Weather with Cliff Mass
9:12 am
Fri March 15, 2013

Thank the rain shadow, get ready for Gore-Tex weekend

Expect more of what we've been seeing all week — "clouds, showers, breaks," says KPLU weather expert Cliff Mass, a professor of atmospheric science at the University of  Washington.

"Wear Gore-Tex or some raincoat," says Mass, adding, "There will be plenty of breaks."

The temperatures will remain mild until Saturday evening when colder air will head into the region. The new front will bring some welcome snow to Cascade ski areas, says Mass.

"The freezing level will drop from 4,000 to 5,000 feet, to 1,000 – 1,500 feet," he says. "They can easily get 2 to 6 inches."

Why relatively dry? Thank the rain shadow

Rivers were flooding around Western Washington, but the Seattle area remained much drier. Why?

Thank the rain shadow, says Mass.

What, exactly, is a rain shadow?

It typically occurs near mountains. The peaks act as a barrier against moist air, forcing the air to rise and form a cold front, chilling the windward side of the mountain and shielding the other.

“When the air is coming down off the mountains, that’s where you tend to have a rain shadow,” says Mass.  “The air tends to be warm, the relative humidities drop and you tend to have relatively dry conditions.”

Some of these rain shadows are permanent.

“But the interesting thing is sometimes these rain shadows move,” says Mass, adding it happens when the wind direction changes.

“So the rain shadow that’s normally giving those dry skies over Sequim and Whidbey Island moves down over us. And so we get dry. And that’s why we were so dry this week,” he says.

During the peak of the rain shadow, Mass says, the Puget Sound area only saw “a few hundredth of an inch of rain.”

So, in sum: thank you, rain shadow! 

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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 weekly podcast of “Weather with Cliff Mass” interviews.