Rare weather alert issued for Seattle, Portland, Vancouver and Salem

The Storm Prediction Center (SPC) has issued a level 2 out of 5, or Slight Risk, for severe thunderstorms in parts of the Pacific Northwest from Wednesday afternoon through Thursday morning, March 26–27. The peak threat is from Wednesday afternoon into the evening, with some residual activity that could linger past midnight.

This unusual weather pattern is capable of producing strong winds, large hail, and even isolated tornadoes.

This is particularly rare situation as such weather conditions are generally not expected in this region.

Historical SPC data shows severe weather outlooks in this region are more typical in late spring or summer, if at all, due to the need for sufficient warmth, moisture, and instability—conditions this event unusually meets due to a potent eastern Pacific trough/low approaching the coast.

A strong upper-level low-pressure system and jet stream are moving toward the Oregon coast. These features are expected to increase atmospheric lift and moisture, while cold air aloft will contribute to instability favorable for thunderstorm development.

Forecasts indicate high Convective Available Potential Energy (CAPE) and favorable wind shear, both supportive of supercell thunderstorm development. Storms are expected to initiate in western Oregon in the late afternoon or early evening on Wednesday, then move north-northeast into Washington.

Storm impacts are forecast to begin between 14:00 and 15:00 PDT (21:00–22:00 UTC) on Wednesday, with hail posing the greatest threat. The Slight Risk zone spans 69 700 km² (26 910 mi²), affecting a population of 7 286 713. The most populous areas within this zone include Seattle, Portland, Tacoma, Vancouver (Washington), and Salem (Oregon).

Meteorological outlook

A strong upper-level low-pressure system off the Pacific Northwest coast is moving eastward. A powerful jet stream associated with the system, with winds of approximately 185 km/h (115 mph) at the 500 hPa level, is expected to approach the Oregon coast late Wednesday afternoon.

As the system moves inland, it is expected to trigger strong upward motion, leading to rising air and the development of thunderstorms.

An associated cold front will advance inland from the Pacific Ocean, approaching the Oregon and Washington coastlines. Surface moisture, with dew points around 13 °C (mid-50s °F), will be sufficient for thunderstorm development, although overall moisture content is not particularly high, with precipitable water values near 25 mm (1 inch).

Cold air aloft, combined with surface heating, will create a strong temperature gradient (steep lapse rates), resulting in atmospheric instability. CAPE is forecast to reach approximately 1 000 J/kg, which is supportive of strong thunderstorm updrafts.

Strong wind shear will support the development of rotating supercells, increasing the potential for severe weather across the Pacific Northwest.

References:

¹ Mar 26, 2025 0600 UTC Day 1 Convective Outlook – NWS/SPC – March 26, 2025