Heading for La Nina Snow?

Posted by Rufus La Lone on

Black Friday 2021
Lots of rain will continue to fall over the far NW corner of the PNW; trending drier for the rest.  Let’s go through the latest outlook.
From now until late Tue Nov 30, rain & showers will be almost continuous for portions of Vancouver Is, southern BC and the NW corner of WA.  Heed Nat’l Wx Service alerts as to flooding issues.  For SW WA, OR and ID, additional precip is charting for Saturday & early Monday.  Fog & dry conditions are likely to develop by Tue, excluding the far NW.
Ponder Points.  Here’s where the models begin to chart a mixed message (happening frequently this fall, for sure) - 
   1st scenario: generally dry and gradually cooler conditions for Wed Dec 1 through the end of next week.  Maybe some showers over the weekend of Dec 4,5, with a N-NE wind developing as cold High Pressure dome drops deep into the heartland, east of the Rockies.  Mild-to-warm (esp for OR) Sunday & Mon, Dec 5,6 before it turns colder with snow teasing Patrons north of Bellingham by Dec 9th.
   2nd scenario: coldest air of the season begins to drop into the PNW, with increasing chance for low level SNOW during the weekend of Dec 4,5.  A Low develops over northern CA, drawing 'snow-cold' air over OR & WA for snow showers becoming likely at the surface.  Ground may not be frozen yet, so accumulations may delay; cold Fraser Gap outflow - strong wind - could set up sometime that weekend, too.  (The pattern for Dec 4-12, 2021 has similarities to that of our very snowy Dec 2008.) Cold wx for much of CA, too, with the 2nd scenario.
Overall, both solutions do project our weather turning colder with very low snow levels as the week of Dec 13 gets under way.  
La Nina.  We are in a rather COLD La Nina pattern.  The image below (as of Nov 23, 2021) illustrates the cold (blue color) sea surface temperatures from the west coast of South America on east across the central tropical Pacific.  The Pacific Ocean offshore of the PNW is also running colder than average.  This is our second fall/winter with a La Nina pattern in place.  Yes, back-to-back La Nina winters do happen, as this will be the 6th time since 1999.  While not a certainty every time, La Nina winters generally produce more low level snowfall and above average precipitation in the PNW.  As the chart below indicates sea surface temps in the ‘middle’ of the tropical Pacific 0.75 - 2.0 degrees C colder than average.  We’ll see how all this plays out over the next 3 months. For now, be prepared for the chance for low elevation SNOW as the month of December gets underway.  It is probable. 
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