Thursday, March 21, 2013

How Low Can It Snow?

Spring is here...and often we get some of our coldest air aloft during the early part of this season.

Behind yesterday's strong front, we are getting some very cold air moving in aloft, with temperatures at roughly 5000 ft (850 hPa) dropping to -6 or -8C. Those temperatures are much colder than usual and cold enough, in the presence of heavy precipitation, to bring snow levels down to near sea level...and certainly to the tops of higher hills (500-1000 ft).    The cold air aloft, coupled with the warming of the surface (yes the sun IS getting stronger), produces a lot of convective clouds and showers (convection...associated with cumulus and cumulonimbus clouds... is forced by large changes in temperature with height).  Such convection produces intermittent heavy precipitation and is being enhanced by convergence zones in the lee of the Olympics and upslope on our local terrain.

Why do heavy showers cause the snow level to descend?  Melting of snow falling into warmer air is the big reason.  Evaporative cooling can be important as well if the air below is not saturated (100% relative humidity).

The latest visible satellite and radar imagery really shows the instability clouds and showers.



A proof, here is a picture earlier today of a light dusting  of snow at Clearview, northeast of Seattle near Mill Creek, provided by Lori McConnell.

Take a look at the predicted temperatures for 8 AM on Friday at roughly 3000 ft (925 hPa) from the UW WRF model--the blue is cold air air!


And here is the 24-h snowfall predicted by our flagship, super-high resolution WRF model...1.3 km horizontal grid spacing for the period ending 5 AM on Friday.   Some snow getting real low---nothing heavy of course....   so Jim Forman of KING-TV can keep his parka in the closet.   More of a change over NW Washington and over central Puget Sound where the convergence zone will be active.

And there will be lots of snow in the Cascades... some locations (particularly those feeling the enhancement of the convergence zone) could get a foot or more.

View from Peter Benda's House in Bellevue on Thursday
View from Peter Benda's house in Bellevueon Thursday

5 comments:

Gpacharlie said...

Snowing right now at 4pm at 1000' near North Bend.


Unknown said...

Rain & snow mix in Duvall at 428'...

Dan said...

Just as a point of clarification. Is it evaporative cooling that cools the air as the snow falls, or the heat of fusion of the melting ice crystals? Or a combination of each? If it takes 334 joules to melt a gram of ice, it seems like just the melting process alone will absorb a considerable amount of energy from the atmosphere as the snow falls through the warm air.

Ansel said...

I got about 1.5 inches of snow on the Bothell - Mill Creek line. More than all winter.

snapdragon said...

Hazel Dell (SW WA) got sun, hail, wind, rain, snow mixed with rain, snow not mixed with rain, and snow mixed with rain and hail today. All within about 40 minutes.
Oh, and yesterday there was a tornado in Hockinson. It was only an F0, but still...