December 27, 2010

Wednesday Lowland Snow??

Snow at the top of Seattle Hills and rain near sea level often occurs
during marginal snow situations

There has been talk for nearly a week for the potential for lowland snow on Wednesday. I will analyze the situation below, but let me make it very clear at the outset--this is a VERY, VERY different situation than November 22nd. You will not see a powerful arctic blast associated with strong high pressure in British Columbia and a major coastal low over SW Washington. Temperatures will be far more marginal. Far less icing potential. But there COULD be some interesting a chance for Puget Sound Convergence Zone snow.

A frontal system is moving through now and moderate to heavy snow is falling in the mountains. Between today and tomorrow, perhaps 1-1.5 feet in the mountains (see graphic). Good for skiers, snowboarders, and anyone who likes to play in the snow.

Cascade Mountain snowpack is running close to normal now, while the Olympics are way above normal. But this is not what you are probably interested in.

Tuesday will actually be a fairly nice day over the the lowlands, but the "action" follows later on Tuesday night into Wednesday as cooler air invades the region. For the lowlands, this air has a marine origin and thus the temperatures will be marginal for snow near sea level. This is not the primo cold air from the interior of British Columbia. Furthermore, the ground surfaces are above freezing. Here is the surface chart for 10 AM on Wednesday. The shading indicates temperature at around 1 km above the surface and below freezing is shown by white and blue colors.

A weak trough of low pressure over western Washington, but no low center near the coast and no strong high pressure area and intense cold over southern BC. Where precipitation is heavy enough, some snow showers could reach the surface, but nothing substantial. Here is the forecast 24-h snowfall ending 4 PM on Wednesday:

Snow showers over SW Washington and over the eastern Puget Sound suburbs and western Cascade slopes. Some suggestion of a Puget Sound convergence zone, with enhanced snow showers over the central Sound...but only a few light showers over Seattle. In fact, the forecast winds at 10 AM do indicated a convergence zone...see below.

The Convergence Zone is only transient and doesn't do much. If the CZ is stronger than forecast then more snow could hit the Puget Sound lowlands. However, forecast temperatures are predicted to peak near 40F on Wednesday. This looks marginal to me...only heavy precipitation and the cooling associated with it...something that is not predicted... could bring several inches of snow to Seattle.

We will continue to monitor the evolution of this event, but right now it does not look serious event near sea level. Eastern suburbs could get few inches. Not an icing situation during the day...


  1. Why isn't anyone talking about the winds tonight? By anyone I mean the weather forcasters on the news....Out here in Eatonville the winds have been sustained 15-25 gusting to 45 since about 4:00 pm. I'm not complaining...I love the wind!...I'm just surprised because I haven't heard of any winds being forcast.

  2. Speaking of snow...Click on the URL below to see a wonderful set of satellite photos showing the movement and development of the December 25-27 Major East Coast Snow Storm.

    Art Davidson

  3. Actually, this is in respect to the pineapple express. I have a special interest in it since I spend about half the year on Kauai and the other half in the San Juan Islands. I am a pilot and often have to fly through the results.

    My question is about its origins down in the ITCZ. The present configuration is fairly typical. At about 10° latitude huge bubbles of warm air mass start drifting north across the islands. My question is how does this process get started in such a localized area. East and West of this location for a 1000 miles everything is relatively quiet. So what triggers off this process to get it going? Once it starts I can see how the whole process works.

  4. Impressive radar returns today from portland, medford, and especially eureka. Cool to see it rollin far out in the ocean. Soon that will be available to us I hear.

  5. Yay! It snowed! Only about an inch but everything is covered...for now...It's already 32 and I can hear it dripping. I hope it hangs around long enough for the kids to build a snowman! And I hope we get more! Guess I'll wait until later in the week to run those errands!


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