The Potential for More Lowland Snow

Don't run out to the supermarket yet.  But don't put away your snow shovels or snow boots.  We are about to go into a cooler period, with the potential for snow reaching sea level, and near certainty of the snow level dropping down to 500-1000 ft.

Cooler air, capable of supporting lowland snow is going to move in this weekend--not as cold as last week, but cold enough.  To illustrate this, here is the sea-level pressure and low-level temperature analysis for 4 AM this morning.   Blue is cold, purple is colder, and you don't even want to think about white.  Light blue is cold enough for lowland snow.  Western WA is now too warm for snow, and in fact it is raining.

But here is the same chart, but for the forecast valid at 4 AM Sunday.  Blue and purple has moved in.  Much colder, and the cold air has pushed south to northern CA and Nevada.  Cold air that will give us a real shot of Sea-Tac experiencing the coldest February on record.

This movement of cold air at low levels is associated with the building of high pressure aloft over the eastern Pacific, as illustrated by the upper level (500-hPa pressure level) chart for 10 AM Sunday.  Such a configuration is generally dry, since east of upper level ridges there is generally descending motion and lack of clouds/precipitation.   This is not like a typical El Nino situation, more more representative of La Nina years.  We currently have a very weak and dying El Nino--too weak to have much impact.

But this upper level pattern can bring snow if an upper level disturbance moves southward along the eastern flanks of the ridge.  And the latest model runs suggest the potential for this to happen, first Wednesday and then next weekend.

Let us start with the best global modeling system in the world (the European Center model) and the output of its 51-member ensemble system.   I love to use this....since each forecast starts slightly different and we can examine how sensitive the forecasts are to small differences in initialization. 

This graphic shows accumulated snow at Sea-Tac for the 51 members (top panel) and the average of the ensemble (green lines below).    Most members show light snow starting around February 20th at 0000 UTC (4 PM on Tuesday).  Around an inch. 

And a majority of the runs shows even more snow starting on Friday...bringing the snow total to around 3 inches.  But much more uncertainty with the timing and magnitude of the second pulse.

So, let's examine the Tuesday/Wednesday snow situation in more depth.   Here is the 24-h total snowfall ending 4 PM Wednesday.  Lots of snow in the mountains (our ski areas will brag about the powder available here!), and light snow extending over NW Washington and down to Puget Sound.  Temperatures are marginal for snow near sea level...that is why there is so little around the south Sound, the coast, and the Strait.

What about the ECMWF model for the same period? A nearly identical solution.  Gives us confidence in the prediction.

But what about the potentially bigger event later in the week?  At this point, the uncertainty is simply too large. 

Looking at the ensemble snow forecasts at Seattle from the US system (GEFS), we note that nearly all the models are going for snow on Tuesday afternoon (about an inch, with the solid black line being the ensemble mean, which is often the most skillful prediction).  You can see the Friday/Saturday event, but the forecasts are all over the place.   A lot of uncertainty.  We have to wait to have any confidence in the prediction.

Finally, there is a lot of talk about landslides in the media and some weather information sites.  I believe this is highly unlikely.   Even with all the snow, precipitation has been nearly normal during the past 30 days and much of the water is locked up in snow.   With the cold temperatures, the snow will not melt rapidly.  And there is no hint of any weather feature that will cause rapid melt down (like a strong atmospheric river).   I worry about the exaggeration of risks, both regarding weather prediction and in the climate arena.   It may promote more clicks and attention, but in the end it lessens credibility.

from Cliff Mass Weather and Climate Blog http://bit.ly/2tlTR6u