One Rainy Day, Then a Cold Christmas Weekend

December 17, 2017

9pm Sunday

Today was a mild one and the next two days will be mild too.  High temps will make it up around 50; a typical high is 45 now that we are in our coldest month of the year.

This has been an incredibly dry December so far, less than 1/2″ of rain has fallen in the first 17 days of the month

Driest Decembers Ever Graph 2017

THAT is the one consistent story through the next 7-10 days.  Our dry December is going to continue.   Let’s go over the…

HIGHLIGHTS

  • There is only one rainy/windy weather system in sight for the next 7-10 days.  That’s Tuesday.  Pretty weird to see this happen twice in December.
  • Expect mild temperatures tomorrow and Tuesday, then back around normal Wednesday & Thursday
  • Another blast of east wind arrives Saturday and continues for quite a few days.  That means it’ll be a windy Christmas for the usual east wind locations.  At first ALL of us in the metro area will get the wind Saturday/Sunday.
  • That east wind will be at least somewhat colder than the last event, but could be a bitter cold “arctic blast”.  Models aren’t agreeing how much cold air makes it into the Pacific Northwest
  • Regardless, it’s likely the coldest weather so far this season is on the way for Christmas weekend and into sometime next week.
  • SOME models produce light showers Friday & Friday evening as the first colder air arrives.  This CAN be a setup for mixed rain/snow showers in the lowlands…or we just stay dry.  That’s TBD.  I haven’t even bothered to change to snow tires on my car yet; obviously I’m not freaking out so no need for you to either.
  • Since the airmass coming in is colder than last time, it’s time to wrap your pipes & cover your faucets.

Right now we are in a mild westerly flow since the big upper-level ridge broke down late last week.  But that big ridge is going to rise again.  Check out the Saturday 500mb forecast from this morning’s GFS model…a full-latitude ridge from offshore California into Alaska & Yukon.

Jet Stream Forecast 2017

ALL models show that big ridge this weekend and beyond.  It’s just that some have the pool of cold air closer to us than others; this evening’s GEM model is especially cold. Probably highs around 20 degrees in Portland; that’s at one extreme.  Others just shove a little cool air over us with highs maybe 40 or so over the weekend.  They ALL push a massive blast of polar air into the west-central USA.  I’d hate to be in Denver, Salt Lake, Billings, or Minneapolis for Christmas…brrr!    Check out the ECMWF model meteogram for Denver.  Near 60 Wednesday and then a HIGH around 10 by Saturday:

KDEN_2017121712_ecmwf_min_max_10

So to summarize:  Don’t get too excited about snow for now.  In general a very dry weather pattern is going to persist for another week or more and it’s going to turn colder.

Looking farther out, of course once the cold air gets in here there’s always the threat for Pacific moisture to return and ride over the chilly air.  That’s how we get snow/ice storms (as we all know from last year).  There are hints of that on the ECMWF.  21 of 51 ensemble members from the morning run show at least some sort of sticking snow somewhere in the 15 day period after the 26th.  I’ll be keeping an eye on that of course.

The great news is that the first dumping of snow since Thanksgiving is coming to the ski resorts Tuesday afternoon through Wednesday midday:

RPM Snow Accumulation

Those totals might be a little high, but I expect a solid 10-15″ to freshen things up.  A foot should fall at Government Camp.  This should help salvage a terrible start to the Christmas Break.

Chief Meteorologist Mark Nelsen