Brief Freezing Rain or Snow Monday AM

December 18, 2016

10pm Sunday…

Our cold spell is about to come to an abrupt end in the next 12 hours.  The lowest part of the atmosphere over the western valleys of Oregon and S.W. Washington is still quite chilly this evening with many of us below freezing.

During the night a strong southerly and/or southwesterly flow will be developing.  By 4-7am up at 2,000′ models say the southwest wind will be blowing around 30-40 mph transporting in much milder air.  That’s in advance of a relatively strong cold front approaching from the northwest.

Now typically if we have a juicy front approaching with a dry and cold airmass at the surface, it’s quite possible to get some sort of snow/ice storm.  But this time it’s much different from last week.  This system is coming in from the northwest and gusty southerly wind arrives by tomorrow morning.  The easterly Gorge wind that would often transport very cold and dry air into the Portland metro area has died down to just an easterly breeze that is barely making it out of the Gorge.  I was just at a house about 200′ above the Columbia River in Corbett and I can vouch that it IS just a breeze at this point.  So the “freezer door” is now shut and the atmosphere will quickly moisten and go above freezing overhead by 4-7am.

At the same time precipitation will be arriving.  Between 4-7am SOMETHING frozen will likely fall for many of us.  There could be spots of snow from the metro area north but I think it’s more likely we’ll get areas of freezing rain for a couple of hours at the beginning of the Monday AM commute.  In this case it can’t last long with the mixing southwest wind arriving both above and the Gorge wind nearly dead.  So the main message is:  Be very careful if you are out and about between 4-7am; what appears to be rain may be freezing on the roadways in some spots.  Or there could be light snow accumulation at first as well.  You can see the 4-7am precip forecast from the WRF-GFS this evening:

or_pcp3-15-0000

IF that were all snow, that would imply an inch or so accumulation.  But I think freezing rain is more likely for the spots that remain below freezing since the airmass above appears to be mainly too warm for snow (for most of us).  Check out the snow forecast from our RPM model, which implies at least a dusting for many of us before the changeover to regular rain:

rpm_snow_00z

And the WRF, which holds the snow in the higher elevations only:

or_snow3-15-0000

Beyond 7am temperatures should warm above freezing quickly.  In fact by tomorrow night at this time we’ll be around 50 degrees with a gusty south wind!  It’ll feel “tropical” after the last couple of weeks…

For you folks in the (central/eastern) Gorge, you’ll likely get a mix of freezing rain/snow through tomorrow evening before a changeover to rain tomorrow night.  A gusty WEST wind there by Tuesday means high temps in Hood River and The Dalles will surge well into the 40s…the melt will be on!

Chief Meteorologist Mark Nelsen