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.