In the last two weeks, it seems as though the tropics have exploded.

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MIAMI — 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, numberwise. 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.

Large weather patterns can add spice.

This year, Nolan said, there are three of them — La Niña 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 status as Category 4 on the Saffir-Simpson Hurricane Wind Scale (which goes from 1 to 5, with 5 meaning winds of 155 mph or more). It remained on a path that could sideswipe much of the Eastern Seaboard.

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’s director, said he was confident that Earl would turn when a cold front pushes across the East Coast, forcing Earl out.

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.

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.

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 Niña, 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.