Major changes ahead

The Inland NW is about to see major changes after a very mild December and early January.   A significant storm is likely this Friday lingering into Saturday for some areas, followed by more opportunities for snow and much colder temperatures.

FRIDAY-SATURDAY SNOW

First let's talk about the late week storm.  A low pressure system will drop down from the Gulf of Alaska bringing a burst of moderate to heavy snow to most areas on Friday.  This will be a colder storm compared to the past several ones.  The green shading on the map below indicates high atmospheric moisture over the region.

12z GFS depiction of MSLP and 850-500 mb Relative Humidity valid 4 PM PST Friday, January 10th

The low is expected to track southeast into the region on Saturday.  It's exact track remains uncertain leading to some uncertainty with snow totals Saturday.  Here is one model depiction for Saturday.

12z GFS depiction of MSLP and 850-500 mb Relative Humidity valid 4 PM PST Saturday, January 11th

So how much snow are we talking?  As usual that depends on where you live.  Here is our forecast as of Tuesday afternoon of potential snow amounts Friday and Friday night.  Keep in mind as of this writing we are still 72-96 hours out so these numbers will continue to get fine tuned.  Go to our website, weather.gov/spokane, for your latest forecast.

NWS Forecast issued Tuesday afternoon of potential snow amounts Friday/Friday night

Current forecast shows heavy amounts in the Cascades, NE Washington, and Idaho Panhandle.  The Moses Lake area and Lewiston-Clarkston Valley are a tougher call as temperatures may warm above freezing.  Wenatchee is also a lower confidence point.

Another set of model data forecasters like to look at is ensembles.  This is essentially the same model run several times slightly altering the initial state.  This gives us an idea of confidence in the forecast.  Here is a plume showing snowfall accumulations for the Spokane Airport through the morning of January 15th.

12z GEFS Snowfall Forecast for KGEG (Spokane Airport)

Each gray line is a separate ensemble forecast and note how tightly clustered they are for Friday with an increasing spread in amounts starting Saturday.  For Friday-Saturday combined the model suggests about 6-10" for Spokane.  The dark black line is the average of all members and notice how those snow totals continue to climb into next week.  But also noticed the much larger spread in the forecasts indicating much lower confidence in terms of snow amounts going into next week.  The blue line represents the raw GFS model data which you can see is on the lower end of the various forecasts.

MUCH COLDER NEXT WEEK

Now let's talk temperatures!  It has been a mild stretch of late as noted by December's temperature anomalies.

And the warm temperatures have continued into January.  Moses Lake for the first six days of January is running 16 degrees above normal!  But big changes are coming.  Here is the latest temperature outlook from the Climate Prediction Center highlighting a high risk of much below normal temperatures Jan 15th-16th with a moderate risk continuing through the 19th.  This outlook begins on the 15th and the cold may start a day or two before this. Below normal temperatures will likely continue beyond the 19th.

CPC Risk of Hazardous Temperatures issued January 7th, valid January 15-21st, 2020

What is going to cause this pattern?  A highly amplified pattern with a strong ridge nosing up into the Aleutian Islands, and a deep trough over our region.  The image below shows one ensemble model projections for January 14th.

12z GEFS 500mb heights and anomalies (shaded) valid 4 AM PST Tuesday, January 14th, 2020

Cold northerly flow is expected to draw arctic air into our region as noted by the map below.  These are temperature anomalies up around 4500-5000 feet MSL showing departures from normal of -25 to -20C over southern British Columbia!

12z GEFS 850mb temperature anomaly valid 4 AM PST Tuesday, January 14th, 2020

What does this look like at the surface?  Let's take a look

12z GEFS 2-m air temperature valid 4 AM PST January 14th, 2020

You may have to enlarge the image to see the numbers.  Those pink colors to our north over Canada are surface temperatures of -37 to -20F!  Over our region the model has temperatures in the single digits and teens.  Much colder temperatures for our area, but will that really cold stuff to our north make it down here?  Good question where we again turn to the ensembles.  Here is the GEFS plume of temperatures for Spokane

12z GEFS temperature plume for KGEG (Spokane Airport) valid January 7th-15th, 2020

The model predictions of temperatures are tightly clustered through Saturday (11th) showing colder temperatures but nothing too abnormal with highs on Saturday near freezing.  But next week is a different story.  The solid black line (mean) is trending strongly colder so we have high confidence in the colder temperatures. 

But how cold?  This carries low confidence with much more variability in the members. Most solutions show temperatures falling into the single digits and teens, but some members are quite a bit colder with below 0F and even -10F readings.   Other ensemble modeling systems show similar results.  But before next week is over, single digits are a strong possibility over much of the region, with the potential for below 0F readings.

To summarize, a major change to snowy and cold weather is coming.  Moderate to heavy snow will impact most of the region Friday and may continue into Saturday for some areas.  The weather will then turn colder and potentially continued snowy for some areas.  How much snow and exactly how cold carries low confidence.