Unusual Late Winter Snow & Cold

February 20, 2011

I’ve perused all the models and maps and here are my thoughts this evening:

1.  An unusually cold and snowy pattern; maybe one we haven’t seen this late in 20 years, is headed into the Pacific Northwest the 2nd half of this week (Wednesday and beyond).  It appears that only 5 times in the last 70 years have we seen 1″ or more at PDX on February 24th (Thursday) or later.  We could beat that.  So it’s not UNHEARD OF to get snow this late, but it IS RARE.  Of course that sure does NOT lower the chance of it occurring; just a reflection of the rarity of such cold airmasses coming south this late in the season.

2.  In general, models the last 48 hours have generally converged on a reasonable solution.  That’s after the wacky ECMWF a day or so ago, and the extremely cold GFS.  The solution appears to be somewhere in the middle.

The big picture shows a large upper-level ridge out over the Gulf of Alaska the next 7-10 days, with waves shooting down the backside and into the Western USA.  This has been going on since late last week.  That includes the surprise rain on Friday and now a low dropping south straight west of the coastline.  This is the classic “watch out for snow or an arctic blast” pattern. 

On Wednesday and Thursday a much deeper upper-level trough digs down the coastline out of the Yukon and Alaska.  Very cold air, even for winter, plunges out over the Pacific and rotates in over the Pacific Northwest both of those days.  It’s important to point out that these are “onshore flow” days.  From what I’ve seen, no model shows cold and dry arctic air entering our area until at least Friday.  If you want a classic “frozen snow & east wind” storm, this isn’t it.  It just appears to be a colder version of what we saw the middle of last week.    Tons of snow in the mountains and increasing chance of snow here in the lowest elevations (including the beaches) from late Tuesday night through Thursday night.  Of course temps will be coldest in the early mornings. 

I see the NAM is significantly colder with this onshore flow-heavy snow shower pattern this time around.  For example, on Wednesday PM, NAM has a 516 500-1000 thickness, the GFS just 522.  By Thursday morning, the NAM has a 511 thickness!  The GFS 517.  Considering the air is coming in from the Pacific (after coming out of Western Canada of course), I’m inclined to believe the NAM will warm things up the next 48 hours to what we see on the GFS as it clues in.    That leaves us with snow showers about as low on Wednesday morning as what we saw last week; dependent on heavy showers to get it to stick in the Valleys.  Then a normal late-winter warmup Wednesday afternoon (40-45), then better chance for widespread snow (at 32-33 degrees) Thursday morning.  But even Thursday afternoon, with no arctic air coming in from the north or east, and sunbreaks between showers, our 36 forecast high might be a little cool.    Thursday could still be a 40 degree day, unless we sit under a cluster of snow showers all day. 

On Friday models push the moisture out of here and we get the cold and dry air.  At FOX-12 we have a high of 32…that’s probably too extreme for what I see right now (even though it was my idea…hmmm).  Maybe 35 will be more reasonable; but it makes the point for the general public.

Beyond that, lots of uncertainty, except that the foothills (1,000’+) probably have more snow coming either Sunday or Monday.

For the kids and adults wanting to get a “Snow Day”? 

Possible Wednesday morning, but more likely Thursday or Friday morning.  It’s quite possible each afternoon/evening will see clear roads with temps well above freezing for the later commute.

Chief Meteorologist Mark Nelsen