Christmas Weekend Outlook: Who Gets The Snow?

December 22, 2017

9am Friday

Now that the threat for freezing rain has ended since the lowest elevations are near/above freezing, let’s talk snow.

BIG PICTURE:  This weekend we are in a “battleground” between 1) a cold arctic airmass over the Rockies and east of the Cascades and 2) Pacific moisture coming in from the west and southwest.  Models always struggle with what goes on in this battleground or major clash between airmasses even though they are much better than 20+ years ago.   This is always a tough forecast; but I’ve been through LOTS of these events!  This is my 27th winter forecasting in the Portland area…wow.  This setup is the #1 way we get significant (or sometimes major) ice/snow events in the metro area.  Sure it’s the SETUP, but everything has to be just right (or wrong if you don’t like winter) to get a snow or ice storm here.  And it’s even more rare to get this setup right at Christmas!

HIGHLIGHTS

  • It’s unlikely anything frozen falls the next few days on the Oregon Coast…a bit too mild this time around.
  • We only expect some snowflakes mixed in with the showers TODAY in the lowlands.  No sticking snow in the lowest elevations in the Portland/Vancouver metro area.
  • Hills at/above 1,500′ could see an inch or two, but that’s way up in northern Clark county or other foothills of Cascades/Coast Range.
  • Saturday will be a dry or mainly dry day as cold/dry east wind arrives, bringing chilly air out of the Gorge.  Highs up around 40 as we’ve been forecasting.
  • Sunday is the #1 question for nowI see the possibility of either brief freezing rain early in the day and/or a real snowfall in at least parts of the metro area by afternoon/evening.  Yes, as of now a White Christmas is a possibility in the Portland metro area.
  • Sunday south of the metro area (Woodburn to Eugene) and on the coast should be uneventful.  Likely too “warm” for either freezing rain or snow.
  • Christmas Day:  All models agree we dry out Christmas, it’ll just be a matter of whether we have leftover snow/ice on roads that day or we don’t get any to start with.

GORGE HIGHLIGHTS

  • The central/eastern Gorge turns white tonight with 1-4″ fresh snowfall as cooler air filters in from the east.  That’s east of Bonneville Dam; west of there I think it’ll be pretty quiet, although a dusting is possible by sunrise IF enough cold air shows up.
  • Saturday will be a dry day.  Cold east wind gets going, eventually reaching full-on winter strength by Sunday AM…brrr!  Peak gusts 55-70 mph Sunday…ouch.
  • Sunday turns snowy by midday, expect difficult I-84 travel by Sunday afternoon through Christmas morning.  Another 2-5″ is likely
  • Enjoy your White Christmas!  3-8″ should be on the ground Christmas morning.

 

Today’s forecast is relatively straightforward:  a boundary between a “mild” Pacific airmass and modified cold arctic air is sitting right over the middle of Washington state.  We are on the “mild” side of that through 6-9pm tonight.   The lowest elevations still have some cool air trapped from yesterday’s inversion/fog, but they will gradually “mix out” today and temperatures will rise into the 35-40 degree range.  We aren’t getting a significant onshore flow, but it’s enough to raise the sticking snow level to above 1,500′ by midday/afternoon.

Tonight the showers continue but sticking snow levels remain above 1,000′.  Temperatures remain in the mid 30s in the metro area (above freezing).  Easterly wind arrives in the Gorge and that means a switch to all snow out there.  You can see the forecast snow totals now through Saturday PM.  A nice 8-12″ on the way for Mt. Hood too!  That may help the ski areas open a few more lifts:

 

By Sunday morning we have cold modified arctic air pouring out of the Gorge on strong east wind into the metro area.  At the same time a juicy low pressure area and approaching frontal system is offshore, heading toward the Pacific Northwest.  You can see the cold surface high pressure eastside and the approaching system on the 7am Sunday map:

 

The forecast is a mess at this point (Sunday) because the GFS model (and Canadian GEM) continue to insist that surface low heads NORTH of us and into Western Washington. Note the liquid rain forecast for 10pm Christmas Eve:

That brings mild southerly flow over us, ending the frozen precip threat everywhere except in the Gorge by Christmas Eve evening.  As of now the GFS suggests there will be no interesting snow/ice episode this weekend except MAYBE patchy freezing rain early Sunday AM before we wake up.  But it would only take a slightly southern move of this system to bring snow into the metro area on Sunday.

On the other hand the NAM and ECMWF models bring the surface low either right over us or to the SOUTH of us Sunday and into Christmas Morning.  That’s quite a bit different!  If this is the case, we could easily get a real snow storm Christmas Eve and into Christmas Morning.  These models imply a rare White Christmas for the metro area and only the 3rd in Portland’s history!  Note the total snowfall product from the ECMWF for 4am Christmas Morning.  You can ignore the heavier stuff in the NW metro area, that’s partly due to terrain.  But you see a general snowfall across much of the central/north metro area due to cold air pouring out of the Gorge getting overrun by Pacific moisture; our classic snow/ice storm setup.

A good 75% of the time eventually models seems to settle on the more northern approach (and warmer) solution for Sunday and Christmas morning.  But my inner child wants a snowstorm Christmas Eve and 2-4″ snow sitting on the ground Christmas morning.

By Saturday morning we should have a much better idea what’s going to happen Sunday.  I’m home through the weekend, not in some tropical location, so I’ll keep on top of it through Christmas.

Chief Meteorologist Mark Nelsen


No Freezing Rain Today

December 22, 2017

7am Friday…

freezing rain

Good morning!  There’s some good news in the forecast.  Temperatures are not cold enough to support freezing rain in the metro area this morning; you don’t need to worry about that west of the Cascades.  We didn’t really think that was a big concern, as my coworkers have been saying on-air the past 24 hours and I didn’t even mention it on a blog post a few days ago for that reason.

web_temperature_ORWA

ODOT road temperature sensors are all well above freezing as well.

That said, at this moment we have a cold enough airmass overhead to support incoming showers being of the “white” variety.  Shower action is starting to pick up on radar and much of that will fall as a rain/snow mix in the lowlands, or all snow today for the hills.  I’m looking at that closely and will post within the next two hours.  Lots to talk about between now and Christmas Day!

Chief Meteorologist Mark Nelsen