Weekend Storm

This weekend we are monitoring the potential for weather impacts across the region as an Atmospheric River takes aim at the area.   While the weather will differ depending on your location, there are several items we are monitoring including moderate to heavy rain, some mountain snow, wintry mix potential for the Methow Valley, and windy conditions.

Here is one model depiction of an Atmospheric River aimed at the region Saturday night.

00Z Dec 16th GEFS forecast of Integrated WVT valid 10 PM PST Dec 19th, 2020

But you may be asking, what is an Atmospheric River?  Basically it is a column of very moist air containing high amounts of water vapor that gets transported towards the west coast by moderate to strong winds.   The graphic below contains lots of information, but worth a read for those wanting to know more, with this link also providing more information.

So what kind of impacts will this lead to this weekend?  First let's begin with the warmer temperatures.  

WARMER TEMPERATURES

Some of the raw model data suggests Spokane could reach 50 degrees as noted by the big upward spike in temperatures Saturday night into Sunday

Various model forecasts of surface temperatures for Spokane Airport through Dec 20th, 2020

The Tri-Cities area could reach 60 degrees.

Various model forecasts of surface temperatures for Pasco through Dec 20th, 2020

And the warmup isn't just for Spokane.  This model depiction during the peak of the warmer air shows freezing levels possibly reaching 6000-9000 feet MSL.

12z Dec 16th UW WRF-GFS forecast of freezing level (MSL) valid 4 AM PST Dec 20th

WINDY CONDITIONS

In addition to the warmer temperatures, models show an area of enhanced winds across the Columbia Basin, Palouse, and up into the Spokane area. 

12z Dec 16th GFS model forecast of 850mb winds (shaded) and heights valid 4 AM PST Dec 20th

These are winds at about 4500 feet MSL, which is about 2000-3000 feet above ground level.  For those looking closely at the legend, those red colors correspond to 50-65 kt (55-70 MPH) winds!   Winds at this level can represent wind gust potential but it all comes down to mixing of the lower atmosphere as to wind gusts that are experienced at the surface.  Typically in these more stable atmosphere regimes the higher gusts don't reach the surface with model guidance suggesting wind gusts of 40-55 MPH being a more probable outcome, but something we will continue to monitor.

PRECIPITATION

What about precipitation?  Here is one model depiction starting 4 PM Saturday.

12z Dec 16th GFS 6 hour precipitation rate and precip type valid 4 PM PST Dec 19th

It looks like precipitation type is mainly rain with mountain snow up near the Canadian border.  Some model output suggest some of the mountain passes may start off as snow with possible accumulations.  Also note the pink area in the above graphic around the Methow Valley where cold air often has a harder time scrubbing out.  This denotes a potential wintry mix.

Fast forward now to 4 AM Sunday.

12z Dec 16th GFS 6 hour precipitation rate and precip type valid 4 AM PST Dec 20th

As warmer air surges north, rain is the main precipitation type even up to the Canadian border, with rain intensity increasing in the Cascades as noted by the orange and red colors.  What is interesting though is the lack of significant precipitation in Central and Eastern Washington.  While these Atmospheric Rivers have an abundance of moisture, the strong winds often "ring out" much of the precipitation over the Cascades, with downslope flow off the Cascades leading to a drier conditions in Eastern Washington and especially Central Washington.  Here is one model depiction of 24 hour precipitation highlighting this

12z Dec 19th GFS forecast of 24 hour precipitation ending 10 AM PST Sunday, Dec 20th

SNOW MELT AND HYDROLOGY OUTLOOK

Here is the latest snow analysis as of this morning, Dec 16th.

Modeled Snow Water Equivalent (SWE) valid 8 AM PST December 16th, 2020

The warmer temperatures and breezy to windy weather this weekend will melt much of the lower level snow pack leading to rises on small streams.  Thankfully the incoming Atmospheric River will be of shorter duration (24-36 hours) over our region.  This combined with the lower stream flows to start should keep the flooding threat low.  Although the best combination of warmer temperatures, snow melt will be over the Palouse area so will be monitoring Paradise Creek.  The Cascades will see a burst of heavy precipitation, but the deep snow pack may absorb much of the rain for the East Slopes of the Cascades.  But in Western Washington, some rivers may reach flood stage based on forecasts as of this morning.

River Forecast Center Max Flood forecast page.  See link below for latest forecasts

For the latest river forecasts, click here

In summary, expect an active weekend with a wide variety of expected weather.  Stay informed of latest forecasts.  And for those wondering about having a White Christmas, not much has changed with our thinking since the last blog post through the 20th, which can be found here.  There is low confidence with the Dec 21nd-22nd forecast as models are not in good agreement with a possible weather system during this period.  Yet another opportunity for precipitation may arrive on Christmas Eve or Christmas Day so stay tuned for updates.

White Christmas this year?

Many of you may be wondering about odds of a White Christmas this year.  Snow has covered the ground recently in many areas, but will it stick around until Christmas?  What about the outlook for Christmas Day?

First let's start with the climatological odds of a White Christmas.  As always, the odds depend on where you live as the map below shows

Odds of a White Christmas, defined as 1" of snow or more on the ground, based on historical data shows the odds increase as you travel northward in the Columbia Basin up into the Spokane/Coeur d'Alene area.  And along the East Slopes of the Cascades, northern mountains, and Idaho Panhandle mountains the odds are very high (>90%).  There is a great article explaining this map here that has more background information, including an interactive map.

You may be saying ok these are the historic odds, but what about this year?  Let's start with where we have snow.  The map below from December 13th shows water content in the snow, which isn't much where most people live.  

Modeled Snow Water Equivalent (SWE) valid 4 AM PST Dec 13th, 2020

And the latest CoCoRaHS map from today (Dec 14th) shows many of these locations only have between 1-4" of snow on the ground, except for higher amounts around Republic and the upper Methow Valley.

CoCoRaHS map of Max Observed Snow Depth Dec 13-14th, 2020

For an interactive version of this map, click here

Why all this talk about what is on the ground now?  Because many areas may have a hard time keeping it.  More on that in a minute.  First it is worth mentioning that some areas will get more snow this week.   But snow levels will be gradually increasing.  The latest forecast for your location can be found at weather.gov/spokane

Temperatures are forecast to warm up this week, especially over the weekend.  This may melt much of the snow over the region.

Here are the forecast temperatures this week for Spokane, Omak, Lewiston, and Wenatchee.

NWS Forecast Temperatures issued 3 AM PST Dec 14th, 2020

Why the big warmup this weekend?  A mild Pacific jet stream aimed at the region

12z/Dec 14th GEFS forecast of 250mb winds (shaded), and MSLP valid 4 AM PST Dec 20th, 2020

And this particular model solution depicts warm southwest winds across much of the area.

12z/Dec 14th GFS forecast of 850mb winds (shaded) and heights valid 4 AM PDT Dec 20th, 2020

Mild temperatures and wind can melt snow very quickly.  Thus, there is a good chance of a complete snow melt off across the Columbia Basin, Spokane/Coeur d'Alene area, southward through the Palouse if these model solutions verify.  Tougher call for the northern valleys.  The Cascade valleys should have enough of an established snowpack to keep snow on the ground.

This takes us through December 20th.  So for areas that lose all their snow this weekend, will the snow return in time for Christmas?  This is getting far out into the forecast, but there has been some hint of a potential system with lower snow levels Dec 21-22nd.

12z/Dec 14th GEFS forecast of 500mb heights and anomaly valid 10 PM PST Dec 21th, 2020

Confidence isn't high, but some areas could get some snow out of it.  After that there are signs of high pressure trying to build over the region in time for Christmas.  But it's not very strong which could open the door to some weaker systems.  Of course this is more than a week out so confidence in the details is low.

12z/Dec 14th GEFS forecast of 500mb heights and anomaly valid 4 AM PST Dec 25th, 2020

In conclusion, a white Christmas defined at 1" or more of snow on the ground is a good bet for the Cascade valleys, and possible for the northern valleys into the Idaho Panhandle.  From the Wenatchee area through the Columbia Basin, Spokane/Coeur d'Alene area, Palouse, and Lewiston-Clarkston Valley a White Christmas is possible if new snow coats the landscape after the weekend warm up.