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Weatherblogging: Tropical Storm Isaac Threatens To Disrupt RNC

Four years ago, Hurricane Gustav’s landfall along the Louisiana coast delayed the start of the Republican National Convention in Minneapolis. Four years later, Republicans decided to convene the RNC in Florida during the heart of hurricane season. So naturally, there's a tropical cyclone heading in the general direction of the RNC.

Luckily for readers, while not tracking election data, I like to follow tropical weather. This one is called Isaac (coincidentally, my editor's name) and it's currently a disorganized and weak tropical storm in the eastern Caribbean with 45 mph winds. Dry air to the northeast of the storm appears to be hindering intensification, but warm waters and favorable environmental conditions should allow Isaac to strengthen into a hurricane over the next 48 hours. Computer models agree that the storm is likely to track toward the west-northwest in the general direction of Florida over the next five days, and the official National Hurricane Center forecast currently places the storm near the coast of southwestern Florida in 120 hours—the morning of the first day of the RNC.

But Isaac’s exact intensity and path are uncertain at this early stage, not only because of the intrinsic uncertainty associated with long range weather forecasting, but also because of factors specific to Tropical Storm Isaac. The extent that Isaac turns north depends on a ridge of high pressure currently to the north of the storm. The prevailing winds around the ridge are currently pushing Isaac to the west, but models show that this ridge could weaken as a mid-upper level trough moves through the eastern United States. If the ridge weakens, Isaac would move north and potentially toward Florida.

The computer models agree that the ridge will remain intact over the next two or three days and weaken thereafter, but they disagree about where the ridge blocking Isaac’s northward advance will break. If the ridge stays relatively strong, Isaac would continue west into the Gulf. If the ridge breaks over the Atlantic, Isaac could advance north into the Atlantic and miss Florida to the east. But if it breaks over the northern Gulf Coast, the storm could move over Florida. Different models currently advocate each of these solutions and the National Hurricane Center forecast currently splits the difference. But since any of these outcomes are possible, Isaac could potentially leave RNC unscathed. And of course, even if every model agreed on the same trajectory, the intrinsic uncertainty associated with long range forecasting would still give great cause to question whether the storm would head right down the middle of the “cone of uncertainty.”

Adding to the forecasting challenge are Cuba and Hispaniola, two large Caribbean islands that lay in Isaac’s path. If Isaac’s core tracks over either of these two mountainous landmasses for a prolonged period of time, the storm will be removed from the warm water that fuels tropical cyclones and the circulation will be severely disrupted by rugged terrain. But if Isaac remains inland a shorter period or avoids land altogether, then Isaac could retain more of its strength and organization as it heads toward a possible rendezvous with Florida. Similarly, the Florida peninsula's northwest-southeast axis is parallel to the storm's forecasted northwesterly path. So if the storm tracks slightly to the east, it would run up the length of the Florida peninsula and weaken over land. Were it to track west, it could stay over the Gulf and retain it's strength while lashing the west coast of Florida.

All things considered, the NHC forecast calls for Isaac to approach Florida on Monday as a Category 1 Hurricane with maximum sustained winds of 80 mph. Between heavy wind and rain, power outages, and canceled flights, a near pass by Isaac would seriously disrupt the RNC, although I can't say I know what would force them to cancel or reschedule the final days. But long-range intensity forecasts are arguably less accurate than long-range forecast paths, and that’s especially true when the latter is a function of the former due to possible interaction with land. With all of these variables, the National Hurricane Center gives most of the southern Florida peninsula about a 20 percent chance of experiencing tropical storm-force conditions in the next 120 hours, although those odds will continue to increase unless something nudges the storm off of its path. As always, follow the official forecasts, but perhaps I'll be writing dorky posts like this over the next few days illuminating the factors influencing the storm's changing path and strength.