Wet and windy storm this week before winter returns next week

For those wondering what happened to La Nina, a more winter like pattern may be returning next week.  So you may want to keep this nearby because it is still winter.  

Later we will discuss what may cause this pattern change and what it could mean.  But before we get to that, we have another storm this week to talk about so let's start with that.

Monday-Wednesday Storm

Another mild and wet storm is on tap for Monday through Wednesday with two surges of moisture as another atmospheric river takes aim at the region.  The first moisture surge arrives from the southwest Monday through Tuesday morning as noted by the influx of moisture from this model run.

UW WRF-GFS Forecast of Integrated Water Vapor Transport valid 4 AM PST Tuesday January 12th, 2021

For most towns precipitation will fall as rain.  One exception is the North Cascades and the Okanogan Highlands especially the Methow Valley where heavy snow is possible.  Also we will be monitoring areas near the Canadian border especially the higher terrain.  Here is the European model extreme forecast index of snow highlighting these areas Monday night into Tuesday.

ECMWF Extreme Snowfall forecast valid 4 PM PST January 11th to 4 PM PST January 12th, 2021

It looks like the moisture feed continues Tuesday night into Wednesday with another significant slug of moisture but with higher snow levels.

UW WRF-GFS Forecast of Integrated Water Vapor Transport valid 4 AM PST Wednesday January 13th, 2021

How much precipitation are we talking?  Rain totals of 0.75" to 1.50" is possible across Eastern Washington and north Idaho with 0.25-0.75" across the Columbia Basin into the Lewiston-Clarkston Valley.  

What about wind?  This is something we will need to keep a close eye on.  Windy conditions are possible Tuesday night into Wednesday across the Columbia Basin, Palouse, and West Plains.  Models haven't been consistent on the wind speeds, but with strong winds above the surface (60-75 MPH at 4500-5000 feet MSL) we will be monitoring the potential for some of this to mix down to the surface.  Here is one model depiction of those winds.

Jan 10th 18z GFS run of 850mb wind speeds (shaded) and heights.  

Return to winter next week

January has been off to a mild start.  But we are looking at likely changes next week.  Here is a look at the first nine days of January highlighting the mild start.

Climate Toolbox image showing average temperature anomaly January 1st-8th, 2021

And the mild temperatures will continue this week.  The Tuesday and Wednesday system will have plenty of mild air associated with it.  Here is an animation from an ensemble system that shows the mild pattern dominating this week, but look at those cooler temperatures over western North America during the Jan 21-26th time frame!

12z/Jan 10th GEFS 2 meter temperature anomaly (degrees C) valid Jan 11th-26th, 2021

What could cause such a change?  

12z/Jan 10th GEFS 500mb height and anomaly (shaded) valid Jan 11th-26th, 2021

Note the big pattern change that occurs during the period.  We start off with low pressure in the Gulf of Alaska with a prevailing southwest flow bringing in the mild temperatures.  But during the Jan 21-26th period there is a noteworthy shift in the pattern, with high pressure building in the eastern Pacific bringing a cooler north to northwest flow to the area.  

Now this is just one modeling system, but other models run out of Canada and Europe show a similar change, and this change has been indicated by these models over the past several days which does yield more confidence in the change.

What will this mean?  Looks like a good bet for more winter like temperatures.  And potentially some precipitation to go with it likely in the form of snow.  Stay tuned for updates as we get closer.