Stages for Snow

Posted by Rufus La Lone on

Friday December 3
Waiting - for Christmas, for snow.  One we are sure of, the other, well, let’s give it a try.  Challenging forecast to ramble through, so better fill that Mug.  Ready?  
Mild conditions across much of the PNW will yield to a much colder, typical December pattern soon.  It will happen in stages.  First, to the relief of many in NW WA & BC, the incessant rains have let up, yielding time to repair, replace, rebuild.  That said, a weak system will move onshore Saturday, ushering in some precip, mainly north of Portland.  As that Low moves SE, colder air will fill in, setting up the chance for snow above 1,000 ft, and a snow/rain mix at the surface north of Seattle.  It will continue mild over Oregon.
The next, much stronger storm, will arrive from the NW by Monday morning.  This Low will generate a fair amount of rain & wind for WA & OR; the Coastal Mountains of Canada will likely get a good shot of snow.  By Tue, it will dry out during daylight hours.  
Second stage, in a manner of speaking, starts overnight Tue - this one will drop the freezing level rather low, esp over western WA, so expect snow showers mixing down to 1,000 ft by Wed night, and even though showers will diminish by Thu, the freezing level may drop lower still.  Portions of western WA could get some snow in the foothills, although the ground will not be frozen to hold very long.  A weak trough may arrive Thu night Dec 9, drawing in more cold air and dropping snow levels to sea level or very near so in many west side locations, esp north of Portland (or even Salem).  Not a lot of moisture with this disturbance, so rain/snow mixed at the surface will be light on Fri Dec 10; should be all snow in any showers north of Chehalis.  If not foggy in your location, a hard frost is likely Saturday morning, the 11th.  
Third stage.  Pooled, cold air over the Gulf of Alaska will sag south even more by Monday Dec 13.  The transition could be Sunday the 12th, with increasing chance for rain as a surface Low moves south along the coastal waters.  If the Low turns inland early (as some model runs suggest), the air mass will be cold enough for very low elevation snow, esp western WA; a few model runs track the Low farther south into the San Francisco area, keeping much of the PNW on the narrow, warmer edge of the broad upper level Low for an extra day or two.  We’ll see.  Rain and showers will persist on/off through Tue Dec 14.  
Middle-to-end of the week, Dec 15-17:  Fourth stage: heavy rain over central/southern OR as another push of cold air arrives from the NW.  This one may have the needed cold air aloft to spark SNOW at all elevations from Eugene north.  If not, snow will be everywhere on one’s path west, south, north or east over any terrain over 500-1,000 ft.  Chain time.  A weak series of fronts may push in from the NW, each with enough cold air support to threaten low level snow or rain/snow mixed for all areas west of the Cascades as the week ends.  
The weekend of Dec 18,19 is trending chilly, with snow or rain/snow mixed depending on elevation.  Precipitation could be heavy at times, which could drive snow down to the surface periodically.  
Second Cup: Various global weather models have been struggling on a consistent solution for the Dec 12-19 period, as we have noted here for several forecasts.  Anyway, we must report that the overall pattern remains on a trend for COLDER air to begin arriving into the PNW from the NW over the eastern Pacific (not from our usual ‘Arctic outflows’ out of the Fraser Gap or Columbia River Gorge).  Snow at the surface in the PNW requires quite a few atmospheric attributes to align just right — a few of these do properly align during the period in question.  We usually lack moisture when the air aloft is cold enough for snow.  The cycle discussed here is right on the edge of ‘cold enough’ while also having plenty of moisture.  We recall December 2008 when a similar combination was charting - and verified.  Will December 2021 be a repeat?  Close call for now.
For our California Patrons - heavy rain and surprisingly low elevation snow is possible during the period discussed above.  Be safe!
Our quip today reminds us that forecasting is always tough, even for the best of us:  “The phonograph is not of any commercial value.” -Thomas Edison, 1915.
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