Why the NW is experiencing a summer of thunderstorms, flash floods

Jul 20, 2012

"This is the summer of thunderstorms," says Cliff Mass, KPLU weather expert and meteorologist at the University of Washington.

An unusual pattern of low pressure keeps recurring over the western U.S., bringing the rain and lightning. Temperatures are about ten degrees below normal for this part of July--which is typically the sunniest part of the year in the Puget Sound area.

East of the mountains, there have been two flash floods, as of this morning.

Forced by an upper level trough, heavy convective showers are moving through western Washington this morning, but they should dry out by mid-afternoon, says Mass. Saturday will be relatively dry with morning low clouds. Listen to the conversation for details on the weekend forecast.

The real heavy weather will likely continue on the east side of the Cascades, however, bringing more flash flooding.

In his blog post on the subject, Mass writes:

East of the Cascade crest, heavy thunderstorms have caused a series of flash floods and more thunderstorms are on the way.  Eastern Washington and Oregon are quite vulnerable to such storms, and one of the great flash floods in U.S. history---resulting in the deaths of hundreds--occurred over northwest Oregon in the town of Heppner (in 1903).

Why the vulnerability? Eastern Washington and Oregon get a fair number of thunderstorms each summer and some can bring an inch or two over one hour.  Then there is the geography--lots of valley and canyons where water can concentrate and an arid landscape that has not adjusted to heavy rainfall.  Plus, folks unaccustomed to heavy rain and the proper response.

In the video below, a thunderstorm caused a flash flood north of Malott, Wash., along U.S. Highway 97 the afternoon of July 15, 2012. Water is pouring over the highway instead of through the culvert under the road. Guardrail, half the two-lane road and shoulder were washed away.

Former Seattle Post-Intelligencer editorial page editor Joann Byrd wrote a history of the Heppner flood. Below is an excerpt of the publisher's synopsis of the book:  

June 14, 1903, was a typical, hot Sunday in Heppner, a small farm town in northeastern Oregon. People went to church, ate dinner, and relaxed with family and friends. But late that afternoon, calamity struck when a violent thunderstorm brought heavy rain and hail to the mountains and bare hills south of town. When the fierce downpour reached Heppner, people gathered their children and hurried inside. Most everyone closed their doors and windows against the racket.

The thunder and pounding hail masked the sound of something they likely could not have imagined: a roaring, two-story wall of water raging toward town. Within an hour, one of every five people in the prosperous town of 1,300 would lose their lives as the floodwaters pulled apart and carried away nearly everything in their path. The center of town was devastated. Enormous drifts of debris, tangled around bodies, snaked down the valley. The telegraph was down, the railroads were out, and the mayor was in Portland.

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Do you have a weather question? Cliff Mass and Keith Seinfeld occasionally answer reader questions on the air. Share yours here (at the bottom of the page, where you can sort the questions by "newest to oldest").

The weekly KPLU feature "Weather with Cliff Mass" airs every Friday at 9 a.m. immediately following BirdNote, and repeats twice on Friday afternoons during All Things Considered. The feature is hosted by KPLU’s Science and Health reporter Keith Seinfeld. Cliff Mass is a University of Washington Professor of Atmospheric Sciences, a renowned Seattle weather prognosticator, and a popular weather blogger. You can also subscribe to a podcast of “Weather with Cliff Mass” shows.