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