Quick Evening Update

December 4, 2013

9:30pm Wednesday…

Not much change in 00z models tonight with most of them just skirting the Portland metro area with snow later Thursday night and Friday morning.  If there was any movement, the precipitation is slightly closer to us.  Here’s the WRF-GFS model showing accumulations of snow as close as the western suburbs and down around Woodburn and then south.



Still looks great for at least 1″, maybe more, in the areas mentioned in the previous blog posting.

Our 00z RPM is even more white than its earlier run, producing a swath of decent snow all down I-5 from Olympia to Medford!  I’m ignoring it…for now.

Models are now pushing even colder air than expected down out of Canada later Friday through Sunday.  I see the NAM and GFS both have 850mb temps down to the -12 to -15 range.  That would be about 10 degrees fahrenheit at 4,000!  We haven’t been below the -12 to -13 range since before 2007…I don’t know when.  The lowest in December 2009 was -12.3 at Salem, and -12.5 in November 2010.  February 2011 and December 2008 only bottomed out around -10.  The 2008 number surprised me.  Of course what models predict and what actually shows up on the sounding can be different.

Regardless, we have a true “arctic air” weekend coming up here in the Pacific Northwest.  High temps only in the upper 20s to 30 in the metro area.  Low temps in areas with bare ground and calm wind should be in the 10-15 degree range.  If we get snow cover down the valley, it’s going to be the coldest we’ve seen down there in many years…into the single digits!

Chief Meteorologist Mark Nelsen



Episode 35: Portland’s Deep Freeze & Metro Snow Chances

December 4, 2013

Fresh Podcast Today!

Northwest Weather Podcast

Snow Picture

Episode 35: Portland’s Deep Freeze & Metro Snow Chances

The temps are cold enough and the moisture is on its way, but the big question is… WHERE WILL IT HIT?  In this week’s episode, the guys break down our chances of snow in Oregon on Thursday night/Friday morning and take a look at some of the coldest overnight temperatures we’ve been talking about in four years.

Snow ForecastIn this holiday special, the guys will also answer your news and weather questions and play a rousing game of “Cities of the Week.”

Thanks for listening this year!  We will be back with more episodes in 2014.


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Snow In The Valley Friday Morning; Then Very Cold!

December 4, 2013

11am Wednesday…

I think there’s a good chance you will get a snow day Friday if you live in the central/southern Willamette Valley from Salem down to Eugene AND on the central Oregon Coast!  Models are in total agreement on the track of a cold low pressure system that will slide down the coastline Thursday night into Friday.  They haven’t changed much in the past 24 hours, slightly farther south on last night’s runs, slightly farther north on the snow this morning.  I’ve looked at all the info this morning, and nothing significantly different comes out between now and later this evening;  so a good time to post..



  • Clear and cold again tonight, in fact a bit colder than last night.  Notice a few of the official stations made it down into the upper teens.  I expect about a 5 degree cooling for tonight.  That means PDX could drop below 20 for the first time since February 2011.
  • High clouds thicken up Thursday, as a result our coldest day so far, just creeping a few degrees above freezing
  • Snow will spread onto the beaches (yes, the coastline!) later tomorrow evening and inland before sunrise Friday.
  • At this point it appears accumulating snow will most likely stay SOUTH of a line from about Seaside to Newberg to Mt. Hood.  Yes, that means just flurries at best for the Portland metro area Thursday night and Friday morning.
  • 1-3″  is possible from Tillamook to Newport on the coast, and in the Valley from McMinnville to Salem and south during this time (Friday AM)
  • 2-4″ is possible Albany, Corvallis, and Eugene (south Valley)
  • Avoid travel south on I-5 during the day Friday between Salem and Eugene
  • Any northward movement of the whole system in future model runs would put accumulating snow into the Portland metro area.  There is still (fading) hope!
  • Gusty east wind pours out of the Gorge Friday, bringing a frigid arctic airmass west of the Cascades, the coldest we’ve seen in 4 years.  High temps probably won’t rise above freezing Saturday and Sunday; our coldest weekend we’ve seen in a long time…but sunny!


Amazing model agreement this morning on the big picture considering we’re 36-48 hours out.  Look at the Friday morning forecast from the GEM, GFS, NAM, & ECMWF.


All have the low pressure system sliding down the coast, and deepening (always good) as it moves into southcentral Oregon by Friday afternoon.  They all show a good shot of precipitation just to the south of the Portland area centered on sunrise Friday.  Looks like a very good time to be in Corvallis, Lebanon, Albany, and Eugene!

Now the problem is that we are right on the edge of the precipitation band here in Portland.  I tend to lean on the dry side too because the low-level atmosphere will be very dry in the metro area due to a strong and gusty east wind.  Take a look at the surface map off the UW WRF-GFS for 7am Friday.  That’s beautiful!  A classic snowstorm setup with the low pressure sliding down the coast pulling cold and dry arctic air through the Gorge.  Not quite easterly downslope windstorm material, but it’ll be a windy morning here.


Here are the snowfall accumulation graphics from the UW’s models, the NAM-MM5 and WRF-GFS:


Technically the first one is just precipitation accumulation, but there is no snowfall accumulation graphic for that model.  Notice the WRF-GFS pushes the snowfall slightly farther north.  Also, to really send you geeks over the cliff, note the 2-4″+ snowfall out over the ocean offshore going to waste on the turtles and whales!  This really points out that you can have lots of cold air, but need moisture to get snow, and they often don’t line up correctly in our area.

In this setup, with an east/northeast wind over the Willamette Valley ahead of the low, there will be a upslope component on the west side of the valley.  Assuming the moisture makes it far enough north, I could see Gaston, Willamina, and Dallas getting better totals than in the valley itself.

Our RPM is still having issues, developing flurries and/or light snow even up in SW Washington as the low moves south Thursday evening.  Notice it’s snow total from the 12z run doesn’t show that, but the 15z does.  At least it got rid of all the precipitation farther north:


Remember that this is unusual territory for us, having a system drop out of Canada, spin up a surface low, throw moisture over the cold air and deposit snow in our area.  If models are off by just a bit, we could get snow here in Portland.  At least we still have the 00z models tonight and 12z tomorrow to look for any trends.

Here comes the cold air…all 12z models REALLY COLD with the airmass behind this thing.  Take a look at our 12z RPM text graphic.


I don’t think I’ve ever seen it show negative dewpoints like that.  And I haven’t seen it show such cold temps for this spot since 2009 either.  Wow.  Probably overdone a bit on those dewpoints, but this will definitely be the coldest since 2009.  When the wind goes calm Saturday night in the valley where snow cover is on the ground?  I think single digits are real likely down there.  Here in the metro area we’ll be in the 10-15 degree range…Brrr!

We removed any precipitation out of the forecast until at least Tuesday.  The general forecast pattern would get us out of the cold air quickly at that time with maybe only some ice in the Gorge…we’ll see.  That’s still 6 days away.

Chief Meteorologist Mark Nelsen