A Modest Snow Event Later Today; A Potentially Major Event Later Tomorrow (But Substantial Uncertainty)

Sometimes the atmosphere has "juice".  Lots of vertical motion, lots of precipitation, the potential for a major, problematic event.

Later today we will have a snow event with little juice, but one that we have a good idea of what will happen:  light snow (around 1 inch) sometime after 4 PM.  We have a lot of confidence in that forecast.

But then there is tomorrow.  Later in the day, a much stronger system will approach, with a lot of "juice".   But temperatures will be on the margin, leaving the potential for a "wintry mix" for central Puget Sound southward.   We could also get a major snow dump.  Or we could get mixed rain and snow.  We could get freezing rain over the south Sound.   A lot of uncertainty...and I will try to unravel that in this blog.

We start this morning with some of the coldest temperatures of the winter in many western WA locations (see below, click to expand).  Many areas away from the water were in the teens (even 18F at my location in NE Seattle), and some single digits in colder valleys and the south Sound.   The ground is below freezing and rock-hard ice are on many roadways.

A weak upper level disturbance is approaching (see the clouds over BC in an 8:30 AM satellite picture).  Some low clouds are covering portions of western WA and if you look closely you can see the snow on the image.

Associated with the upper level disturbance will be a surface low (see below) that will cross the northern portion of WA.  Not an optimal position to produce a lot of snow over the lowlands without "juice".

The latest UW WRF forecast (and everyone else's) is for light snow over the Puget Sound lowlands later today (see the snowfall total ending 7 AM Monday).  Roughly an inch, but with heavier amounts over NW WA and in a band south of Seattle.  More near Olympia in the mountains.   A big issue will be downslope flow on the Olympics and the mountains of Vancouver Is:  not rainshadowing but snow shadowing.

And our high-tech, high-resolution UW ensemble forecast of snow at Sea-Tac shows most of the solutions are for about an inch or two, with the majority of the action before 10 PM tonight.

Bottom line.  Light snow will start between 4 PM and 7 PM today.  Light, powdery snow will fall on a cold surface, so melting will be minimal.  With salt, plowing, and a bit of attention, local department of transportations should be able to ensure the main roads are in good shape by commute time tomorrow.  Here at the UW, many of the roads are icy and some buildings don't have cleared paths--attention will be needed to make it ready for tomorrow.

But all this is a warm up for the real-threat tomorrow.

A much stronger system is approaching, with a strong warm front, a well-defined cold front, and much more upper level support.  Much more juice.  The forecast for 1 PM tomorrow shows the sea level pressure and temperature around 2500 ft.  Blue is generally cold enough for snow.

The low will slowly creep towards NW Washington (see same map for 4 AM on Tuesday), with warmer, moist air invading southwest WA, while cold air well be sucked into NW Washington by the differences in pressure (low along the coast and high pressure over southern  BC)

Over SW Washington, precipitation will turn to rain.  Over NW WA there will be snow, and in between there will be wet snow, freezing rain, and everything in between.   

The 24 hour snowfall predicted by the UW system shows this storm, with lots of snow (3+ inches)  north of Seattle, much less to the south.  Huge amounts of snow in the Cascades.

The latest European Center forecast for the same period is generally similar (note that is lower resolution than the UW model).  Mountains get hammered, 2-6 inches around Seattle.

If the temperatures were cold enough, this system would produce a foot of snow, but temperature is the problem.  

But things are more complicated.  Our computer models tend to mix out low-level cold air too quickly, reducing snowfall.  So there could be more.   And god help us if the models are wrong and the low went farther south.  Then we would be talking about the snow apocalypse. 

OK, what is my bottom line with the next storm.

Snow will reach the south Sound in the mid afternoon (around 2-3 PM) and move into central Puget Sound a hour later (3-4 PM).   Snow will be falling during the rush hour.   The evolution will depend where you are.  Transition to rain over SW WA, all snow over NW WA.  In central Puget Sound there may be a transition wet mixed, wintry precipitation during the evening, but there will be at least 3-5 inches of new snow.   If the cold air holds, 5-8 inches.  And expect wet snow, not the fluffy stuff we will have tonight, and a good chance of transitioning to cold rain on Tuesday morning.  What a mess.

Sorry I can't be more definitive...but this one is very hard.  More tomorrow.

from Cliff Mass Weather and Climate Blog https://cliffmass.blogspot.com/2019/02/a-modest-snow-event-later-today.html