The powerful Cape Verde hurricanes that pose the greatest threat to the Southeast start as thunderstorms rolling off the West African coast, a fact sometimes overlooked about storms widely thought of as ocean events. Hurricane Arthur was one of those rare freaks - a storm that started out going the other way, rolling off the Southeast coast last week.
The last time that happened in the Lowcountry was a decade ago, when one of those systems rolled off the coast, stalled, spun up into Hurricane Gaston and came back ashore in 2004.
But even as seasoned forecasters in the spring were calling for an average or less busy storm season, at least one group saw Arthur coming. AccuWeather.com, a private forecasting company, predicted in May an increased likelihood something like that might happen. And in late June, Dan Kottlowski of AccuWeather said conditions were in place for it to happen for the Fourth of July weekend.
"Heading into the season, the jet stream was abnormally far south. As it moved (farther north) it had the potential to leave pieces of energy in place off the northern Gulf of Mexico and off the Southeast coast, and that's in fact how Arthur formed," said AccuWeather.com meteorologist Andy Mussoline on Monday.
Hurricane prediction is an imprecise science; forecasters try to read a soup of climate factors such as thermohaline circulation, sea surface temperature anomalies and the multivariate El Niņo southern oscillation index.
Jet streams are-high altitude wind currents. It's generally accepted that where the jet stream blows affects the likelihood of hurricanes seasonally and how they recurve, or move. But forecasting storm development in the "pieces" the stream leaves, or residual pockets of disturbed air, is edgy stuff.
"Hurricane formation relies on a set of six so-called necessary conditions," said meteorologist Lian Xie, of North Carolina State University, whose team produces one of the better regarded seasonal forecasts. "An abnormally southern position of westerly jet stream may provide the initial disturbance that is needed if other conditions are met ... but it alone is not a sufficient mechanism for tropical cyclogenesis (development)."
Early season storms aren't too common off the Southeast coast. On average, only one or two named storms form in the entire Atlantic Basin through July.
Folly Beach surfer Annie O'Brien called Arthur's passing "promising for the next few months," and hoped more storms would kick up good surfing waves. And less than a week after Arthur, National Hurricane Center forecasters on Monday morning were watching another patch of bad weather off the Carolina coast. It dissipated by evening.
The basin is quiet again and expected to stay that way at least through the next few days. Forecasters are sticking so far to the average-or-slightly-below-average preseason forecast. Although AccuWeather.com doesn't rule out another Arthur, "we do think the jet stream is beginning to 'normalize'," Mussoline said.
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