Tropical Storm Isaac Gathers Steam, Approaches Keys

By Andrew Freedman

Tropical Storm Isaac is bearing down on the Florida Keys, after having swept across Haiti on Friday and skirted the northern coast of Cuba, maintaining much of its strength in the process. The storm is expected to be at or near hurricane strength, with sustained winds of at least 74 mph, when it makes its closest pass to the Florida Keys on Sunday evening. But there is a lot of uncertainty as to where it will head from there. A more northwesterly track, favored by some of the computer models on Saturday night, would bring the storm ashore as a powerful hurricane in coastal Louisiana or Alabama. A track closer to western Florida, with a landfall in the Florida Panhandle, would more seriously affect the Tampa area, the site of the Republican National Convention, which kicks off on Monday. 

Computer model track forecast for T.S. Isaac from the GFS model ensemble, run on Saturday night. Click on the image for a larger version.
Credit: NCAR.

The National Hurricane Center is warning Gulf Coast residents from coastal Louisiana to Florida to make preparations for potential hurricane or tropical storm conditions, along with flooding rainfall, during the next few days. 

Because the storm is already so large, even if the storm’s center stays off the coast of Tampa and other parts of western Florida, it is still likely to bring heavy rain and wind to much of the state, particularly along the west coast. As of Saturday night, tropical storm force winds (between 39-to-73 mph) extended nearly 200 miles away from the storm’s center, and Isaac could become an even larger storm with time.

Infrared satellite image of T.S. Isaac as it skirted the north coast of Cuba on Saturday night.
Credit: NOAA.

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 1921. If the center of the storm passes to the west of Tampa, the city would see impacts from a storm surge driven by strong winds from the southwest. The NHC is forecasting a storm surge of 3 to 5 feet in Tampa Bay, and 5 to 7 feet for locations closer to the storm in southwestern Florida. (The surge in areas that the storm makes its final landfall will likely be even higher than this.)

The track forecast is a tricky one because the storm's movement will be determined by the evolution of weather systems across the lower 48 states. The storm is caught in between two High Pressure systems and is also interacting with another low pressure system near Cuba, and the airflow in between them will help draw it northward. However, the exact position of these weather systems will be crucial to determining how far east or west the storm gets before making landfall.

As for the storm’s intensity, conditions will be favorable for strengthening during the next few days. The Hurricane Center forecast calls for Isaac to have sustained winds of 105 mph when it makes landfall, but intensity forecasts several days in advance are highly uncertain, and storms traversing the Gulf of Mexico have a shown a propensity for rapid intensification, which is difficult to forecast.