Friday, November 18, 2016

Active weather pattern setting up near Thanksgiving

Those that are traveling over the Thanksgiving Holiday weekend (Nov 23-27) may be impacted by several storms moving into the region.  The mountain passes will be especially impacted by snow and roads may resemble something like this...

Since this forecast period is still several days out, we will try not to get too specific with the details, but the pattern certainly looks favorable for mountain snow, which will impact travel at times over the mountain passes.  Some of the lower elevations over the Inland Northwest could see some snow as well, with colder valleys such as the Methow and up near the Canadian border most at risk.  Typical warmer spots such as near the foothills of the Blue Mountains (Pomeroy area) and Lewiston-Clarkston valley are under the lowest threat.  But again, this is still several days out so please keep informed of the latest forecasts.

So, what are the models showing?  They all show (except the Canadian model as of Friday night) an active northwest flow pattern with multiple storms tracking through beginning as early as Tuesday night and continuing through the holiday weekend.

Let's begin with the first system, arriving Tuesday night into Wednesday.  Keep in mind that Wednesday is one of the busiest travel days of the year.

Tuesday night/Wednesday:
Nov 19th/00z GFS model forecast of 500mb heights and  700-500mb RH 12z Wed (4 AM PST Wednesday, Nov 23rd)

This is the weakest of the incoming storms but will still likely produce accumulating snow over the mountain passes as it tracks through.  Valley snow (mainly above 2000 feet) is also possible depending on timing.  A morning arrival during the coldest part of the day would be more favorable for this.  Here is what the University of Washington WRF-GFS is showing.

Nov 19th/00z UW WRF-GFS 24 hour snow accumulation ending 4 PM Wednesday

This suggest the potential for 2-4 inches of snow Tuesday night into Wednesday in the mountains, and possibly light accumulations for the northern valleys as well.  Keep in mind this is still several days out so precise locations and amounts will probably change in the models between now and then.

Thursday (Thanksgiving):
Nov 19th/00z GFS model forecast of 500mb heights and  700-500mb RH 18z Thu (10 AM PST Thursday, Nov 24th)
A stronger storm as a deep low sends a moist frontal system through the region.   Some models dig the system a bit further south and delay precipitation until Thursday night, time will tell.  Of course timing could change even more between now and then.

Nov 19th/00z GFS model forecast of 500mb heights and  700-500mb RH 18z Fri (10 AM PST Friday, Nov 25th)
The system moves inland with continued unsettled weather.  The snow continues in the mountains, with a chance of rain and snow for the lower elevations.

Nov 19th/00z GFS model forecast of 500mb heights and  700-500mb RH 12z Sat (4 AM PST Saturday, Nov 26th)
Here comes the next storm.  Another moist frontal system tracks through with significant snow to the mountains, too early to call rain vs. snow in the lower elevations.

Nov 19th/00z GFS model forecast of 500mb heights and  700-500mb RH 18z Sat (10 AM PST Sunday, Nov 27th)

The storm moves inland and thus the pattern remains unsettled.

Now keep in mind the model forecasts are still several days out and usually don't get the precise timing and strength details correct this far out into the forecast.  The main message is that the pattern looks very active for the Thanksgiving Holiday weekend and travel impacts are a high possibility.  So keep informed of the latest forecasts before traveling this Thanksgiving.   The upcoming pattern looks very good for the ski resorts.

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