Get Ready For The Coldest Week So Far!

December 26, 2014

It has been very mild December; in fact as of this evening it’s the 2nd warmest December on record at PDX!  But the last 3 days of the month are going to be totally different.  That’s Monday-Wednesday of next week.  Remember the yo-yo temperatures in November?  We saw rapid swings from warm to record cold to warm, and then back to cold at the end of the month.  We’re going to see that happen starting Monday again.

First, let’s get this weekend out-of-the-way.  A cool and wet system is dropping in overnight from the northwest.  This has a beautiful west/northwest flow behind it to pound the Cascades with what could be their best snow so far this season.   We really need it.  MarkSnow_MtHood_Totals  Although some snow has fallen this week and allowed Meadows/Timberline to open at least half their runs, we’re still way behind.  RPM_72_SNOW_00z  Our RPM forecast and WRF-GFS are about the same…showing a solid 12-18″ with possibly up to 2 feet in spots by the time we dry out Monday  Here in the lowlands that means mainly wet with a breezy south/southwest wind both days.

What’s changing Monday?  A strong ridge of high pressure magnifies quickly later Sunday-Monday over Alaska, sending a couple of upper-level disturbances plunging south along the West Coast.  The carve out a sharp upper-level trough over us which eventually ends up in Southern California later Tuesday/Wednesday. gfs_500mb_4pmMonday That is the view at 500mb (~18,000′) Monday evening.  What happens down below is even more interesting.  A HUGE surface high pressure area drops south through Western Canada and into western half of the USA.  Take a look at that monster Monday evening at 10pm:  gfs_1060mb_Mon10pm

It’s covering about a third of the country already with a central pressure right around 1060 millibars…expect your home barometer to skyrocket early in the upcoming week!  Of course cold high pressure to the north and east means gusty northeast wind for ALL OF US (not just in the Gorge) later Monday-Wednesday…probably not as strong as the damaging northeast wind in mid-November, but it’ll still turn windy and cold starting late Monday.

There is a slight chance that as the cold air starts arriving from the east Monday we’ll still have some showers around that could be mainly snow.  But almost always in this situation the cold air slowly trickles in (at first) while skies clear…as a result don’t get your hopes up for any sort of snow Monday.

How cold is it going to get?  One clue is in the 850 mb temps.  Models have been pretty consistent showing -8 to -10 over the northern Willamette Valley.  Seems that with sunshine and easterly well-mixed wind Tuesday we would see high temps around 35-40.  But, check out how cold it is up against the east slopes of the Cascades:  gfs_950_10pmMon

That -15 to -17 celsius at 5,000′ means the ski areas will be in the upper single digits or low teens at best late Monday through Tuesday…Brrrr!  That cold air will be funneled over the Cascades and through the Gorge, cooling us even further.  As a result I think we’ll only be around freezing or just a few degrees above on Tuesday, Wednesday, and New Year’s Day.  And in this pattern there is no obvious warming coming from the west or south, so the airmass will only slowly modify.

I’d tell you to wrap your pipes and sensitive plants, but it was cold twice in November so I assume you already have done that.

Is this going to be a one-shot deal?  Not sure yet.  The 12z ECMWF said YES, any cold air about 9-10 days from now will be diverted to the east or stay up north.  tseries_850t_000-360_Portland  The 12z GFS wasn’t so sure…you can see several ensemble members showing another cold shot.

tseries_850t_000-384_Portland

We’ll have to wait and see.  But for now the screaming message is that our coldest weather so far this cold season is on the way for Tuesday-Thursday.   2014 is going to go out cold!

I’ll probably post again Sunday evening since I’m working again.  But don’t worry about me, I already spent 1/2 hour this evening figuring out which days next summer I’ll take off to make up for it…

Chief Meteorologist Mark Nelsen