Major Winter Storm Ahead

December 6, 2016

5pm Tuesday…

The National Weather Service has issued a Winter Storm Watch for ALL of Western Oregon from the Coast Range to the Valleys FOR THURSDAY.

markwarningswinter-wx

So here we go folks…at this point it appears that Thursday we’ll see the biggest snow/ice event we’ve seen since early February 2014.  Yes, we’ve gone two winters without a major snow/ice event.  Although we DID have an all day icy/snow episode early last January that gave us 1″ of snow.

What’s going on?  It’s our classic snow/ice storm pattern:

  1. Cold and dry air is moving in now, and by tomorrow at this time the freezer door will be wide open.  That means east wind gusts 40-50 mph as that cold/dry air pours out of the Gorge from Eastern Washington.
  2. Thursday morning a low pressure system slides north along the coastline, pushing abundant moisture OVER THE TOP of the cold air mass in place
  3. The air mass should be cold enough to support some mix of frozen precipitation (snow or freezing rain) all day in the Metro area, Gorge, & Kelso/Longview.
  4. Likely cold enough for brief snow than freezing rain for at least the first part of the day in the Willamette Valley.

This should give you a good idea of what we’re expecting…

 

marksnow_otherareas

marksnow_otherareas2

Once cold air is stuck in the Columbia River Gorge, it’s extremely difficult to modify (warm) that air mass.  It takes several days IF we don’t get a warming westerly wind.  And as of this evening no model shows a warming west wind in the Gorge Friday.  So I think the Gorge will see the worst weather…likely blizzard conditions Thursday as the snow falls heavily.

markfreezingrain_gorge

Of course you all want to know…“how much snow am I going to get at my house?”.  The short answer is that we don’t know yet, but it’s fair to say in the metro area 1″-5″ is reasonable and a Trace-4″ in the Willamette Valley.  That’s a real rough guess.

Except for the Gorge and Cascades, that snow forecast is a real problem.  That’s because above-freezing air will be moving in up around 2,000-4,000′.  Once that occurs, then it’ll just be freezing rain and/or ice pellets until the temperature at the ground goes above freezing.  And warmer air above will arrive at different times in different parts of the valley.  So let’s say it remains as all snow all day Thursday for you…probably 5-7″ of snow!  But if the changeover happens quickly, you might only get a dusting.  Generally, the closer to the Coast Range you are (due to cold air piling up against eastern slopes) and closer to the metro area/Gorge, the better change you’ll hang on to the snow longer.  Of course models have a tough time predicting snow totals to start, but this really screws up those snow maps!  Our RPM, not usually a good performer in these east wind snow events, just shows very light totals before changeover, even in the metro area:

rpm_snowaccum

The WRF-GFS does a little better:

wrf_24hrsnow

It shows the heavier snowfall up against the Coast Range AND down into the Valley.  Supposedly Clark County gets left out…r.i.g.h.t…  See the problem here?  I guarantee the next run of the same model will look different.  It’s still almost 2 days out and we’re going to see some last-minute twists and changes in the forecast the next day or so.  There is still time to figure things out.  We do know there is plenty of moisture coming in from Thursday morning through Friday morning.  Check out the RPM rain forecast!  If it’s all snow, you can get about 10″ of snow for each 1″ of rain.  You can see the possibility in the Gorge can’t you?  Theoretically 20″ of snow could fall (and has in past snow storms there!)

rpm_12km_precip_nwor

Of course once we get into the cold air then we look for a warming south or westerly wind to bring those temps above freezing.  Now that the unreliable GFS model has come around to the ECMWF on timing (morning start Thursday), we can compare them a bit more closely.  The GFS brings a deep low up toward the NW Washington coastline, turning our wind gusty southerly Thursday evening/night.  The ECMWF barely gives us a breath of southerly wind late Thursday night and never gets rid of the easterly gradient in the Gorge!  That’s due to a much weaker low just offshore, with the main low center much farther west.  For now I’m going with a slow warmup like the more reliable ECMWF.  That means it’s still frozen in the Gorge Friday morning and we ever so slowly warm up Thursday night and Friday in the metro area.

To sum it up:

I’m quite confident that a big storm is coming for the Gorge and much of the metro area.  Thursday will be one of those days you may not make it into work or school.  Friday is a question mark…there could still be a lot of snow/ice left around.

More tomorrow!  Chief Meteorologist Mark Nelsen