the intensification process.

Rather interestingly, latest analysis from CIMSS indicates the beginnings of an upper-level anticyclone, which is currently centered about 50 miles to the southeast of the depression. No doubt, this development has occurred because of the rapidly lessening influence of the persistent upper low. The 6z GFS suggests that this anticyclone will remain collocated with TD3 for at least the next 18 hours, after which point it is forecast to detach from the cyclone and retreat northeastward and gradually open, ultimately becoming absorbed into the western Atlantic subtropical ridge. My current thinking is that the anticyclone will benefit the depression enough to bring it to 45 mph before it approaches southern Florida or the keys. Obviously, the intensity could certainly be higher than this, depending on how well-established, persistent, and strong the anticyclone becomes.

The system, should it go through the Florida Straits, rather than through south Florida, should emerge into the eastern Gulf of Mexico in around 24 hours. Upper-level winds are not forecast to be ideal for intensification, and may only be marginally favorable. However, the shear will primarily be southerly, which would tend to have less of an impact on the cyclone than it ordinarily would. Overall, atmospheric and oceanic conditions conditions appear favorable enough for a 65 mph tropical storm, but this will depend upon the evolution of the upper wind environment across the Gulf.

A large weakness is evident in the subtropical ridge to the north of the cyclone, which is responsible for the current WNW motion. I expect this general motion to continue throughout most of the forecast period, though a more poleward turn could occur by around 60 hours as the system nears the coast of central Louisiana. Just how much of a poleward turn this system takes will depend largely upon the evolution of the aforementioned upper low, which is forecast to continue retreating westward into the Gulf, and eventually move inland by around 54 hours. This is impossible to tell at this point, so residents from Galveston eastward to Biloxi should carefully monitor the progress of this system over the next several days. Another factor regarding the eventual Gulf Coast landfall location of the depression will depend upon a cold front currently moving across the central United States.

My current thinking is that this trough and associated front will at least dent the western extent of the Atlantic subtropical ridge during the next two days. All in all, based upon model analysis as well as other data, I expect the cyclone to ultimately make landfall along the west-central Louisiana coast, just west of Vermilion Bay, though this is obviously not certain. Again, residents across the aforementioned areas should closely monitor the progress of this system over the coming days.

My forecast track agrees well with the NHC's current one, except it's just a little bit to the east.

Invest 98L

Invest 98L continues churning across the Bay of Campeche. Visible satellite animations imply the continued development of deep convection near the center, with some bands of heavy thunderstorms to the north of the circulation. These rains are impacting portions of central Mexico to the south of Tampico, an area just recently battered by Hurricane Alex and Tropical Depression Two. Based on animation of visible satellite imagery and 850 mb vorticity data from CIMSS, I assume that the surface center is located near 20N 95W. An upper-level anticyclone continues to be centered aloft, which favors continued development until the system moves inland. Land interaction is the only inhibiting factor I can see to tropical cyclogenesis, and the system should move inland to the south of Tampico by tonight or early tomorrow morning. The farther south this system goes, the less time it will have over water.

I give this system a 50% chance of becoming a tropical depression prior to moving inland. Were it not for land interaction, I'd go higher. Residents from Veracruz to Tampico should closely monitor the progress of this system. Regardless of development, locally heavy rainfall will overspread the aforementioned areas, and possibly even extreme southern Texas over the next day or two.

Elsewhere

The 12z ECMWF isn't out yet, but the 0z run was still indicating the possibility of our first Cape Verde storm on July 25. The 6z GFS also continues to reflect this possibility. Upper-level winds still appear rather conducive for anything that might happen to form in this area.