Tropical weather analysis - August 4, 2010

Invest 92L

A tropical wave over the eastern Caribbean Sea was designated "Invest 92L" yesterday evening by the National Hurricane Center. Infrared satellite animations indicate a marked decrease in heavy thunderstorm activity associated with this feature. The likely cause of this is an area of dry air to the east, noted on water vapor animations.

Despite the recent convective downturn, however, CIMSS wind shear analysis indicates an anticyclone has developed aloft over the wave axis, which I estimate based on satellite animations and CIMSS 850 mb vorticity analysis is near 14N 76W.

This system should continue moving generally W throughout the remainder of its lifetime, eventually running into Central America in about two days, with perhaps a slight WNW component developing after 12 hours as the ridge to the north weakens somewhat. This system could take a track very similar to Hurricane Alex earlier in the year. The system should continue WNW even after landfall, and will enter the Bay of Campeche in the Gulf of Mexico in about three days or so, and ultimately make a second landfall from central Mexico to south Texas. The main legacy from this system will be heavy rains across Central America, Mexico, and south Texas.

An alternate scenario is for the system to move a bit more southward (though I'd still expect a slight N component of motion even with this arrangement) and eventually move into the Eastern Pacific.

Though upper-level winds favor development over the next seven days, I am not expecting any development from this feature. None of the computer models even remotely hint at its development, either.

Remnants of Colin

The remnants of former Tropical Storm Colin are located in the western Atlantic Ocean, several hundred miles northeast of Puerto Rico. Infrared satellite animations indicate that deep convection has remained fairly persistent across the system over the last couple of hours. Indeed, this system has been producing persistent deep convection over at least the last 18 hours. This can probably be attributed to a significant slowing of the forward speed over the last 24 hours, as Colin has gradually moved underneath the influence of a strong weakness in the western Atlantic subtropical ridge.

The center of Colin is still not particularly well-defined based on various satellite animations, along with various microwave satellite images from the Navy Research Lab (NRL). Unfortunately, a recent ASCAT pass failed to capture all of Colin, but it too suggests that the circulation is poorly defined, with no evidence of westerly winds along the southern side of the circulation center.

This system has some dry air to contend with to the west, being induced by an upper-level cold cored low pressure system centered near 26N 73W. Additionally, CIMSS analysis depicts 20 kt of westerly vertical shear blowing across the system.

The track for this system has finally become more clear, with the models now unanimously calling for recurvature out to sea underneath the influence of a strong cold front currently moving across the central United States. This system will further erode the western periphery of the Atlantic subtropical ridge, and allow the system to turn N by 36-48 hours, with a gradual deceleration in the forward speed from now until 48 hours.

Colin is moving NW at the moment, but as previously noted, a turn to the N is expected over the next day or two as the ridge weakens further with the approach of the aforementioned front. Thereafter, a gradual increase in forward speed, particularly after 72 hours, is expected as the system becomes well-embedded within the westerlies. The only land areas potentially threatened by this system are Bermuda and the Canadian Maritimes. The models suggest that the former will be impacted by the system in three days, and Colin be a hurricane as it approaches that island. Residents there should be carefully monitoring the progress of this system over the next several days.

Upper-level winds should begin to ease somewhat by tomorrow, and particularly after 36 hours. It is still possible that Colin will become a hurricane, and I actually find it quite likely. Its best chance would be after 72 hours, when the shear will be at its lowest.

Elsewhere

Elsewhere, a tropical wave along 30W is the next threat to develop. Most of the models are developing this feature. Right now its track is uncertain, but it could follow a path out to sea like Colin. This is highly speculative, though. Upper-level winds favor gradual development over the next several days.