Recurvature Key to Katia

Whether Tropical Storm Katia threatens the U.S. as a hurricane will depend on how the storm “recurves” over the Atlantic, says Robert Muir Wood, chief research officer with Risk Management Solutions (RMS).

Recurvature is the concept that the prevailing motion of the atmosphere south of the equator causes a near constant wind from the east, while a westerly wind is expected in the northern portion of the globe. The transition point between the easterly flow to a westerly direction will cause a storm system to recurve away from the U.S. mainland.

“It’s a fundamental principal of large scale atmospheric flows. Hurricanes don’t drive themselves. They are moved along by these forces,” Muir Wood says. “It’s related to everything from how sailing ships crossed the Atlantic to the reason it takes more time to fly from London to New York. “

The key whether Katia – and most tropical storm systems – make landfall in the U.S. is where the recurvature takes place. Muir Wood explains that the center of recurvature is “not a fixed constant” and can move between Florida and New York depending on the forces of high and low pressure systems.

A strong high in the Azores and it’s likely that a hurricane will recurve back into the Atlantic without hitting then U.S. But if a there is a high pressure system near the Caribbean – a so-called “Bermuda High”—it’s more likely that a storm systems will make U.S. landfall.

“High pressure systems tend to be more stable and can drive the recurvature,” Muir Wood says. “For example, there was a very stable Bermuda High in 2004.” 2004 is considered one of the most costly Atlantic hurricane seasons, with large storms such as Hurricane Charlie and Hurricane Ivan causing nearly $50 billion in losses.

A report issued Thursday by RMS says that it is too early at this time to determine a forecast track and if the storm will be a threat to the .U.S. “Currently, model guidance is in agreement as to a slow intensification towards major hurricane status in the next few days. Intensity guidance has reduced the strength of Katia from previous model runs, as a consequence of the wind shear,” the statement says.

But if recurvature theory stands up, the storm is more likely to head out into the Atlantic and possibly threaten Bermuda. “Katia is moving fairly slowly and the orientation is going too far to the north,” Muir-Wood says. “That means it will almost inevitably swing around away from the U.S. But it’s far too early to say for sure.”


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