Weathercatch: Why thunderstorms are uncommon in Pacific Northwest

Last year on May 29, a thunderstorm rolled into parts of Eastern Washington and North Idaho causing sporadic power outages and several reports of minor hail damage to cars. Though the storm caused no significant or widespread damage, the event was big news for residents who witnessed it.

Why? Because thunderstorms rarely occur in the Pacific Northwest.

Loud rumbles and spectacular lightning bolts get far more notice in Spokane than in Chicago or Miami. Where the Windy City gets an average of 39 thunderstorms each year, the Lilac City gets 11.

On multiple days this May, severe thunderstorms struck from Tennessee to Texas and tornadoes swept parts of Oklahoma and Colorado. In comparison, shorted-lived, feeble-sounding thunder rumbled over the Inland Northwest on May 4.

In the central and eastern U.S., all the right conditions can easily align to create thunderstorms. But not so in the Pacific Northwest – a storm-chaser’s paradise it’s not.

A key ingredient missing here is humidity, or plenty of water vapor in the low levels of the atmosphere. Unlike the warm Gulf of Mexico, air blowing off the chilly Pacific waters is cool. Because cooler air near the ground provides a relatively small amount of vertical temperature differences, the air is less likely to rush upward and produce thunderclouds. Also, cooler air holds less moisture than warm air. A lower amount of moisture means there’s less fuel to kick-start storms.

High-pressure systems that anchor themselves off the West Coast in late spring through summer also hinder the development of thunderstorms in the region.

On those rare occasions when churning storms do get served up in Eastern Washington, chances are they’re the result of warm, moist and unstable air flowing from the south.

So while other parts of the country are breeding grounds for thunderstorms, that’s not the case here. Thanks to cool water and dry air, jarring kabooms and high-voltage skies aren’t ordinary, they’re extraordinary.

Nic Loyd is a meteorologist with WSU’s AgWeatherNet. Linda Weiford is a WSU newswriter and weather geek. Contact: linda.weiford@wsu.edu.