Rare “December Drought” & LOTS of Chilly East Wind On The Way

December 3, 2017

7pm Sunday

We’re drying out this evening and now you can say GOODBYE to the rain for quite a while.  Meteorological Winter (December-February) is going to start out very dry.  It’s possible we won’t see any significant rain for more than 10 days!

The typical westerly jet stream (our storm machine) we see in winter is heading much farther north this week and beyond.  A strong upper-level ridge of high pressure is developing along the west coast of North America and it’s going to stick around for an unusually long period of time.    You can see on the forecast chart from the ECMWF model for this coming Wednesday:

The lines are the actual model ensemble average (51 different runs of the same model) and the colors represent deviation from normal for this time of year.  Huge above average heights all along the West Coast, and well below normal in the Eastern USA.  Then let’s move ahead to ONE WEEK LATER…same thing!  If this pans out, expect dry conditions all across the far western USA for the next 10 days.  All models are in good agreement with this scenario as well.

Looking farther ahead, here’s the view at Day #15, Monday the 18th.

This is the beginning of the first week of Christmas Vacation for lots of kids (some like mine don’t start that 1st week until Friday the 22nd).  The ridge is still there, but a bit weaker, and more likely we’d see some mild westerly flow breaking through at times.  All global models show this same scenario through the next two weeks.  Either a dry & warm upper-level ridge over us or wet at times & warm (this last map).  NONE show a cold and wet or cold and dry pattern in the mountains.  Let’s cover the main effects from this upcoming weather pattern:

 

INVERSIONS

Notice I said it’s a warm UPPER-LEVEL ridge.  At our latitude in winter when it warms up overhead with strong high pressure, it won’t be warm down in the valleys where most of us live.  A strong inversion will form under the warm air in the mountains; that starts Tuesday.  That’s because the long nights and very weak daytime sunshine doesn’t allow the surface layer of cold nighttime air to warm up.  Assuming these stagnant conditions continue, it will actually cool a bit over a period of days or a week.  That’s why you’ll notice my current 7 Day forecast is warmest on Tuesday (when the first east wind arrives), then turns cooler later this week and into next weekend.  East of the Cascades a cool pool of air will form below about 3-4,000′ in the Columbia Basin.  That pool of cold air is heavy and dense.  It’ll be trapped over there except for one spot it can move at sea-level…through the Columbia River Gorge.  Yes, we have unending days of…

EAST WIND

It’s time!  Every winter those of you at the west end of the Gorge and east Portland metro area suffer through long periods of cold east wind.  That begins Tuesday afternoon and continues until further notice.  I can’t tell you when it will stop this time around…sometime the following week maybe?  At first you can expect temperatures in the 40s out there (Tuesday), but then as the cold air gets established east of the Cascades the airmass will cool.  By next weekend you’ll be only in the 30s with east wind gusts in the 60-80 mph range anytime beyond Wednesday.  It’s going to be a long haul folks!  Tie everything down.  Rumor says Wednesday could be a “Vista House Day” for the weather geeks.  But…there is one huge benefit to that dry east wind…

SUNSHINE

Yep, we’ve got day after day of sunshine coming.  In the metro area we’ll start with areas of fog Tuesday, but Wednesday and beyond it’ll be too dry to support fog for us.  This is the cool/crisp weather I personally prefer in December IF we can’t get any good storms.  If you live from Salem south in the valley it’s possible your fog lingers all day Tuesday and part of the day Wednesday, it depends on how much drier air works into the valley midweek.

TIME TO WRAP PIPES?

I don’t think you need to do it although why not do it now for the season and be done with it?  In calm areas this week I could see low temperatures drop into the 22-26 degree range (after Wednesday).  Windy areas may not drop to freezing at all.  Those temperatures generally aren’t cold enough to cause big issues, but again, you could just get it done for the season.

CASCADE SKIING

We are very lucky some snow showed up this week, because I don’t see a pattern that brings snow to the ski resorts in the next 10 days.  Timberline & Meadows are open, with 2.5-3 foot bases.  Good enough with modern grooming techniques and short sunny days around 45-55 degrees won’t melt much but that snowpack will become a little…er…consolidated (an icy brick).  Again, groomed runs should be just fine though.   Hoodoo, Skibowl, & Willamette Pass all need more snow to open.  Hopefully some surprise can show up near the start of Christmas Break.

Is this a rare event?  Yes and no.  I’ve gone back and looked over the records.  Let’s assume we go 10 days without rain (a relatively big assumption at this point).  That has only happened in 7 Decembers out of around 80 years of records at PDX.  We had 11 consecutive dry days in 2009, and 14 in 2005 and 12 in 1989.  All 3 were followed by great skiing the 2nd part of winter.  It also happened in 1993 and 1980, those two weren’t very good snow seasons in the Cascades.

Enjoy the next week!  Chief Meteorologist Mark Nelsen