Rain. Rain. Puh-leeze, go away!

Such was the collective prayer of about 1,500 Beach Boys fans crowded into the visitor center at Longwood Gardens on Thursday. They waited two hours for the delayed start of the outdoor concert as yet another thunderstorm lashed trees and heaped buckets of rain across the region.

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"We handed out towels," director of marketing Marnie Conley said.

With 11 days left in this month, and more drenchings predicted, meteorologists expect to chalk up these soggy dog days as the wettest month ever recorded in Philadelphia.

It already is the wettest August, having sloshed more than 12.76 inches of rain onto the official recording station at Philadelphia International Airport, said Dean Iovino, a meteorologist with the National Weather Service in Mount Holly.

"If you think back to July" - which was the hottest month on record in Philadelphia - "it was mostly dry, warm summer air," said Iovino, even though most of us probably remember it as humid.

In the last few weeks, he said, cooler air from Canada has been colliding with warm air coming up from the South on a boundary line that sits above Southeastern Pennsylvania, New Jersey, and New York.

That clash of hot and cool air "is causing precipitation," he said, "with thunderstorms on what seem like an almost daily basis."

After the ground becomes supersaturated, and the weather turns fair for a day or more, there also is something of a terrarium effect.

"The sun comes out and some of the moisture evaporates into the atmosphere," said Iovino, "which helps fuel the next storm."

That dynamic is always true. So why such extreme weather this year?

Scientists point to various studies showing that water vapor in the atmosphere is on the increase, particularly in the Northern Hemisphere's higher latitudes, and that monster precipitation events have become more common. The World Meteorological Organization recently reported that the first decade of the 21st century was the wettest in the period studied, dating back to 1850.

Another factor making this August so wet and wild is the effect of alternating masses of high- and low-pressure air in the atmosphere, scientists say. High-pressure air, which is heavier, alternates with lighter low-pressure air to drive winds that spin up storms.

To become the wettest month ever in Philadelphia, August will have to drown out September 1999, which was doused with 13.07 inches. That statistic was driven largely by Hurricane Floyd, which dumped six inches of the total in Philadelphia by itself. This year, by contrast, the inundation has been more or less spread out, although pockets have been hit by hurricane-like rainfalls, without the accompanying hurricane-force winds.

Nonetheless, local residents have felt the fury. Across the region, August storms have downed trees and power lines, washed away roads, overflowed dams, and drowned crops.

Though Eastern Pennsylvania took the hardest hit, with a series of flash-flood warnings in an arc stretching from State College, Williamsport, and Scranton to Philadelphia, South Jersey was clobbered, too.

Cumberland County, the part of New Jersey that took the brunt of last week's worst weather, estimated damages at $20 million. Gov. Christie is expected to call on President Obama to provide federal relief.

Cumberland's Seabrook Farms was bombed by nearly 11 inches of rain Sunday into Monday - the kind of freak event that happens once in 200 years, a Weather Service meteorologist said.

At the Jersey Shore, vacationers took advantage of the tumultuous weather to visit Cape May Lighthouse, antiques shops, and other indoor attractions.

North, at Island Surf in Beach Haven, the rain wiped out business for two days because no one rents bikes or takes surf lessons in thunderstorms. Said store manager Matt Boyd: "It's been pretty rough."

Contact staff writer Michael Matza at 215-854-2541 or mmatza@phillynews.com.
Inquirer staff writer Barbara Boyer contributed to this article.