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Frenetic winds, dry air help fuel blazes

Fri., July 11, 2008

High winds that slammed the Inland Northwest and fanned fires Thursday originated from an unusually strong summer cold front that swept south from British Columbia during the afternoon.

Gusts in Spokane peaked at 47 mph between 4 and 5 p.m. but were subsiding following passage of the cold front.

The wild weather was accompanied by bone-dry air: Humidity dropped to 10 percent in Spokane during the afternoon, adding to the fire danger.

Kerry Jones, warning coordination meteorologist for the National Weather Service in Spokane, said the wind was stirred by severe contrasts in temperature and in air pressure.

In Omak, where the front passed at midafternoon, the temperature dropped from 80 degrees to 64 degrees between 3 and 6 p.m.

“That contrast generates strong winds,” Jones said. “For July, that’s a pretty strong front.”

North-to-south valleys along the Canadian border acted as channels for the wind. A weather measuring site reported a gust of 74 mph at Okanogan, Wash. A trained weather spotter reported that trees up to a foot in diameter were toppled near Colville.

The high of 83 degrees in Spokane was accompanied by relatively low air pressure along the ground, which is typical for sunny days this time of year.

The difference in air pressure between Omak and Spokane at 5 p.m. was 8 millibars on the barometer, a fairly strong pressure gradient, Jones said. Some locations saw air pressure increases of 6 millibars an hour as the cold front passed from north to south.

The heavier cold air moves over the landscape like an invisible mound with the cold air accelerating downward off the mound into a trough of low pressure. That displaces the lighter, warmer air, which is swept off ahead of the front prior to the front’s arrival.

Winds are expected to be light today with a high of 79 degrees. Relative humidity levels should increase, Jones said, and that ought to help firefighting efforts.

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