Snowy weather ahead!

Those that are looking for more snow, we have some good news for you!  The Inland Northwest is entering an active winter pattern with cold and snowy weather.  While we are confident of this pattern, a complex situation is developing.  Let's take a closer look at the pattern, beginning with Thursday.

00z/Dec 7th GFS model forecast for 500mb heights and 700-500mb Relative Humidity 21z THU (1 PM PST THU)

The bright green areas indicate areas of high moisture content with snow expected to develop across the region late in the afternoon or evening on Thursday.

How about Friday?

00z/Dec 7th GFS model forecast for 500mb heights and 700-500mb Relative Humidity 21z FRI (1 PM PST FRI)

The low moves closer to the region with lots of moisture streaming over cold air still in place.

And Saturday?

00z/Dec 7th GFS model forecast for 500mb heights and 700-500mb Relative Humidity 18z SAT (10 AM PST SAT)

Still looks unsettled, and, Sunday?

00z/Dec 7th GFS model forecast for 500mb heights and 700-500mb Relative Humidity 18z SUN (10 AM PST SUN)

Possibly a "relative" before another system seen here approaching the coast.

So why the prolonged period of active weather?  We are in the battleground area between the Polar Jet stream coming around a strong blocking ridge of high pressure over Alaska, and a Pacific Jet Stream.  Where the two meet equals snowy weather.  Here is one GFS model run for Sunday illustrating this complex pattern.

Would you like proof of the frigid air to our north?  Well here you go!

18Z GFS/Dec 6th forecast of 850mb temperatures and heights valid 18z SUN (10 AM PST SUN)

Given this complex pattern, just small variations in the jet stream can lead to big shifts in where the heaviest snow falls.   To help with this, we can look at the ensembles where each member makes a prediction.  In this case, we will look at the snow forecast for Spokane.

GEFS Snow accumulation forecast for Spokane WA through 4 PM Sunday.

As you can see, all solutions show snow beginning to accumulate late Thursday afternoon or evening, and show snow persisting at times at varying intensities through the weekend.  By4 PM Sunday, most solutions show anywhere from 6 to 10 inches.  However again given the complexity of this situation I wouldn't be surprised to see this range come down or up slightly.  Also this doesn't take into account compaction which can lead to a snow depth (snow on ground) not as high compared to the total snowfall.

Here is one model solution showing geographically the potential amounts of precipitation through the weekend.

00z GFS/Dec7th forecast of total qpf through 00z Monday (4 PM Sunday)

Given the westerly flow providing upslope into the higher terrain, the Cascades crest and the Central Panhandle Mountains will get hammered with heavy snow.  This solution indicates the potential for 2-4 inches of liquid translating to at least 2-4 FEET of snow along the Cascade crest and 1-2 FEET over Lookout Pass.   Travel over the mountains will be especially hazardous.

So, how long will this pattern last you ask?  Good question.  The latest CFS Model shows December as a whole finishing colder and wetter than normal, take a look.

This active winter pattern should last into next week and possibly beyond.  Low confidence of the details so stay tuned!