Posted by Rufus La Lone on

Wednesday December 14
Winter pattern on the way that could impact Christmas holiday travel.  Be prepared.  Subject to change, of course.
The 1st clip of cold air will infiltrate NW WA & BC this weekend.  A weak surface disturbance arriving late Sunday will increase the chance for low elevation snow (500-1,000 ft) or even to the sea level for areas north of Mt Vernon.  This increases on Monday & Tuesday as a stronger push of moisture moves in and heads south.  SNOW SHOWERS at the surface is probable on Tue., extending across all of the Puget Sound region.  Rain or rain/snow mixed possible for NW OR & SW WA Tue, depending on track of the Low; however, models suggest colder air will follow the weak front, so precip could turn to all snow late Tue from north of Salem.  The Columbia Gorge and eastern basins will be wintery.  
Turning notably COLDER by the middle of next week, as modified Arctic Air arrives.  The Fraser Gap outflow will be substantial, as we discussed in our last forecast.  
A 2nd, much colder air mass continues to drop south out of the interior of Canada by Thu morning, setting up strong east-to-northeast winds and chance for FROZEN precipitation moving over the top by sometime Thu Dec 22.  Moisture will arrive at the same time that the coldest air settles in east of the Cascades - and heads into OR, as well.  
SNOW ahead of sleet and then freezing rain - starting from western OR (roughly Salem) moving north - is looking possible on Thu Dec 22.  Snow could be HEAVY over western WA and in the Columbia Gorge, east Portland before turning to SLEET and eventually freezing rain overnight. Yikes.   Puget Sound could be hammered with frozen precip, as the high moisture laden front moves north.  The event could last through mid-day on Friday before the front crosses the Cascades.
Travel may be interrupted both on land and by air.  Consider options, just in case.
We will update this outlook in detail on Friday Dec 16.  This event may NOT happen, but we consider the chance for some sort of winter wx that impacts pre-Christmas travel (Salem north into southern BC) to be probable enough to warrant planning alternative travel or the timing of travel.  
No alarm bells just yet.  Please.
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