Drier Than Normal Next 7 Days, But Turning COLD!

December 28, 2016

6pm Wednesday…

Today was an amazingly boring weather day, with just areas of low clouds/fog in the valley and some sunshine central/east metro area.  Not much happens tonight, Thursday, Friday, or the 1st half of Saturday.  Take a look at the 3 day rain forecast from our RPM:

rpm_rain_nworegon

Yes, pretty light stuff with a weak cold front that comes through Thursday night.  That means the daylight hours tomorrow and Friday should be dry.  Saturday morning will be frosty and cold with temps below freezing under a cool airmass.

Things start to get interesting just about the time all you folks start partying New Year’s Eve (2nd half of Saturday).  The upper-level flow begins coming in from the north, and a disturbance is dropping south through British Columbia.  Take a look at the 500mb chart for 4am Sunday:

gfs_namer_090_500_vort_ht

You see the disturbance carving out an upper-level trough right over us.  Air is flowing from the arctic south straight into the Pacific Northwest.  At the surface, there is a surface low pressure area developing over southern British Columbia.  This map is right at 1am…one hour into the New Year.  Note two things:  first, the airmass is quite chilly, snow could fall even below 1,000′ in this pattern.  The 2nd, a tremendous westerly to southwesterly flow is on west of the Cascades due to that developing low.  That gusty mixing wind will likely keep sticking snow above 1,000′.

wrf_sun1am_deeplownorth

There will be plenty of showers, with the ECMWF showing the potential for 8-15″ snow in the Cascades, that’s due to a very strong westerly wind between 2,000-6,000′.  Perfect for orographic lift…the west slopes of the Cascades will get pounding by heavy precipitation for about 12 hours from late Saturday through late morning Sunday.

ecmwf_snow_sundaypm

By 7pm Sunday the surface low pressure center is weakening and dropping south over Western Oregon.  At this time the surface wind is turning north and northeast as very cold (and very dry!) arctic air invades the Pacific Northwest:

wrf_sun7pm_arcticboundary

The WRF-GFS is even more interesting, check out the 24 hour snow total ending Sunday PM:

wrf_snow_endingsundaypm

Notice how far west the snow is pushed down into the foothills and even far eastern Willamette Valley!  That’s due to the strong westerly flow pushing up against the west slopes of the Cascades.  It’s only noticeable if you regularly look at these maps.  This says significant snow could fall even down to or below 1,000′ on the east side of the Willamette Valley.  For sure the hills will get a dusting, but in this pattern I think significant snow down on the valley floor is unlikely.  But if you live in Sandy, Estacada, Sublimity, Mill City, Sweet Home etc…?  You could see a few inches of snow on the ground by Sunday AM or midday.  After that ALL models agree we dry out quickly as a the dry arctic airmass moves in.

To summarize:

  1. A significant snowfall is unlikely in the western valleys and Portland metro area New Year’s Eve & Day
  2. But we could see a dusting of snow just about anywhere early New Year’s Day, most likely that’ll happen on the hills
  3. Roads may get covered with snow above 500′ or so near/in the Cascade Foothills by New Year’s Morning.
  4. Coast Range roads will likely be snowy/icy later Saturday night through Sunday.
  5. There is relatively good model agreement on the general progress of these weather events Saturday/Sunday

Beyond Sunday…

Models are in excellent agreement that we’re into a cold and dry pattern for at least 3 days…Monday/Tuesday/Wednesday.  How cold?  Now at least the GFS has come around to the colder ECMWF/GEM models, showing 850mb temps dropping down to the -12 to -13 range Monday night and Tuesday with offshore easterly flow.  These numbers are in the same category as the December 2009 and December 2013 cold spells.  Not a massive 1990-style “Mr. Cold Miser Has Arrived” blast, but definitely an outbreak of arctic air.  So prepare for the coldest airmass we’ve seen in 3 years.  It’s going to be in place over us much of next week.  That means wrap pipes and protect your pets/livestock/animals.  High temps will likely only get up to around 30 degrees even with full sunshine.  That’s assuming we get little/no snow cover on the ground.  If some part of the Willamette Valley ends up with 2″ or so (not expected for now), temperatures would be a good 5+ degrees colder.  East wind will also be blowing through the Gorge, although it doesn’t appear to be a real strong wind for now.  Lows should reach down into the 10-15 degree range, even here in Portland.  It’s about time!

Our forecast looks like this for lows:

data-driven-7day-forecast-lows

Note the lowest temperature we’ve seen each winter from my OMSI presentation back in October.  During the last December arctic blast we saw a few upper single digits in the metro area and then lower teens.  That’s why I have that 12 degree forecast, especially with the east wind not looking too strong.

picture1

There is very good agreement on this second part of the forecast this next week…the cold part.  Check out the ECMWF ensemble chart from this morning:

tseries_850t_000-360_portland

Look at all those lines very close together through next Thursday, the 5th.  Then things break down, but the trend is to add some moisture back into the forecast as the pattern stays cold.  Yes, we could be looking at some sort of snow/ice event the end of next week.  That’s still 8-10 days away though…

Chief Meteorologist Mark Nelsen