Nine years ago Thursday, a rare storm complex barged across the Inland Northwest, bringing heavy rains, damaging winds and a barrage of lightning.
It was a squall line, a group of thunderstorms arranged in a long line that can extend hundreds of miles and span several states at once. The leading edge of one can advance quickly, turning calm blue skies into a sea of brooding gray and triggering a sudden surge in wind. And if you happen to be driving when the downpour hits, visibility can drop significantly in a matter of seconds. The atmospheric turmoil also has the potential to cause flash floods.
Squall lines are unusual in this part of the country. Instead, they typically form east of the Rockies, due to higher humidity levels and more atmospheric instability.
But on Aug. 25, 2013, “what came through eastern Washington and northern Idaho was definitely a rare, strong squall line,” according to the National Weather Service Spokane, which conducted an analysis of the severe weather event.
Like many squall lines, this one formed ahead of a cold front. Cool air collided with a persistent warm air mass, producing instability in the atmosphere combined with the lifting of low-level air high into the sky and leading to the rapid growth of clouds. The first storms formed in central Oregon and began moving north toward Washington that afternoon, with plenty of moisture and instability to create more storms in a linear formation along the way.
Meanwhile, the National Weather Service in Spokane issued 13 severe thunderstorm warnings for Eastern Washington and the Idaho Panhandle throughout the evening.
The drama began to unfold at dinnertime as the squall entered the Tri-Cities area and the skies turned dark. Just before 6 p.m. a weather station southeast of Kennewick recorded a sudden 26 mph wind gust followed by 0.21 inches of rainfall in only 15 minutes. It also registered a temperature drop of nearly 20 degrees in an hour. Roughly 34 miles to the north, a weather station in Mesa recorded a wind gust of 48 mph, a steep temperature drop and a similar burst of rain.
The squall band descended on the Spokane area shortly before 9 p.m. with the sudden arrival of gusty winds, a marked temperature drop and a third of an inch of rain that fell within a half-hour. Winds exceeding 60 mph were reported west of Spokane near Airway Heights. Reports of property damage from falling trees extended from Grand Coulee to North Idaho, according to the weather service analysis. Also, a Sandpoint man was killed after a 200-foot tree fell on his tent at a campground in the Priest Lake Ranger District.
Predicting the formation of a squall line can be difficult. Although meteorologists can predict where severe weather is likely to occur, it’s challenging to know if individual storms will assemble in a long band and if enough wind power will develop to push it a long distance. Thankfully, computer models and remote-sensing devices such as radar and satellite imagery often make it possible to warn of an advancing squall line within several hours of its arrival.
Nic Loyd is a meteorologist in Washington state. Linda Weiford is a writer in Moscow, Idaho, who’s also a weather geek. Contact: firstname.lastname@example.org.
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