Cliff Mass: July 29 historically ‘driest day of the year’ for Seattle before rain today

July 29 — one of the driest dates in Seattle history — surprised many within the city when raindrops started to fall. “Measurable” rain has only been recorded on this day six times at the Federal Building weather station and just four times at Seattle-Tacoma International Airport (SEA).

“Traditionally, climatologically, today is the driest day of the year,” Cliff Mass, an Atmospheric Sciences professor at the University of Washington (UW), said on “The Jason Rantz Show” on KTTH 770 AM. “And this isn’t going back 30 years. We’re talking about Seattle right now. It’s a very dry day. It rarely rains. The most rain we ever had was 0.17 inches.”

Despite the shock to some meteorologists, Mass stated the weather models had been showing this surprise rainfall for a while.

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“Our forecasting models have gotten very good now, so three or four days ahead, we kind of know what’s going on,” Mass said. “And even the week ahead, it was suggesting something we knew is common, but it’s been so dry. A very dry July and late June.”

Mass described the mild downpour as the consequence of a weak disturbance off the Pacific. High pressure is associated with warmer conditions and drier conditions, which the region had for several weeks, but a “trough of lower pressure” moved in off the Pacific Ocean.

“I think what’s really notable is how dry it is here. At the end of July and beginning of August, we were one of the driest places in the whole U.S.,” Mass added. “So we go from being one of the wettest places in the U.S. to one of the driest places. I think that’s interesting.”

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During the middle of the winter, Western Washington is in a “jet stream” of very moist air that is forced to rise in the mountains, creating a ton of moisture. During the summer, the jet stream travels north into Alaska and further north, creating high pressure and very dry conditions.

“Now the one thing we don’t have, that the rest of the country has, is thunderstorms,” Mass said. “If you’re in the central part of the U.S., the summer is pretty wet from thunderstorms. We don’t get thunderstorms.”

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