Flurry of storms is typical for time of year, experts say

In the last two weeks, it seems as though the tropics have exploded: Hurricane Earl, big, scary and steaming toward North Carolina and a string of states. Tropical Storm Fiona, nearing hurricane strength but heading into hostile conditions and out to sea.

Add the latest wave to roll off Africa, which got itself together enough Wednesday to quickly move from Depression No. 9 to Tropical Storm Gaston. Its future power and path remain highly uncertain. There are more tropical waves behind it.

The flurry suggests otherwise but hurricane season 2010 is actually running pretty much on schedule, number-wise. It's September. It's not unusual for the Atlantic to wing multiple storms down Hurricane Alley.

"It's just the same as it is every year," said David Nolan, a professor of meteorology at the University of Miami's Rosenstiel School of Marine & Atmospheric Science. "We're just coming up to the peak of the hurricane season."

All the ingredients are in place to cook a furious tropical stew, he said: Ocean temperatures at their warmest, low wind shear, a conveyor line of tropical waves off Africa. Some years, like this one, large weather patterns add spice. This year, Nolan said, there are three of them — the familiar La Nina and two "oscillations" that are not exactly household names, the Madden-Julian and the Atlantic Multidecadal.

Hurricane Earl, only the fifth in a year federal forecasters predicted would churn out 14 to 20 named storms, regained its Category 4 strength after weakening slightly and remained on a path that could sideswipe much of the Eastern Seaboard. Hurricane warnings were posted for most of the North Carolina coast, and watches as far north as Massachusetts.

The National Hurricane Center expected the 135-mph storm to spin close to the coast by late Thursday as a major storm, then veer off and skirt the coast on up to the Canadian Maritimes, gradually weakening along the way.

Bill Read, the hurricane center director, said he was confident that Earl would turn when a cold front pushes across the East Coast, forcing Earl out. But the timing of that turn was going to be a close call, he said, particularly for the Outer Banks of North Carolina.

If it turns offshore, residents might see no more than broken tree limbs and rip currents in surf too rough for swimming anyway. But, Read said, "If it comes in later, then the center of the storm or the eye could make landfall across the extreme eastern tip of North Carolina, and for them, that would be the worst-case scenario."

That could mean widespread power outages, flooding from storm surge and millions of dollars or more in property damage and losses. Governors in North Carolina, Virginia and Maryland declared emergencies and evacuations were ordered for two of the string of Outer Banks islands, Hatteras and Ocracoke.

Despite the fine weather, Brittany Grippaldi and her family decided to pack their Ford Explorer in Hatteras and head home to New Jersey.

"It's sad because reality hasn't really set in because it is so beautiful out. It's like, 'Oh, I don't want to leave this,' but it's like the calm before the storm," Grippaldi told The Associated Press.

If the experts are right, it could be a drill repeated a number of times this year.

Tim LaRow, an associate research scientist at the Florida State University Center for Ocean-Atmospheric Prediction Studies, helped develop a computer model with an impressive record of pre-seasonal forecasting. He predicted 17 named storms for 2010, 10 turning into hurricanes.

He's still "semi-confident" about the prediction. Though the season started with average numbers, the tropics have turned more hurricane friendly toward the end of summer — as they usually do. Historically, Sept. 10 is the peak of the season.

That's when tropical waves — big, disorganized storm masses generated by temperature differences between the northern part of Africa and the continent's equatorial zone — begin pinwheeling west into the Atlantic. As the ocean warms up — and its temperatures now are as high as they have been since 2005, which produced a record number of storms — it fuels the tropical cyclone development.

"It's not going to be unusual over the next month to see a lot of storms forming from African waves," he said.

There are several weather patterns aiding development as well, scientists say.

Much of the ocean remains in what meteorologists call the warm phase of the Atlantic Multidecadal oscillation, a pattern of temperature shifts that can last 20 to 40 years. Warm phases like this one, in its 16th year, tend to produce more storms.

Another weather pattern on the opposite side of the globe also adds fuel: La Nina, which is marked by cooling temperatures in the Eastern Pacific Ocean, typically tends to reduce wind shear, making it easier for storms in the Atlantic to form and strengthen.

The third is dubbed the Madden-Julian oscillation, which Nolan said amounted to a constantly moving boundary line — with one side favoring storm development, the other suppressing it. He said that helped explain the lag between Tropical Storm Colin, which formed July 29, and Hurricane Danielle, which formed Aug. 19, and grew into an intense Category 4 storm before curving out to sea.

"There is an internal variation to each hurricane season," Nolan said. "Each season has about two or three weeks on and two or three weeks off."

For the moment, at least, it appears solidly on.

Though experts predict storm numbers based on atmospheric conditions, they can only guess at strikes based on past statistics. The odds of multiple strikes also go up during active years — with a 90 percent chance of a landfall on the East Coast and 80 percent on the Gulf Coast — as do the risk factors for Caribbean countries.

That's because ever-shifting and unpredictable regional weather systems dictate where hurricanes go. They are, said James Franklin, chief of hurricane specialists at the center in West Miami-Dade, "passengers" not drivers on atmospheric steering currents.

A large seasonal front famously known as "the Bermuda High" initially drove Earl west, said LaRow, until the storm found its edge and began curving more to the north where its next direction will be dictated by that approaching cold front.

Danielle and Fiona also curved north along the edge of the Bermuda High but LaRow cautioned that trend may not continue. The National Weather Service forecasts that high pressure ridge growing stronger and expanding westward toward the mainland. That could, at least for a time, discourage storms from curving harmlessly into the Atlantic, LaRow said.