March 06, 2009

Complex situation

This is the kind of forecast where one can make big errors because there are so many degrees of freedom and things happening at once. So lets start with what we are sure about.

We are fairly certain that:

(1) a cold front is heading for us and will cross western Washington between 4 and 7 AM.
(2) some light precipitation will accompany the front
(3) that much colder air is behind the front the freezing level will plummet. The snow level will decline to only a few hundred meters above the surface very quickly and by the end of the day will reach sea level.
(4) that the mountains will get more heavy least a foot.
(5) the the freezing level is sufficiently low that the high eastern suburbs should get more snow than the lowlands near Puget Sound
(6) that the air will get colder on Sunday and will hold in until Tuesday
(7) that outflow will develop in Fraser River Valley, push into Bellingham, and give upslope snow on the ne side of the Olympics
(8) that there will be substantial rain/snow shadowing due to the Olympics
(9) that a Puget Sound convergence zone will form sometime tomorrow and produce moderate snow in the eastern suburbs and quite possibly down to sea level.
(10) that outside the convergence zone and the upslope on the mountains, precipitation will be generally absent or light after the front goes through in the central Puget Sound region.

You see, we know quite a lot. But there is plenty we don't know...when and to what degree will the convergence zone set up? Will it be cold enough for snow near sea level if the convergence zone sets up early. Those are the the million dollar questions. So you got to be ready for the white stuff. Or not.

My own take is that the convergence zone will set up by 9-11 AM over central Puget Sound and that there will be snow above 200-300 ft, with wet snow/rain down to sea level. The convergence zone will hand around for much of the day...although it could shift in position and intensity with time. Later in the day and Sunday AM, the convergence zone will shift north over Snohomish County. Anyway, the position and strength of the convergence zone is a hard keep this uncertainty in mind. Watch the weather radar in the morning to see what happens in real time.

If anyone is interested, KCTS-channel 9 in Seattle-- will replay my weather program at 1 PM. And if you want to help a good cause, you might make a pledge to keep public TV going.


  1. For when it's replaying, is that Saturday? Just asking since no date is given.

    Can you post a new snowfall forecast map :S

  2. Well, I have one more thing to add to the 'we know' list.

    (11) We know that we are glad to have the radar back for this event.


  3. We know that we WILL see snow in Sammamish.

    Thanks for mentioning the "East Side". Our area needs its own TV station, or at least its own news/weather hour. The current ones are too Seattle-centric.

  4. Me again. I'd love to see a map of current conditions. I look at the DOT cameras and can see a clear delineation. North of I90, it's pouring rain. South of I90, it's ... NOT!. It would be fun to see that on a map.

    For awhile we (Sammamish) were dry, but now the rain has moved in.

  5. Started off as a rain snow mix on Union Hill in Redmond (600') but now it is just plain old snowing. As Cliff said last time, "It's Showtime."

  6. Mixed rain and wet snow falling here in Fremont near sea level.

  7. Here 580 feet in Sammamish, it's slushing. I'm thinking of grabbing a glass, some cherry flavoring (and my raincoat) and going outside for a slushy.

  8. Replying to Josh (and going beyond...)

    (1) Yes, the next rebroadcast is today (Saturday) 1:00-2:30 PST;

    (2) it is also being rebroadcast 4 more times in March and 4 or 5 times in April, as a major feature of the KCTS9 Spring Pledge Drive; and

    (3) better yet, all of us who have Comcast On Demand can watch or tape or CDR it ANY TIME, ANY DAY, at least until April 23: check the KCTS9 website for details (or I could post this later as a followup comment -- I emailed this to Cliff yesterday...

    Bob Moore Cliff fan

  9. Snowflakes were falling at 7:15 A.M. at ~500 feet near Novelty Hill Road. The warm ground prevented accumulation. Snow fall stopped about 7:45.

  10. A mix of mist, and big snow clots in Alki at 200'. All snow is melting. This is convergence snow.

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  12. Nothing here yet in Eatonville. It's 8:31 am, 37.2 degrees (down from 40), dewpoint of 33, SW "breeze" of 8. I can even see some blue sky peaking through the clouds, which are moving at a pretty good clip! We still have some snow left over from Thursday (in the shady spots). Hoping to get a bunch today/tonight/tomorrow to say good bye to winter! A little wind would be fun too! Stay warm, dry, and safe everyone! Enjoy!

  13. Lake Stevens
    Had a good bout of rain this morning and now the wind has picked up pretty good, 15 to 20mph from the west (which is unusual, most of our wind comes from the east).

    It's dry now though, sitting around mid 30's.

    No clue what the rest of today has in store for us up here, but I'm hoping for snow!


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