After two days of unseasonably warm weather, the Pacific Northwest faces a threat of isolated severe thunderstorms Wednesday that could unleash lightning, large hail, damaging winds, heavy downpours of rain and maybe even spawn a brief tornado.
The Storm Prediction Center issued what the National Weather Service calls a level-two threat rating for the Interstate 5 corridor that straddles Western Washington and Oregon and includes Seattle and Portland. Other areas of the states and portions of Idaho are in a slightly lower threat level.
While the risk may be only slight, it’s significant in a region where severe thunderstorms are rare.
“This is about as high-end of an outlook as we typically get here in the Pacific Northwest, only once or less per year west of the Cascades,” said Lee Picard, a meteorologist with the Weather Service office in Portland.
Chance for thunderstorms comes after warm spell.
Thunderstorms can form when warm, moist air clashes with cooler air, something that rarely happens in the Pacific Northwest, which is dominated by cooler air coming off the Pacific Ocean and doesn’t have a source of warm, moist air. Thunderstorms are far more common across the Southern states, which see a consistent flow of warm water vapor from the Gulf.
But all the ingredients are coming together on Wednesday: The Pacific Northwest has been experiencing unusually warm temperatures in the 70s this week amid a streak of springtime heat along the West Coast. Warm air near the ground will collide with a cool, wet storm on Wednesday, creating unstable conditions in the atmosphere that can generate thunderstorms.
Thunderstorms are considered severe when they produce winds of 58 miles per hour or greater and hail at least one inch in diameter. Winds with the storms on Wednesday could reach 60 m.p.h. and hail could measure up to two and a half inches in diameter. The threat is highest in the afternoon and evening.
“There aren’t too many events where we’re in the running to have hail the size of quarters, to even bigger,” said Colby Neuman, a meteorologist with the Weather Service. “There is a chance hail could be the size of golf balls with a storm or two.”
The last time the Pacific Northwest saw significant large hail was in August 2014, when a few thunderstorms produced dime-to-nickel-size hail. One dropped ice as big as golf balls.
Tornadoes are even less common than thunderstorms in the Pacific Northwest. Over the last decade, weak ones have occurred about two or three times a year in Washington and Oregon, meteorologists said. They’re usually a zero, the lowest level on the Enhanced Fujita scale, which runs up to five. On Dec. 18, 2018, an EF2 struck near the Seattle suburb of Port Orchard. Even more notable, on April 5, 1972, an EF3 ripped through Vancouver, Wash., damaging homes and killing six people.
On Wednesday, there’s about a 2 percent chance of a brief tornado forming, forecasters said. “I think it’s important to say there is a small threat and people should be prepared and know how to respond,” Mr. Neuman said. The Weather Service advises people to go to their basement or an interior room of their home if tornado warnings are issued.
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