Sunday, March 5, 2017

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.


11 comments:

Cole Bazemore said...

About half an inch of snow at Pacific Beach. Large, wet flakes blanketing the beach which is unusual and quite beautiful.

Bob said...

On Sunday Mar 5 at 12:30 pm on, LGX was showing the most perfectly formed "hook" I have ever seen in the PNW. If you have access to archived LGX data, you could pull out an amazing frame of extra-equatorial hurricane influences. Thanks for all your good work educating the public.

Mike P. said...

The low pushing onshore at 3pm is causing some very strong winds in the Olympia/Tumwater area right now. Wasn't in the forecast, but potential for downed limbs or power outages here on the Westside if it keeps up for very long...

Catherine Cruver said...

Why are the forecasts so off for Tacoma. Is it the proximity to water? Tacoma is a city of 200,000 but no one forecasts the weather for here. We have had snow exactly twice but we are told we have had many snow events.

Gary said...

At 1354 on the NW radar loop (http://www.atmos.washington.edu/weather/radar.shtml#) a nested hook of precip appears at the coast near Long Beach. We had some nice gusts here in Olympia around that time :-).

Leif said...

IMO the reason for the locked in Jet Stream is because of the loss of Arctic Ice and the exposed open seas have warmed the arctic air. The cold is then forced to move over the coldest remaining portion of the earth which is the land mass. Then warm southern air is sucked in from the North Atlantic and Pacific. Then down over the continents.

Kevin Unknown said...

Leif - How are the arctic ice levels this year compared to the ice levels two years ago during the winter of the BLOB? The level of ice two years ago couldn't have been significantly more than this year but we were locked in a pattern of continuous high pressure over the eastern pacific that led to the elevated water temperatures.

granitix said...

Morning showers have pushed us to 3" in the hills behind Longview @750'. Still pretty, but who'd have thought this could get monotonous?

Lori said...

Snow in Clearview, Tuesday, 9:45 am. I usually love the snow, but I'm hoping this is the end of it!

mig said...

If it were closer to the winter solstice, I'm guessing it would be cold enough for the current weather pattern would bring the Puget Sound lowlands LOTS of snow?

Moral Individual said...

Leif and Kevin Unknown: The cold weather is most likely being caused by the lack of sea ice and that impact on the temps at the top of the world. Please read the following http://nsidc.org/arcticseaicenews/ Cold air being displaced from the top of the world, an unstable polar vortex and that Omega high in the Gulf of Alaska have most likely contributed to lower temps here. I am thankful the jet dipped as this whole state would have been buried by snow if we had gotten the combo of arctic air and the string of storms that went into California.