Friday, August 28, 2015

Rain: Extreme times

Four hours, 7-plus inches of rain, three records. Was it real, and is nature telling us something?

Rain: Extreme times

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What impressed us most about the wettest day in Philadelphia in 142 years of recordkeeping was the rapidity with which it all happened.

Of the 8.02 inches of rain measured in the 24-hour period, 7.35 of that fell just between 3 and 7 p.m. That, alone, was a respectable-rainstorm better than the 6.63 inches wrung out from the remnants of Hurricane Floyd, on Sept. 16, 1999.

Floyd, you might recall, was a region-wide disaster, and the airport total that day didn't approach the double-digit totals reported elsewhere.

Yesterday, the rains took dead aim on two narrow corridors, one slicing through eastern Delaware County, the airport, and the Jersey towns along the river. A second extended from central Delaware into South Jersey. We'll focus on the first.

The Philadelphia totals -- which set records for any calendar-day and July 28 and made this month the wettest July -- were so outrageous that we wondered whether the official National Weather Service gauge at Philadelphia International Airport was hallucinating.

But Gary Szatkowski, meteorologist in charge of the National Weather Service in Mount Holly, who happened to be filling in and working a forecast shift last night, said radar corroborated that the airport gauge was smack n the bull's-eye.

What that image, posted on Facebook, shows is that the rainfall was intensely concentrated. Other areas of the city and Delaware County -- locations not far from the airport -- had far-more prosaic amounts of rain. For the 24-hour period, Pottstown weighed in with under an inch.

The concentrated deluges did set off some flooding, but not the widespread flooding that one might expect, given the rain amounts.

Szatkowski said the region actually was fortunate in at least one sense. Had the extreme rains targeted areas  20 or 30 miles to the northwest, the result might have been "catastrophic."

Coming after the wettest June on record, and less than two years after the amazingly wet August-September period of 2011, yesterday's deluge would appear to fit into the hypothesis that a warmer world means more heavy-precipitation events.

See this study, published in April.

Szatkowski was cautious. "If weather is getting more extreme, this certainly supports that theory," he said.

"It doesn’t prove that theory.”

In any event, we will say that we often hear from loyal readers complaining that the airport gauge somehow missed the deluges in their neighborhoods.

We've received no such calls today.

 

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Inquirer Weather Columnist
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Tony Wood Inquirer Weather Columnist
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