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Weathercatch: A mighty wind: How last week’s windstorm gained its strength

Comstock Park on Spokane’s South Hill lost many trees, toppled by strong winds in the overnight hours Jan. 13. Winds of up to 70 miles an hour swept through Spokane and brought down hundreds of trees made easier also by loosened earth from previous days’ rain. Felled trees knocked out power to tens of thousands of customers. (Jesse Tinsley/The Spokesman-Review)
By Nic Loyd and Linda Weiford For The Spokesman-Review

First we got soaked; then we got wind-whipped. The heavy rain was expected. The deadly winds, not so much.

The windstorm that pounded the Spokane area and North Idaho on Jan. 13 was partly due to the tail end of an atmospheric river that made landfall on the Washington coast less than 36 hours earlier. Because strong prevailing winds carry these giant rivers in the sky from the tropical Pacific Ocean, winds were anticipated when this recent atmospheric river came onshore. But why did they reach such brutal force here in the Inland Northwest – some 400 miles away?

Waning winds from the atmospheric river were fueled by a fast-moving low-pressure system that descended from southwestern Canada. The strongest winds in our region occurred as the pattern transitioned from warm, wet conditions produced by the atmospheric river to cold, dry conditions produced by the low-pressure system.

As that Wednesday morning was unfolding, winds peaked at 71 mph at Spokane International Airport, 70 mph in Post Falls and 61 in Coeur d’Alene. In a matter of a few hours, the windstorm killed two people, uprooted trees, downed power lines and knocked out power to tens of thousands of people in the Spokane and Coeur d’Alene regions. Damage was so severe that Spokane’s mayor declared a civil emergency to aid in clean-up efforts.

For many residents, the storm felt eerily similar to the historic windstorm that slammed the Inland Northwest just five years ago, when near-hurricane force wind gusts brought the Spokane area to a standstill on Nov. 17, 2015. As was the case in last week’s storm, wind gusts exceeded 70 mph, two people were killed by felled trees, and tens of thousands were left without power.

Although both storms were caused by a convergence of weather systems, no atmospheric river was associated with the November 2015 tempest. That’s one big difference between the two wind events. Another difference? Where strong sustained winds blew for only a few hours in the recent storm, they blew considerably longer during the 2015 event – from early afternoon until 8 p.m., according to National Weather Service data.

The ultimate impact of last week’s windstorm was made worse by heavy rains unleashed by the atmospheric river a day earlier on Jan. 12. Because the ground was saturated, tree roots had less adhesion to surrounding soil when powerful winds struck the morning of Jan. 13. Hence, a number of stately ponderosa pine, spruce and fir trees were toppled.

The windstorm’s roar was eventually replaced with relative calm as the system shifted east across Montana and into the Dakotas. This week has been pleasant and dry, with morning frost, patches of fog and much-needed breaks of sunshine.

The abnormally mild temperatures that have dominated winter so far are expected to dip as a cold air mass from Canada spills into our region today. Its arrival could signal the start of a cooling trend that brings temperatures near or even below what we normally see this time of year.

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Nic Loyd is a meteorologist in Washington state. Linda Weiford is a writer in Moscow, Idaho, who’s also a weather geek. Contact: ldweiford@gmail.com