Unusual cold and last chance for lowland snow

This has been an unusually cold late-winter period over the Pacific Northwest, with a threat of (light) lowland snow during the next few days.   But the sun is rapidly strengthening and the snow threat will rapidly fade in the next few weeks.  On the other hand, the Cascades will be buried with more snow:  good news for skiers and water resources this summer.

To get you in a proper (chilly) mood, here is the departure of the average daily temperature from normal over the past two weeks over the western U.S..   Virtually the entire West Coast was 2-4F below normal, with large section of northern CA and southern Oregon 4-7F below normal.

Here in Seattle, the temperatures have been well below normal the  past two weeks, with NO days hitting the normal maximum (red line below)

Why such cold temperatures?  Because the upper level flow pattern has been locked in an anomalous configuration, with a trough over western North America causing the jet stream to push south into California ( see upper tropospheric-300 hPa pressure level- winds (colors) and heights), while downstream a ridge of high pressure has pushed warm air into the central and eastern U.S.

The latest forecasts are emphatic about keeping the Northwest U.S. colder than normal this week, a prediction reflected in the latest NOAA Climate Prediction Center 6-10 day forecasts (see below).

 OK, so what about snow here in the Northwest lowlands?   Because of a rapidly strengthening sun, we are in a losing battle near sea level.  Here are the extreme daily snowfalls at Sea-Tac Airport from (1945-2016).  In early March, there have been some 5 inch snowfalls, but by mid-March the records are only a few inches.  Past mid-April, you are out of luck.  Even now, the chances of accumulated snow is small, as the roads are starting to warm.

The atmosphere has cooled aloft during the last day and the freezing level has dropped to around 1700 ft, which means a snow level of around 700 ft.  But heavier precipitation could bring snow down to a few hundred feet today.

The latest WRF snow forecasts reflect our marginal temperature for snow near sea-level.  Remember, this  is NOT accumulation, but snowfall.  A lot of this snow will melt quickly when it hits the ground.

For the next 24 h (ending 4 AM Monday, UW WRF model is predicting some light snow along the coast (which is already happening), moderate snow over the mountains, and light snow in a convergence zone around Everett and over the south Sound.   Little near Seattle.   Why?  Rainshadowing in the lee of the Olympics.

 The next 24h has a similar pattern.

With the jet stream going south of Washington, the heaviest precipitation will be over Oregon and California during the next few days.  To illustrate, here is the forecast 72h precipitation total starting 4 AM Sunday (today)--with some areas getting 2-5 inches.   Good for water resources.