Tropical weather analysis - August 5, 2010

Tropical Storm Colin

Tropical Storm Colin managed to regenerate into a tropical cyclone this afternoon, after a reconnaissance flight confirmed the existence of a closed surface circulation. As of the 7:30 PM AST advisory from the National Hurricane Center, Colin was estimated to have maximum sustained surface winds of 60 mph, and a minimum central pressure of 1005 mb. Colin is moving NNW at 20 mph, and is located near 25.6°N 66.3°W. It's difficult to ascertain how much vertical shear the cyclone is experiencing, because the CIMSS shear product, along with many of its other products, are down for the moment. However, it is quite evident that Colin is undergoing some southwesterly vertical shear associated with an upper-level low centered to the north of the Bahamas.

Water vapor loops reveal this low quite nicely, and also indicate that it is steadily entraining dry air into the western semicircle of Colin. Infrared satellite loops suggest that Colin has an almost fully exposed surface center. This is not unexpected, given that shear is likely on the order of 20 to 30 kt at the moment. This, combined with the dry air, has ensured that Colin will generate only meager and disorganized convection, which will continue to be blown northeast, away from the center.

I expect Colin to turn to the N within the next 6-12 hours as the low- to mid-level ridge weakens further, followed by a turn to the NE or NNE by around 42 hours with an increase in forward speed as the cyclone becomes well-embedded within the southwesterly flow associated with a mid- to upper-level trough approaching from the west. Models are in generally good agreement on this. Colin could pass very close to Bermuda on Saturday.

Watches and warnings

A tropical storm warning is in effect for:

Bermuda

A tropical storm warning means that tropical storm force winds are expected within the warning area in 36 hours.

Based on the above, interests in and around Bermuda should closely monitor the progress of Colin over the next couple of days. Even if Colin doesn't come as close to the island as currently forecast, large swells will affect the island over the next couple days. The National Hurricane Center is giving Bermuda a 50% chance of receiving tropical storm force winds from Colin, and this is a reasonable forecast.

Upper-level winds are expected to remain unfavorable for significant intensification until around 24 hours, at which point they could become more favorable. By 72 hours, the shear will be near zero, and will remain that way throughout the remainder of the forecast period. Hence, I expect that Colin will attain hurricane status, and it is possible (though unlikely, IMO) that this will happen before the cyclone nears Bermuda. A hurricane watch or warning could be required for that island over the next day or so, depending upon the evolution of Colin.

I expect Colin will peak with winds of around 85 mph late on day three or early on day four. Thereafter, the cyclone will encounter sub-26C SSTs. That, combined with interaction with the aforementioned trough, should ultimately induce extratropical transition by day five. Residents along Atlantic Canada should also closely monitor the progress of Colin, even though the current NHC forecast track, as well as my own forecast, keeps the center comfortably away from that area.

Invest 93L

A vigorous tropical wave located several hundred miles west of the Cape Verde Islands is showing signs of organization. This wave was recently labeled" Invest 93L" by the National Hurricane Center. Current model consensus is for a general NW motion until around four to five days, when a turn to the N and then NE is expected as the subtropical ridge weakens further with the approach of another upper-level trough. I see no reason to disagree with this forecast for now, and I am not expecting any significant changes in the models over the next several days, as the current synoptic scale pattern favors predominant troughing along the east coast. Upper-level winds are forecast to improve in as little as 24 hours, and the 18z GFS maintains an anticyclone aloft over the system throughout much of the forecast period.

Given this, and current organization trends, I expect this wave to continue to slowly develop, becoming a tropical cyclone by 48 hours. It is possible that this system could eventually attain hurricane status.