Unless you're under ‘the blue hole,’ we’re in for more rain

Oct 21, 2011

The Puget Sound region will be in for increasing rain with temperatures hanging up around the mid-50s through the weekend but letting up Sunday afternoon until Tuesday when there'll be more sun, KPLU's weatherman Cliff Mass says.

... unless you live or play under the 'blue hole' over Sequim, Port Angeles, Port Townsend – that area northeast of the Olympics.

We've all heard it's sunnier in those areas, but now we have scientific, radiation-measuring instruments that prove it: Yes, it is sunnier in Sequim than it is in Seattle or Tacoma or Bellevue ... by far. These instruments measure the amount of radiation, light and warmth from the sun, that hits the ground in the region, Mass said, and it's hitting the ground a lot there.

The phenomenon causing the "blue hole" or the "rain shadow" lets Sequim get by with around 15 to 16 inches of rain a year, while Seattle gets about 38 inches. Cities north and south of Seattle get even more.

"The reason we have this rain shadow and this sunny area is because of down-slope flow off of the Olympics," Mass says. "During most of our big weather events the winds are from the southwest, so the wind goes up the southern, southwest side of the Olympics and you get lots of clouds and rain. But as the air descends down the northeast side, the air tends to warm as it’s compressed. The clouds evaporate, and of course you get less precipitation as well."

Of course the granddaddy of all rain shadows is east of the Cascades! Once you hit Cle Elum on Interstate-90, the clouds tend to burn off. This weekend along the Columbia River, it'll be a mixture of clouds and sun, with temps in the mid-60's.

On the Web:

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 Health and Science reporter Keith Seinfeld. Cliff Mass is a University of Washington Professor of Atmospheric Sciences and renowned Seattle weather prognosticator.Check out the podcast of the show. You can also listen to a podcast of this and previous "Weather with Cliff Mass" shows.