Sunday, July 5, 2015

Attack of the thundersnow

Putting a charge into snow totals

Attack of the thundersnow

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By yesterday morning, the computer models suggested that the Philadelphia region would have a target on its back after dark.

That turned out to be correct, but the final totals ended up being far higher than forecast in what turned out to be a way-overperforming storm.

Officially, the Philadelphia International Airport storm total came in at 15.1, making it one of the all-time biggest storms and worthy of mention with the blockbusters of last winter.

What happened? It had a lot to do with those rumbles of thunder heard throughout the region and the haunting yellow-ish flashes that electrified the sky.

We all know now about the snow ambush in the morning that left up to 6 inches in Chester and Montgomery Counties.

But only about half that landed at the airport. Thus about a foot fell in a six-hour period after dark when rain changed to snow.

The snowfall got a tremendous lift, literally, from updrafts rising so furiously that they set off lightning and thunder. That's a sign of one powerful storm.

Snow falls when warm air rises over cold air and condenses. It falls heavily when the air is rising robustly.

If the rising air creates enough friction, lightning results, said Bob Wanton, meteorologist at the National Weather Service office in Philadelphia.

When it happens in a winter storm, it's a phenomenon called thundersnow. Thunderstorms, obviously, are more common in the warm seasons when more heat energy is available.

But lightning and thunder can occur in winter storms, as we found out last night, and when it does, accumulations can go off the charts. "You can get snowbursts," said Wanton.

It's possible that some areas were getting 3 and 4 inches an hour, he said. Late last night, his colleague, Mike Gorse, put out a statement warning that a heavy bands accompanied by lightning and thunder had set up from Wilmington to New York.

During the great blizzard of Feb. 11, 1983, a thundersnow burst over New York City shook out 5 inches in an hour, trapping motorists trying to get out of the Lincoln Tunnel.

One may wonder what this winter can do for an encore. Wanton says it appears the pattern is staying active. A couple of clipper systems are due this weekend with perhaps light snow, and another storm is possible next week.

Inquirer Weather Columnist
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About this blog

Everyone talks about the weather, and here we write about it.

When we’re around and conditions warrant, we’ll keep you updated about what’s coming, but we will do our best always to discuss weather and climate developments in context and remind you that nothing in the atmosphere happens in a vacuum.

Tony Wood has been writing about the atmosphere for The Inquirer for 26 years.

Reach Tony at twood@phillynews.com.

Tony Wood Inquirer Weather Columnist
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