Monday, December 8, 2008
A Radical Weather Shift on Friday
Well, the big story is the major weather transition on Friday. I mean major. Today we have a residual Puget Sound convergence zone that is now dying with some weak showers in the mountains. This event produced a dusting at Snoqualmie Pass and several inches at Stevens. The snowpack right now is very poor, with the higher terrain only gaining about 20% of normal for this time of the year (see figure).
Tomorrow a warm front will bring showers for mainly the northern portion of the state...and some light snow to the N. Cascades about 3000 ft. This precipitation will continue on into Wednesday. Thursday will be generally dry. The big story in Friday. There will be a major reconfiguration of the large scale atmospheric pattern. The ridge over us will strengthen and move westward and we will be thrust into much colder northerly flow (see upper air chart for 7 PM on Friday night)--and a trough of lower pressure will move southward over region. We are pretty much guaranteed much, much colder air over us...and you should prepare for it. This kind of pattern makes plumbers very happy, as bursting pipes bring lots of business.
The mountains will get significant snow as the trough moves through. The question, of course, is lowland snow. I have studied this issue for a long time....and right now the flow situation is not ideal for much snow at low levels on Friday over Puget Sound. The threat is actually much higher to our south...Portland and the Willamette Valley. Anyway, no use speculating more on this until we have more confidence in the forecast...that will be Wednesday.
Posted by Cliff Mass at 9:36 AM