January 26, 2009

Rainshadow and snow/rain

I will update in more depth later...but another Pacific disturbance is approaching (see figure) and we will get precipitation...the question is the how much and whether it will be rain or snow. Because the disturbance is moving towards us from the northwest, there will be a strong westerly component as is approaches and moves over us. Westerly flow produces a rainshadow over the central Puget Sound region...as demonstrated last month. The air over us is cold and dry, dry enough the there will be lots of initial evaporative cooling as the precipitation falls into it from above. Evaporative cooling only acts for a short while..until the air becomes saturated. And then the movement of warm air into the region associated with the system will take over and turn the precipitation into rain.

The current run of the high-resolution local model provides the following 24-h snowfall ending 4 PM tomorrow (see figure). The mountains do well (6-18 inches) and light snow is found over portions of NW and SW Washington, and over the Kitsap/Hood Canal area. But little over Seattle. By mid-day all the precipitation over the lowlands should turn to rain. Strong winds will also occur over Puget Sound and NW Washington as low pressure moves north of us tomorrow morning and afternoon.

Here is the latest 24-h snow from the Monday night run...but from the coarser 12-km resolution version. Pretty much the same story.


  1. So Seattle should see no snow at all?

    If any, when do you think it will start?

  2. I think that we'll just get snow in the morning, which will shift over to rain in the afternoon. Just a prediction.

  3. It will be fun to watch the rainshadow develop. I am feeling done with the really cold precip for this winter! The mountains can have it, that's fine by me. :-)

  4. No way does Seattle get any sort of accumulating snow tomorrow.

    Best case scenario: snow flurries around 8 a.m. (similar to what we saw yesterday).

    Worst case (and most likely) scenario: cloudy with light rain through mid-morning.

    The Convergence Zone looks like it won't even bring snow to 500 feet.

    Still, it's the snowiest winter at SeaTac since 1985-86.

  5. It's not snow, it is the white stuff! Where's Danger Jim?

  6. It looks like Olympia is right on the edge of between 1 - 2 1/2 inches. Any ideas of if that might shift the 2 1/2 over into the Olympia area?

    I love snow.

  7. My very non-scientific hunch based on years of experience here in Sammamish is that we'll get 2 inches tomorrow morning here, and there'll be no snow anywhere else


  8. Hey, another letdown courtesy of the stupid mountains. Awesome!

  9. 30f outside in lake stevens right now. I do expect some snow to fall over night and into the morning, but don't believe it'll stick much or accumulate much at least.

  10. Olalla: A sunny but chilly day here today, the kind of day on which frost only disappears in those places where the sun shone. The little bit of snow that came yesterday morning is still there in shady spots.

    I'm curious how much snow will accumulate before it changes to rain. It seems that the relative humidity is quite low.

    - Pete

  11. Cliff (or anyone else) - where can we find the precip forecast maps that you used as the images for today?

  12. Use this address to view forecast maps - http://www.atmos.washington.edu/mm5rt/

  13. Boooooooo, nothing up in Sammamish. Booooooooooooooooo.


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