Wind, Snow, and Cold: Very Active Weather in the Northwest

This morning winds are gusting to 40 mph and more around the region (see max gust map below), with scattered power outages in Seattle and vicinity.

The reason?  A low pressure center is moving eastward to the north of Puget Sound, resulting in a large pressure gradient...and thus strong winds.

A strong front is associated with the low, and with powerful westerly winds aloft (some reaching 70-100 mph!), massive snow amounts are falling in the Cascades, with the big trouble in the passes (chains required and delays in Snoqualmie).

But this active weather is just a "warm up" for what is about to happen. (In fact, just the opposite of a warm up!).

Tonight and tomorrow morning,  modified arctic air is going to push in, with a huge pressure gradient down the Fraser River Valley, a conduit between the cold, high pressure in the BC interior and the warmer, lower pressure conditions in western Washington.    Very strong winds will exit the Fraser Valley producing gale-forced flow from north of Bellingham, across the San Juans, and towards the Olympic Peninsula (see forecast gusts at 8 AM tomorrow).  Wow.

The latest forecast model output suggests snow on the windward side of the Olympics and light snow even over Seattle as the leading edge of the cold northerlies push southward during the morning hours.  For Puget Sound residents, LIGHT is the operative word.

Here is the forecast 24-h snowfall (not accumulation) ending 4 PM Sunday.   Lots in the Cascades and less than an inch over parts of Puget Sound, with perhaps 1-2 inches in places south of Seattle. More snow over the San Juans.

But this is only one model forecast.  We should turn to ensembles (running many forecasts to judge our confidence in the snow prediction).   Here is a "plume diagram" showing 15 accumulated snow forecasts for Sea-Tac airport (time on the x axis and snow total on the y axis). A LOT of variation and thus uncertainty, with the ensemble mean being about half an inch.  The heaviest is 1.4 inches. 

But then cold air moves in and the skies clear...and the models are much more in agreement regarding temperature (see forecast below for Sea-Tac).  Dropping to the low 20s on Monday morning--and even colder away from the Sound.

Time to go outside and disconnect all hoses.    This will be coldest air we have had in over a year.