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.

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.

Thank you for your patience while we verify access. If you are in Reader mode please exit and log into your Times account, or subscribe for all of The Times.

Thank you for your patience while we verify access.

Already a subscriber? Log in.

Want all of The Times? Subscribe.