Wednesday, July 1, 2015

Rain: More deluges?

Downpours possible Thursday, weather service says.

Rain: More deluges?

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With its forecast updates, the National Weather Service has issued a "hazardous weather outlook" for the potential of thunderstorm downpours for Thursday.

Naturally, the weather service cautions that this far out, it is impossible to say precisely where any storms would focus their energies.

For now, the Thursday outlook contains the benign wording: "New rainfall amounts between a tenth and quarter of an inch," but it also includes that rather menacing disclaimer, "except higher amounts possible in thunderstorms."

What occurred yesterday provided further proof that forecasts can't capture all the possibilities.

One weather service forecaster commeted whimsically, "'Gee, I didn't put in 7 inches of rain for Philadelphia,'" said Gary Szatkowski, meteorologist in charge of the National Weather Service office in Mount Holly, who was on duty yesterday.

"'You wouldn't have been allowed,'" the boss told the forecaster.

That prediction still would have been off the mark: Over 8 inches of rain was measured at the official station at Philadelphia International Airport, with the heaviest rains falling in a very narrow stripe between 3 and 7 p.m.

The prodigious amounts along that corridor had something to do with "training," a phenomenon in which strong storms keep ganging up on the same area, said Szatkowski.

They also might have been related to the above-normal water temperatures, according to Jim Eberwine, the retired former marine forecaster at the weather service.

Sea-surface temperatures at the Shore were around 77 yesterday afternoon, about 3 degrees above normal. On the Delaware Bay near the entrance to the Delaware River, they were around 80.

Eberwine said that  a "slight south to southeast wind" in the late morning and early afternoon took an easterly turn later the day. The southerly and eastelry winds transported warm, moist air to provide an additional resource for the storms to mine.

Eberwine believes that no one could have foreseen the result.

He said that if he had walked into the Mount Holly office yesterday and someone said, "'I think we will set an all-time 24 hour rainfall amount.' I would have said OK, how are the Phillies doing?'"

Inquirer Weather Columnist
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Tony Wood Inquirer Weather Columnist
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