December 11, 2009

Quick Update

Latest models are in. Some of you will see snow. But no big snowstorm that will last for days.

The first with snow will be the northern Willamette Valley and Portland environs, which should catch some light snow from precipitation moving up from the south. It won't get north of Centralia.

Then on Sunday an upper trough will move through bringing a surge of cooler air from the north and snow showers..mainly over the northern side of the state. There will be considerable rain (snow) shadowing over the lowlands south of Seattle, but Puget Sound convergence snow and other snowbands will be found over the northern half of western WA. Seattle should be prepared and SDOT should be particularly staged north of the ship canal. Wherever the convergence zone band sets up could get several inches.

Then there is a break early Monday, but then a warm front will approach during the afternoon. The cool air will hold in a while, particularly over NW WA, and there will be some initial snow--perhaps several inches in some locations---before it all turns to rain on Tuesday. Warmer air will move in on Tuesday and life will be back to normal. LOTS of snow in the mountains and eastern Washington on Tuesday.

This is a very difficult forecast with the snow hardly being uniform over the region. There is uncertainty...but the above is my best shot. We are not talking about a major snowstorm!

PS: I will be giving a public lecture--"The Secrets of Snow"--at Third Place Books in Lake Forest Park on December 17th at 7 PM, followed by a book signing. In this talk I will describe the unusual circumstances that have to come together for us to get a big snowstorm, details of convergence zone snows, the Fraser River Gap flow, arctic fronts, and more. My favorite topic.


  1. What's a trough, Cliff? Keep up the good work!

  2. Thanks for this forecast. Sounds like at 300' feet elevation in Bellevue, we should count on maybe a flurry and then some rain. Yawn.

    We'll still read you EVERY day, Cliff, even though you're not delivering on that "crazy weather" excitement! LOL

  3. Thanks for tackling what's turned out to be a very hard forecast. Last year it seemed as if there was good model agreement on the cold and snow, for the most part, but this one has been tricky.

    I live in the heart of the convergence zone and picking up a few inches over Sunday and Monday sounds like fun, not enough to delay work, but pretty if it happens. My son's also just old enough to go out and play in it a bit.

  4. Seems like a good shot Cliff. Let it ride!!

  5. Hey Cliff, I've often heard you and other meteorologists predict snow "north of the ship canal." What's up with that?

    Is there a true micro-climate phenomenon that applies to Ballard, View Ridge, and Shoreline, or is that phrase just a handy way of saying "north Seattle?"

    BTW, must be great to work on weather computer models for an area where a 10 mile difference can make a huge difference in the amount of precipitation one gets. I grew up in the South where ones entire State was either hot, or cool, or wet, or dry, and you could see the changes coming days and days in advance.

  6. Hey, Cliff-

    Are we looking at a rain-on-snow event next week in the mountains, or will it be mostly snow up there?

  7. Kelly,

    I'm obviously not Cliff, but to answer your question, "north of the Ship Canal" is used often in Seattle weather-speak because that is where the best chances for a Puget Sound Covergence Zone to form are. The Zone typically sets up between Everett and North Seattle, and usually fizzles as it sinks south of the Ship Canal/State Route 520, leaving areas to the south snow (or rain) free. (The area from SR 520 south to SeaTac airport is sometimes referred to by snow lovers as the "snow hole" of Puget Sound).

  8. Sounds like the Jingle Bell Run at Westlake is going to be cold and wet. Brrr!


  10. I love it! I think we ought to have "Snow Forecasting" as a gambling event and the proceeds would go for a new west coast radar system ;)
    Ok, $2 say's north Marysville will see less than 2" by Monday night.

  11. 4-6 inches in Whatcom County. I will raise your bet to $10

  12. sigh

    I detest the shield Sammamish is under at the moment. Radar shows us right in the empty spot.




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