January 01, 2010

Rainshadow and Year in Review

When you look at the local weather radar imagery from Camano Island (like above), one is struck by the frequency and importance of rainshadowing in the lee of the Olympics and mountains of Vancouver Island. Tonight the winds are moist and from the southwest approaching the Olympics and thus the rainshadow...where air is sinking down the slopes of the barrier...is on the NE side. So if you are on northern Whidbey Island or the NE side of the Olympic Peninsula you are really dry right now, while moderate rain is falling a dozen or so miles away. Wait till we have the new coastal radar--we will really be able to see the whole picture!

By the way, there is plenty of misinformation floating about regarding rainshadows....it is the sinking that does it, not the loss of moisture on the windward side. I spend a lot of time on rainshadow effects in my book for a good reason...we live in rainshadow country!

The image below shows the temperature at Sea-Tac Airport and normal highs and lows for the past four weeks. Today was substantially warmer than normal in the strong SW flow. The cold spell of early December is evident, followed by above normal temps of mid-December. December turned out to be substantially drier than normal at Sea-Tac: 2.75 inches, 2.87 inches below normal. The month ended up colder than normal due to the frigid period early in the month...the mean was roughly 3F below normal.

The dry conditions are having a significant influence on the snowpack, which is now generally 75-85% of normal. And let be honest...being a moderate El Nino year I find it hard to optimistic about mountain snows...and the chances of lowland snow are far less than normal.

My colleagues at the National Weather Service have put together a nice end of year view of 2009 weather events. If you are interested, check it out at:



  1. Good explanation of rainshadow effects. The Year in Review is interesting as I had forgotten some of those events...like the rare Tornado. I reviewed the data for 2009 here Moclips and the total rainfall was 76.20 inches. The high temp was 89F and low temp 16F. High recorded windspeed was only 58mph this year.

  2. Happy New Year, everyone!

    I'm enjoying the warm temps and the 31 mph gusts here in East Bellevue. My car's trunk leaks so I'm thrilled with dry weather. LOL

    Cliff, I just finished re-reading your book... I enjoyed reading it last year right after Christmas, but this year, having watched a year of our weather with that knowledge in my brain, many parts of the book made more sense to me.

    Thanks for all you do. It is much appreciated!

  3. You commented on El Nino effects on precip/snowpack. Where are we re: La Nina? The latest JISAO graphic ends with a warm blip in September. Are we presently warm or cool?


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