Tropical Storm Isaac Remains Weak, Forecast to Intensify

By Andrew Freedman

Tropical Storm Isaac is still on track for a rendezvous with Florida—or another Gulf Coast state—but it may spare Tampa, the site of the Republican National Convention, from a direct hit. However, even a sideswipe from a hurricane can spell trouble for a city as vulnerable to storm surge as Tampa is.

Enhanced satellite image of T.S. Isaac on Thursday afternoon. Click on the image for a larger version.
Credit: NOAA/NHC.

As of 2 p.m. on Thursday, Tropical Storm Isaac was passing to the south of Puerto Rico as a weak tropical storm, with maximum sustained winds of 40 mph. The storm, which is expected to strengthen into a hurricane during the next few days, is moving to the west-northwest, and is expected to pass over or very close to the Dominican Republic and Haiti on Friday. Isaac is then forecast to move across Cuba on Saturday, and be west of Tampa, over the warm waters of the Gulf of Mexico, by Tuesday morning.

The Republican National Convention is a four-day affair that begins Monday in Tampa and it is likely that the storm will bring rain and wind to the city, but computer model projections made Thursday have shifted the storm track westward compared to previous forecasts, raising the likelihood of a landfall in the Florida Panhandle or possibly coastal Alabama, rather than a direct hit on Tampa. However, given the typical errors in long-range track forecasts, Tampa is not out of the woods yet. Five-day forecast track errors for the center of a tropical storm or hurricane average around 225 nautical miles, according to the National Hurricane Center (NHC).

Assuming that Isaac does intensify as forecast, it could be a large and powerful storm by the time it approaches Florida, raising the possibility that much of the Sunshine State will be affected, even though the center of the storm might stay offshore.

As Climate Central reported in June, Tampa is extraordinarily vulnerable to storm surge flooding from hurricanes, and has not had a direct hit from a powerful hurricane since 1922. If the center of the storm passes to the west of Tampa, the city would be on the stronger east side of the storm, which could lead to storm surge impacts even without a direct hit. 

Computer model forecasts for T.S. Isaac. Click on the image for a larger version.
Credit: NCAR.

While computer models have come into better agreement on the storm track, the intensity forecast is a different story. In part this reflects the fact that while major improvements have been made to the accuracy of track forecasts in recent years, intensity forecasts have remained error-prone, particularly beyond a day in advance. The storm has already confounded forecasters’ expectations.

Although the National Hurricane Center (NHC) in Miami has consistently called for Isaac to intensify, observations from hurricane hunter aircraft have indicated that it remains weak, despite an impressive appearance on satellite imagery.

In a forecast discussion on Thursday morning, NHC forecasters said the intensity forecast remains a tricky one, but that all signs still point to the high probability that the storm will intensify sooner or later. “THE STRUCTURE OF ISAAC SUGGESTS THAT ONLY SLOW STRENGTHENING IS LIKELY.  HOWEVER . . . THE ENVIRONMENTAL CONDITIONS ARE FAVORABLE FOR RAPID INTENSIFICATION . . . WHICH COULD OCCUR IF THE CYCLONE DEVELOPS A WELL-DEFINED INNER CORE,” forecasters wrote.

The 2012 Atlantic hurricane season has been an extraordinarily active one so far. In addition to Isaac, Tropical Storm Joyce has formed in the Central Atlantic, which was the second-earliest date for the season’s 10th-named storm. Right now forecasts take Joyce out to sea, with a possibility that it may affect Bermuda next week.