Significant Chance of Lowland Snow Late Sunday and Monday

Note:  I have updated a number of the model runs with the Saturday morning updates.....same story.
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Here we go again.  Another difficult to forecast snow event, with temperature sclose to the margin over the western WA lowlands.  A situation where a number of our models suggest several inches over much of the lowlands west of the Cascades, with those poor suffering folks in Portland getting hit the hardest.

To "warm up", here are the forecast snowfall totals for the 24h ending 4 PM Sunday and 4 PM Tuesday from the UW WRF system . On Sunday there is LOTS of snow over the southern WA and northern WA Cascades....feet.

 Monday brings a huge snow dump over western Oregon...Portland would be hit hard if this is correct.

 For the next day the snow moves northward into Washington.

The snow around Puget Sound begins late Sunday and early Monday in the UW model forecasts.   The situation late Sunday is close to the canonical snow set-up, but not ideal.  As shown by the forecast surface map, very cold air and high pressure is over British Columbia, and a low center is just off of the Columbia River bars.  Such a pattern helps drive cold air into western WA.

At the same time an upper trough is approaching from the west, providing upward motion that drives clouds and precipitation.  This trough extends a bit too southward to optimize snow over western WA:  the strongest lift is over western Oregon at this time.

The above is just ONE solution and there is considerable uncertainty.  We can get an idea of the amount of uncertainty from our ensemble (many forecast) prediction systems.

The U.S. global ensemble (GEFS) snow prediction for Seattle is shown below with the average indicated by the black line and the high resolution GFS member by blue.   The ensemble mean shows a lot of uncertainty in the forecasts (big range of possibilities with the average getting to around 3-4 inches and a range from zero to 8 inches). Snow starts late Sunday and continues for several days.

Another (and even better) ensemble produced by the European Center shows a wide range of possibilities at Seattle, with an average around 7 inches.  About 20% of the ensembles have little to 2 inches of snow, but some get to a foot.

The latest European Center high resolution forecast for the snowfall for the 60h ending 4 PM Monday shows considerable snow inches to a foot between Portland and Seattle. A large north-south snow gradient over Seattle, with MUCH more on the south side of the city.

Why is there substantial uncertainty?  Because the air temperatures are marginal for snow and the models have variations in the forecast details.

Keep tuned...the uncertainty should decline during the next day.  But it looks like the most likely forecast is 2-5 inches of snow over Seattle, with about 20% chance of little snow or over 6 inches.

Precipitation Returns to the Northwest

Potential Snow Even On Monday over the Lowlands:  More Later in a New Blog
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The last week has been relatively dry and cool, but the circulation pattern over the West Coast is about to change as high pressure aloft is replaced by moderate, moist southwesterly flow.

Thursday will be the last dry day for Washington, with a line up of increasingly wet storms invading the State starting Friday.  The forecast for the 72h precipitation ending 4 PM Sunday (see below) says it all: WET, with some of the mountain areas getting 2-5 inches, with more over northern CA.

The big concern right now is about freezing rain and snow in the Columbia Gorge and Portland.   In many ways, the situation is FAR less threatening than in early/mid January and less ominous than feared earlier this week.  The temperatures are much warmer around Portland this time and it is clear that substantially warmer air will invade on Friday, making an extended wintry period unlikely in the Rose City.  Eastern WA will have snow for sure.

The situation at 10 PM Wednesday is shown by a sea level pressure forecast, with low-level air temperature shown as well.  Blue is cold air.     Frigid air is now over eastern Washington, with just moderately below normal temperatures over western WA.  A large pressure difference across the Cascades is producing strong winds in the Columbia Gorge and parts of Portland.

The observations at 8 PM (temp is black, gusts in red), show mid-30s in Portland and teens in eastern WA.  Cold enough to snow.

Tomorrow, a very weak warm front will move northward over Oregon (see forecast precipitation for the 3-h ending 4 PM Thursday).  There could be some very light snow to start, and as warm air surges in aloft, some light freezing drizzle.

 The next pulse of precipitation will come in Friday morning (see forecast at 4 AM) and by that time the temperatures will be above freezing aloft.  Thus, the threat of snow will be over.  There will be a chance of freezing rain over the western side of Gorge into Friday afternoon, after which it will warm sufficiently to be rain even there.

After than, one wet/warm system after another will hit our region.   The precipitation totals forecast by the NWS GFS system for the next 180 h (7.5 days) is amazing (see below), with the West Coast for central CA northward flooded by heavy precipitation.  Temperatures will be cool enough for massive snows in the mountains.  

To illustrate, here is the prediction 72h snowfall ending 4 AM Sunday.  Nearly three feet at higher elevations in the Cascades and more snow over eastern WA.  Lots in the Rockies as well.

As I have said in an earlier blog, the West Coast drought is effectively over.