The storm that slammed the Inland Northwest with near hurricane winds on Tuesday was the product of a unique set of meteorological circumstances.
Considering widespread damage and intense wind measurements, the windstorm could be considered the strongest ever recorded in Spokane with a peak wind gust of 71 mph at Spokane International Airport.
Hurricane speeds start at 75 mph.
Tuesday’s storm blew the strongest wind ever recorded in Spokane for a cyclonic storm, a broad system that combines a warm front, a cold front and deep low pressure at the surface. That’s the typical storm that occurs in the Northwest in the fall, winter and early spring.
It was not, however, the strongest gust ever recorded. The record was set 10 years ago, when Spokane saw a 77 mph wind from a thunderstorm gust front.
National Weather Service forecasters saw the storm days in advance as it formed over the Pacific Ocean, and became concerned when their computer forecasting models consistently called for high winds as the storm drew closer, said agency forecaster Greg Koch.
“We were able to see the potential late last week,” he said.
As the storm arrived on the Pacific Coast on Monday, it lined up to favor high winds over Eastern Washington and North Idaho.
The center of low air pressure along the ground tracked from southern British Columbia into Alberta, dropping the barometer to 980 millibars of air pressure, which is well below an average surface pressure of about 1,000 millibars.
“Pressures that deep don’t often occur,” Koch said.
That deep of a low created a pulling or drawing force that drove winds to what forecasters would call a deep low. Think of it as a hole in the air.
Adding to the power was a 176 mph jet stream in the upper atmosphere that added momentum to the surface winds, Koch said.
“To have that much momentum aloft contributed greatly to the wind in the storm,” he said.
A third factor played into the howling winds. Downward momentum of wind flowing over the Cascades added to the intensity and drew strong winds aloft toward the ground.
But a fourth element played a kicker role: mild temperatures in the 50s that arrived with the storm’s warm front earlier in the day. The rising warm air during the day helped draw the heaviest winds downward.