Friday, December 19, 2008
This weekend storm
I have waited to write this until I have taken a careful look at all the observations and model output. The storm coming in Saturday evening and Sunday morning is potentially much more disruptive than Wednesday's event...and includes both snow and wind.
But before I talk about it how about some fun?...take a look at the latest satellite picture (attached). You see the narrow cloud band over the Sound? That is being forced by a convergence of land breezes on both sides of the Sound over the water. Land breezes occur when the land is colder than the water and the air flows from land to water. Last night with snow on the ground, the land was colder than the relatively warm Puget Sound. The air converges and rises forcing the land breeze convergence zone cloud band. An image from a webcam at the UW shows it (see pic) and if you really like this kind of thing...look at the video (http://www.atmos.washington.edu/cgi-bin/latest.cgi?webcam0_movies). I explain such features in some depth in my book.
Ok, lets get serious. An energetic and wet Pacific weather system is now approaching our coast and will start impacting us on Saturday...particularly later in the day. It will bring a low pressure trough to our coast and will force moist, warmer air above us...producing clouds and precipitation. As pressures fall along the coast on Saturday due to the approaching system the pressure difference across the Cascades will greatly intensify (low pressure west, high pressure associated with the colder air east) and southeasterly winds will increase near crest level. As a result, winds will accelerate over the Cascades, particularly across lower areas in the mountains. The result--very strong easterly winds along the western slopes of the Cascades, particularly around Enumclaw, Black Diamond, North Bend, and Maple Valley. The winds could easily gust to 40 to 80 mph, with serious potential for loss of tree branches, falling trees, and power outages. The winds will start accelerating tomorrow morning and will be powerful by afternoon and peak during the early morning hours Sunday (see graphics for 4 AM Sunday) Weaker easterly winds...but still notable.. will extend towards the Sound.
But the winds are only a warm up. Now lets talk about snow.
The precipitation shield from the incoming system (actually its warm front) will reach the Puget Sound lowlands around dinner time. We will have sufficiently cold air for this to fall as snow. There should be a huge east-west difference in the amount of snow that falls tomorrow night and Sunday morning. The easterly downslope flow causes drying. On the other hand the easterly flow will rise as it approaches the Olympics--producing greatly enhanced snow. As are result, I would expect a huge snowfall gradient by say 4 AM on Sunday AM--with the western Cascade slopes and far eastern suburbs only getting a few inches of snow. (see the graphics of 24-h snowfall from the latest high resolution forecast model). Seattle and west shore of Puget Sound communities should receive 2-6 inches, and the Kitsap hit by 1-2 FEET. If you are living west of the Sound this will be serious. Better buy some food and be prepared to hunker down. The heavy snow will continue there until at least 9 AM Sunday.
But there is so much more.... This includes powerful southeasterlies in the western portion of the Strait that will develop tomorrow evening...with winds gusting to 50-75 mph and substantial snow at the western exit of the Strait. Lots of snow in the mountains--at least a foot. And the potential for freezing rain south of Tacoma and over SW Washington late in the event as warm air moving in aloft causes a layer of air to be above freezing.
This is classic active weather situation where numerous major local weather features occur at once...the complexity caused by our local terrain...that is why we love it here!
Posted by Cliff Mass at 9:13 AM