Severe t-storms possible in Pacific Northwest Wednesday

A cold front approaching the Pacific Northwest from the west on Wednesday is forecast to bring a risk of severe thunderstorms to the region. The greatest risk appears to be along the I-5 Corridor from Everett to near Eugene.

Isolated severe thunderstorms may be observed west of the Cascades from the Lower Mainland to near Medford as well as in Central Oregon and the Columbia Basin. Isolated non-severe thunderstorms may be observed throughout the Pacific Northwest on Wednesday.

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As of Tuesday morning, the Storm Prediction Center has a slight (level 2 out of 5) risk of severe thunderstorms from near Roseburg to near Everett with a marginal (level 1 out of 5) risk for most of the rest of Washington and Oregon.

March severe thunderstorms are rare for the Pacific Northwest. This is the first time the Storm Prediction Center has forecasted a risk of severe thunderstorms in Washington in March and the last time they did so for Oregon was in 2006. They began issuing outlooks of this kind in 1986.

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Storm Prediction Center meteorologists considered upgrading the southern Columbia Basin to a slight risk in their latest update, but opted to not do so due to uncertainty with the severe wind risk there.

Regional synopsis

Wednesday’s thunderstorm chances will be produced by dynamic conditions with the approaching cold front. Ahead of it, south to southwesterly winds will be moving warm and moist air into Washington, Oregon, and British Columbia. Dew points may approach 65°F (18°C) along the western foothills of the Cascades from near Sandy to around Abbotsford.

This moisture will be moved upward by diffluence aloft. Diffluence occurs when winds high in the atmosphere spread apart, creating a fanning-out effect that can lead to upward airflow. In Wednesday’s case, upper level winds will be fanning out right over the same region with the highest dew points.

In addition, storm activity will be enhanced by terrain. As storms approach the Cascade foothills the mountains will help to force air upward, adding to the already existing upward movement from diffluence. All of these together place the best chance of severe thunderstorm activity along the western slopes of the Cascades from near Sandy to near Granite Falls.

Moisture content and diffluence will still exist outside of that corridor. Scattered thunderstorms are possible in most of Washington, Oregon, and Southern British Columbia. Isolated severe thunderstorms are possible in and near the marginal risk area.

Some thunderstorm activity is likely to cross into British Columbia but with minor exceptions it appears B.C. will get the declining leftovers from activity further south.

Because this uplift is going to be caused by the upper level wind pattern instead of surface heating, strong to severe storms are possible even if an area is cloudy for most of the day. That said, surface heating won’t hurt things so cloud cover remains one of the wildcards.

Current modeling suggests most of the Pacific Northwest will be cloudy most of the day Wednesday but if unexpected clearing does occur thunderstorm chances would increase.

A common thunderstorm track east of the Cascades is for storms to form between Klamath Falls and Bend then move northeastward to near Pilot Rock and Walla Walla then onto the Palouse and Idaho Panhandle. If I were storm chasing tomorrow, I think my west of the Cascades target would be near Sumner with my eastern target near Ellensburg.

Weather hazards

In the United States, a severe thunderstorm is defined as having 58 mph (93 km/h) wind gusts, quarter-size hail, and/or a tornado. The criteria is similar in Canada. In addition to these, strong thunderstorms can also bring flash flooding from heavy rain and (of course) lightning strikes.

All severe weather hazards are possible on Wednesday, especially within the Sandy to Granite Falls corridor where the risk appears to be highest. In the slight risk corridor (see map above), the greatest hazard is severe hail. Elsewhere severe wind appears to have the greatest risk. There is a non-zero chance of a tornado along the west slopes of the Cascades.

The Storm Prediction Center forecast even gives the slight risk area a significant risk of severe hail, meaning there may be a few instances of hail the size of limes or larger. This is the first time they have ever issued a significant hail risk in the Pacific Northwest.

A weak short-lived landspout tornado wouldn’t surprise me somewhere within larger region during the thunderstorm development phase. Unfortunately these are difficult to predict with more precision.

With wind being the greatest hazard, people throughout the Northwest should prepare by making sure loose outdoor objects are secure. It is not uncommon for trampolines to be tossed around by thunderstorm gust fronts in the Columbia Basin. Last fall, strong winds blew loose furniture from a high-rise balcony in White Rock into a BC Hydro substation knocking out power for several hours.

Severe weather will not be widespread. A handful of severe thunderstorm warnings wouldn’t surprise me, but most people in the risk area aren’t likely to be included. Remember when thunder roars, go indoors. I have personally seen lightning strike 20 miles (32 km) from a storm.

Dry thunderstorms are not a significant risk this early in the season.

Storm timeline

This section of the article will not address whether or not specific cities will receive thunderstorm activity but is meant to paint the picture of when the best chance of storms is expected to occur.

During the mid-afternoon hours it looks like storms are likely to begin flaring between about Crater Lake and the Three Sisters. These will be expanding northward with storm activity beginning around Portland, Mount Hood, and Mount St. Helens near dinner time.

Some short-range models, such as the 12z HRRR, are particularly aggressive with storm activity during this time period in the area from Battle Ground to Packwood, including Mount St. Helens. Of course, models are never perfect so take it with a grain of salt.

That cluster of storms moves to the eastern suburbs of Tacoma and Seattle over the next few hours. They may remain discrete here, which provides the highest (but still quite low) risk of a tornado. Large hail is also possible. The storms merge together close to sunset to make the primary threat severe wind north of Everett, reaching the Lower Mainland overnight.

The view for Eastern Washington is less clear. Some energy from the Battle Ground-Packwood cluster may make it over to Yakima and Ellensburg overnight. In general, daytime activity in the Columbia Basin appears minimal but nocturnal storm activity with possibly severe wind is possible for places like Hermiston, the Tri-Cities, and Walla Walla.

An isolated storm or two could form in the Columbia Basin during the day. If so, it could become severe. This is a situation where storms are less likely before sunset but those that do form could still be quite robust.

Extended forecast

The low pressure system responsible for Wednesday’s possible thunderstorms is forecast to approach the Oregon coast on Thursday for showery conditions throughout the region. Isolated lightning strikes are possible but with cooler temperatures and lower dew points behind the cold front, a lot of the risk of stronger storms is removed.

The low lingers on into Friday with showers continuing. The Columbia Basin will likely clear out for the weekend with the I-5 Corridor, Lower Mainland, and Vancouver Island possibly seeing some sunshine on Sunday.

Active weather resumes early next week with additional rain chances and generally cool weather. Long-range models indicate a possible return of warm weather during the second week of April but confidence is low.