Gorge Freezing Rain & Snow On The Way This Weekend

December 5, 2018

7pm Wednesday…

It’s been another spectacularly sunny day west of the Cascades under blue sky.  Check out this morning’s GOES-17 image.  If you click on the image (larger view) you can even see Wizard Island in Crater Lake!

Capture

The most important feature for a meteorologist in our area is the large pool of cloud cover below about 4,000′ east of the Cascades.  It fills just about all the Columbia Basin of Eastern Washington and northern Oregon.  That’s the “cold pool” we often talk about in the wintertime.  Cold air is piled up over there.  There is just one little gap through the Cascades where all that chilly air is able to move west of the mountains at sea-level; the Columbia River Gorge.  East wind strengthened today and flowed right over the top of the Cascades too.  The peak gust of 46 mph at PDX around noon was the strongest in a year and a half!  That also gave us our coldest daytime high of this early winter season so far.  That wind strengthens in the Gorge but SHOULD die down a bit in much of the metro area through Friday as a weather system (with its lower pressure) moves toward the coastline.  The return of moisture over the top of cold air in place sets us up for a freezing rain/snow event in the Columbia River Gorge beginning Friday night.

I’ve seen this setup many times.  This isn’t going to be “the big snow/ice storm” out there, but enough to impact your plans if you live east of Troutdale/Camas.

KEY POINTS

  • Cold east wind continues to blow through Sunday night in the western Gorge and east metro area through the weekend
  • Temperatures cool a few more degrees tomorrow & Friday
  • Freezing rain (liquid rain freezing on contact) is likely sometime toward midnight Friday night in the western Gorge (Corbett/Cape Horn to Bonneville). Roads should be very icy within an hour of that rain arrival in those areas.  That may include I-84 assuming temperatures are cold enough at river level.
  • Light snowfall is likely during that time in central/eastern Gorge.  That’s from Cascade Locks to The Dalles in the colder/deeper airmass over there.  Light is the key word, a dusting to 2″ max.  Not a lot of moisture to work with in this first weather system.
  • Saturday is pretty much a dry day…just breezy and chilly again.
  • Heavier freezing rain, possibly some snow eastern Gorge is possible late Saturday night and Sunday.  Could be a significant ice storm in the Central Gorge (Cascade Locks/Stevenson/Hood River) at that time.  I’m not confident it’ll still be cold enough west of Multnomah Falls after a dry day Saturday.  Also some models are holding the moisture off to the west until Sunday morning. We’ll see.
  • The Portland Metro Area will be too “warm” for freezing rain.  But Troutdale/Washougal areas will be very close temperature-wise; a 33-35 degree rain at the coldest point Friday night.

West wind and a general thaw shows up on Monday in the Gorge as a cold front arrives, disrupting the trapped low-level cold airmass eastside and finishing our first “Gorge Event” of the season.

WHY?

  1. COOLING AIRMASS EASTSIDE

The airmass east of the Cascades has cooled the past 24 hours.  By about 3-4 degrees.  More important, the airmass is drier coming through the Gorge.  When precipitation falls into the dry airmass the temperature drops due to evaporative cooling.

The Hood River sounding from WRF-GFS is all below freezing at 4am Saturday = snow in that location, not freezing rain.  Temps are in Celsius along bottom line.

hdrvr.72.0000.snd

EASTERLY FLOW CONTINUES THROUGH WEEKEND

This is a classic setup with a strong surface high pressure stuck east of the Cascades and no low pressure system set to sweep across the region to “knock it down”.  The cool low-level airmass we have in place over us now isn’t going anywhere until Monday.

GOOD MODEL AGREEMENT

The ECMWF and GEM models were first to latch onto a weak system coming in Friday night, then GFS played catch-up.  They all agree the first system has very little moisture to work with but 2nd is wetter.   Notice the light snow by Saturday morning central/eastern Gorge on the WRF-GFS snow forecast.  You can ignore the snow it shows in the western Gorge; the model algorithm paints “snow” anywhere precipitation falls and it’s below freezing under 1,000′ elevation.  But that would be freezing rain in this case.

or_snow72.84.0000

See the 12z ECMWF showing a significant freezing rain event for the western/central Gorge by Sunday PM.

ECMWF Ice Accumulation

It’s only Wednesday evening so some things could still change, but the general sequence of weather events appears to be locked in for this coming weekend.

If you live in the Gorge there isn’t a big snow/ice storm coming but roads could be slippery at times Friday night through Sunday midday.

More tomorrow…

Chief Meteorologist Mark Nelsen