Westmoreland, Butler and Allegheny counties lead Pennsylvania in most tornadoes | TribLIVE.com
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Mary Ann Thomas
Above, a user-submitted photo posted by Tribune-Review news partner WPXI of the storm system as it moved through Westmoreland County earlier Wednesday evening.

It’s certainly not tornado alley, but more twisters have touched down in Westmoreland County in the last 66 years than any other part of Pennsylvania, according to unofficial statistics from the National Weather Service.

The National Weather Service is investigating Wednesday’s potential tornado in Westmoreland County, trying to verify wind speeds, damage and debris dispersion, according to Lee Hendricks, meteorologist with the National Weather Service in Moon Township.

If there was a tornado in the state Wednesday, statistically, it would likely have been in Westmoreland County.

Southwestern Pennsylvania leads the state in number of tornadoes from 1950-2016, according to unofficial statistics from the National Weather Service. The top six counties in the state area are: Westmoreland (32), Butler (21), Mercer (21) Allegheny (16), Beaver (15), Armstrong (12) and Lawrence (12).

Why Southwestern Pennsylvania? Because of the Allegheny Mountains and prevailing winds, according to Hendricks.

“For a tornado we need instability in the atmosphere and warm temperatures and high humidity,” he said.

Specifically, the stream of warm and moist air from the Gulf travels up the west side of the Alleghenies, according to Hendricks, which accounts for the maddening humid conditions in the Pittsburgh area and less so in the eastern part of the state.

The other main ingredient for a tornado is spin in the atmosphere.

“You generally need winds that are counter clockwise rising from the surface up into the atmosphere,” he said.

A twister is not common by any means in the state, which averages on three to five tornadoes a year.

Plus, the tornadoes here are not that strong, registering only an EFO and EF1 on the Fujita tornado scale, with winds 65 to 85 mph and 86 to 110 mph respectively.

Pennsylvania’s only recorded EF5 tornado (winds over 200) — the total destruction category of the Fujita tornado scale — was among 21 tornadoes that ripped through northeastern Ohio and northwestern Pennsylvania on May 31, 1985.

Mary Ann Thomas is a Tribune-Review staff writer. Reach her at 724-226-4691, [email protected] or via Twitter @MaThomas_Trib.

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