Increasing Threat of Lowland Snow Later Today and Monday

I don't want to send folks running to their supermarkets or distract from the big game today, but weather forecast models are suggesting increasing chances of lowland snow later today and tomorrow.  But there are nuances and uncertainties that are very important to communicate, and I will try to do so here.  Then I will run to the store to buy milk, bread and provisions. 😊

But before I talk about the general situation, let's note the significant wintry weather occurring in Whatcom County, north and east of Bellingham.  For most of the region, temperatures are now (7AM) too warm for snow, as shown by the vertical sounding at Sea-Tac, where the freezing level was around 1500 ft at 7 AM.   The snow level (the level where all the snow is melted) can be up to 1000 ft below that (500 ft).   Raining is falling on most of you in the west.

But up in Whatcom County, cold air is seeping into the area through the Fraser River Valley, with cool, northeasterly flow bringing temperatures in the 20s F north and east of Belllingham (see map).

As a result, there has been snow north of Lynden and freezing rain in regions northeast of Bellingham.  Power lines have been taken out by the ice (see the PSE outage map) and the some locations have been covered by ice (see picture by John Bosche)

Today, the air above the region will slowly cool as an upper level disturbance approaches (called an upper level short wave).  This will set the stage for the snowy fun later today and tomorrow.  I have studied Northwest snowstorms for years, and the situation is not optimal for a big snow event, but it is close enough and the models are now insistent enough that significant lowland snow from Seattle to Portland looks highly probable.

In this discussion I will show you the uncertainties, stressing the use of ensembles (many forecasts) to get at those uncertainties.   As noted many times in this blog, the European Center has the best large scale forecasts and their large ensemble is considered the best.   Below is a graphic showing their prediction for Seattle for the next several days. Each horizontal colored line indicates accumulated snow from one of their 51 ensemble forecasts (each is a little different in subtle ways).  Virtually all go for snow, with some showing as much as 10-12 inches.  The ensemble average (or mean) is for about 8 inches (see panel below), with their single high resolution run, a bit more.

Since their high resolution run seems to be on the same page as the ensemble, let's look at its solution, presented in two expanded panels. The heaviest snow is from south Seattle to Vancouver, WA and some of the amounts are impressive 6-15 inches. Not much in Portland.

 The high-resolution UW WRF model is driven by the best US guidance (the GFS).  Here are two forecasts for the 24h ending Monday and Tuesday at 4 AM.   Same general idea, but with less snow by a few inches.

What about the uncertainties?   Here are the ensemble forecasts for snow at Seattle Tacoma Airport from the US (NWS)  GFS (GEFS) ensemble.  A lot of variability (1.5-9 inches), with a mean (black line) of 5 inches,

Both the European Center and U.S. GFS ensemble are relatively course (20-30 km grid spacing), since they are global models.  The only real high-resolution ensemble is run by NCAR (National Center for Atmospheric Research), which runs a small (10 member) ensemble at 3 km grid spacing.  Here are the ensemble forecasts of snow at Sea-Tac from the NCAR system. The real snow starts tonight and there is a little over 4 inch accumulation.  Similar geographic distribution as noted above...and substantial uncertainty as shown by the brackets.  The National Weather Service needs a high resolution ensemble over the U.S. but has been dragging its heals about it...perhaps something to suggest to President Trump.

Finally, the National Weather Service has another, somewhat coarse and poorly designed, ensemble system called SREF.   It also show huge variation of snow accumulation at Sea-Ta, with ensemble mean of around 3 inches, with solutions from a trace to 9 inches.

Why the uncertainties?    A big issue is that temperatures are marginal, so whether snow falls really depends on the exact flow pattern and the precipitation rate (heavier precipitation brings down the snow level).

So what is the take away from all this?

Meteorologists can not give you a specific forecast with absolute certainty....we have to talk about probabilities.   Combining the best tools available to us (ensemble forecast systems at relatively modest resolution and a limited number of high-resolution forecasts),  it appears highly probable that there will be lowland snow from Portland to the Puget Sound lowlands.  In Seattle, there will be some rain and mixed rain/snow showers during the day, but the real action will start this evening and extend into Monday.  There could be some snow today on the higher hills of our region.   The most probable value around Seattle for the event is around 3-5 inches, with more to the south.  In Seattle, there is a perhaps 25% chance of getting only a dusting of snow, with mainly rain falling.  There is a similar chance of getting a major event with 6-12 inches.  These are similar probabilities as were given for the election of President Trump---so they are not negligible.

The mountains are going to be hit hard with feet of snow...that is a near certainty.

I will let all of you decide whether to buy chicken wings for the Superbowl game or milk for tomorrow.  If I were you, I would purchase both.
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New Hawaii Weather Blog

Want to escape the cold winter weather of the Northwest, at least in spirit?
Interested in learning more about Hawaii weather?
You are in luck!

A new Hawaii Weather Blog has been created recently by Professor Steve Businger of the University of  Hawaii.   Steve is an expert on Hawaii weather and his explanations about Hawaii weather features are fascinating.  Check it out here.