Cold, Unstable Air Brings Snow Down to Higher Hills Around the Puget Sound Region

Unusually cold air has moved over the northeast Pacific and the Pacific Northwest, creating unstable conditions and snow descending to the higher hills.

During the past hour (10 AM), snow was reported at Paine Field in Everett (604 ft) and at Sea-Tac Airport (452 ft).  At Peter Benda's house in Bellevue (1300 ft), snow was accumulating in a winter-like scene.

Using the capabilities of the City of Seattle's SNOWWATCH app, we can see that the freezing level this morning has dropped to about 1300 ft (see plot of temperatures above SEA-TAC airport below), which means the snow level would be roughly 300 ft (below that level there would be only rain).

Cold air has moved in aloft, associated with an upper-level low pressure center that is centered west of Vancouver Island (see 500 hPa--around 18,000 ft-- weather map below for 5 AM Friday morning).

Here is a temperature map at roughly 10,000 ft (700 hPa)--you can see the cold air aloft (purple colors) associated with the low.

With cold air aloft and warming temperatures at the surface (it IS late March), there is a very large change of temperature with height and that promotes instability:  having the lower atmosphere break up into upward moving thermals and sinking air around them.   Just like in your hot cereal pot when you turn on the burner.    Upward motion results in cumulus, cumulus congestus, and cumulonimbus clouds, resulting in intermittent heavy showers.  Maybe some soft hail or graupel if you are lucky.

You can see the instability in the visible satellite image this morning.  The small elements are the anvils from convective clouds.

Perhaps a more impressive view is from the infrared satellite image (the shading shows the temperature of the cloud tops, with white being the highest).  Cold, deep convection over and off our coast.

The problem for lowland snow lovers is that low-level air is coming off the warm ocean (which is about 50F), making it too warm for snow, and particularly accumulating snow, to reach sea level.

The threat of light lowland snow over the hills is not over yet.  Tomorrow morning, the upper level low and its surface reflection will approach and move south of us (see surface weather map at 8AM Saturday), with cool air over western Washington.  Very weak onshore flow.  There will be enough upward motion to promote some showers of snow and rain.

The latest UW WRF model run shows some light snow getting close to sea level for the 24h ending 5 PM Saturday.  Enjoy the snowflakes...this could be your last view of them until next winter.

from Cliff Mass Weather and Climate Blog https://ift.tt/2HW8UsP