Saturday, December 31, 2016

1 PM Update

The 1 PM visible satellite image shows the approaching front/low pressure system with substantial convective clouds behind it.   The leading edge of the front is now stretching from Forks, on the NW coast, to NW Washington.


The latest radar indicates showers along the coast and over NW Washington, all of which are reaching the surface as rain.  A few snow flurries have been noted around the region.


The latest sounding above Sea-Tac airport (12:30 PM) shows a freezing level around 950 ft, which means the atmosphere is now close to cold enough for snow to reach sea level.  This morning's model output suggests some warming aloft this afternoon.
However, the time-height cross section above Seattle does not suggest much cooling aloft, which gets me worried, as does the fact that temperatures are cooler than forecast.
Thus, I have some concerns that we might see a switch to snow earlier than forecast over Puget Sound (which was around midnight) and during a period when precipitation is heavier (after midnight there are only a few snow showers).

I am also concerned that the upper level pattern (see 500 hPa heights below for 1 AM Sunday) looks fairly close to the canonical snow pattern.


In such a situation difficult, rapidly changing situation, one turns to the HRRR...the constantly updated NWS High Resolution Rapid Refresh model.   The 1-h snow forecast ending 4 PM shows snow reaching the surface over NW Washington.

 By 7 PM, there is light snow reaching Everett and the eastern Seattle suburbs.  The snow areas are where the precipitation rates are large enough to drive the snow level to the surface.

 The 2 AM and 3 AM forecast show snow over Puget Sound (particularly Snohomish County and north King County in a convergence zone.


This is an extraordinarily difficult forecast because the temperatures are so close to the line for rain versus snow.  The classic western WA dilemma.  And above is only one solution.  There are large uncertainties in the model solutions (see NWS Short Range Ensemble Forecasts of total snow) for Seattle from nearly no snow to several inches.


The bottom line:  there is a good chance of snow showers later this afternoon and evening, particularly north of Everett.    As the atmosphere cools down further later this evening and a convergence zone sets up, several inches could fall north and east of Seattle after midnight.  Will have to watch this situation very carefully during the afternoon, as my colleagues at the NWS will certainly be doing.

29 comments:

BornInCaptivity said...

Lots of flurries all day in N. Tacoma. Nothing sticking.

Liz said...

It's been snowing here in West Seattle for about 45min now, off and on. Thanks for the update!

JoeVashon said...

Moderate snow with minor accumulations on Vashon Island. 34 degrees.

HunterZ said...

Getting some light snow right now (2PM) just west of Lake Sawyer.

lhsouthern said...

It's been snowing off and on in Chehalis all day so far.... nothing is sticking:(

Unknown said...

Light snow starting now in Forest Hills east of Factoria :)

Paul Masterson said...

Snow falling from 2Pm in South Seattle, not sticking as of yet

oneofus said...

Snow showers started at 1:00 pm in Newcastle at 400 ft (Olympus hill). Very small flakes and 35F temp so not sticking.

oneofus said...

Oops, I meant to say 2:00 pm.

ginnaville said...

Looks like some snowy areas just as the New Year's Eve revelers head home. This may not end well for a few folks.

Uplander said...

Snow began at 2:30 pm 2 miles southwest of North Bend, at 1100 ft elevation.

Kevin said...

Snowing lightly in Bellingham since about 1:30 pm. Sticking away from the water, about 1/2 inch so far.

Dave said...

Been snowing pretty hard since 11:00AM at Maple Falls, WA - 25 miles due East of Bellingham. 600 foot elevation. About an inch accumulation.

jjensen554 said...

Top of Finn Hill/Juanita Drive just started snowing

Jennifer said...

Snowing steadily in Carnation as of 6:30pm.

dnunan28 said...

Started blowing out of the South pretty hard with rain around 330 in Tacoma today. Had been snow most of the mid day until 1pm. Afraid it's going to flash freeze once it clears off later. Hoping for snow

Scott K. said...

Had snow in Redmond for a couple of hours. Made a nice slush everywhere, cars, roads, etc.

lisa matson said...

Snowing in Olympia @ 1218 am

Tim said...

0 to 10 inches fell in Metro Vancouver.

John said...

Got about an inch so far in Duvall. Ended about 30 minutes ago. Started around 4:30 pm as a mix, then eventually, around 6, switched over to mainly snow.

Unknown said...

Approximately 1" here in Eatonville this morning...light breeze and 32°F

Maggie Palmer said...

Approximately 1" fell overnight here north of Eatonville...32°F

Soupman said...

It looks like the cold snap will not continue as long as originally advertised unless the predicted warm-up next week-end is short-lived.

Scott K. said...

Talk about hit or miss.

Zero snow in Auburn, but in burien/sea-tac just west there's 5+ inches.

Harrison said...

No model, even the HRRR picked up on the convergence that far south even when the event was 6 hours out. It had the band well north of Sea-Tac (Everett-N-Seattle). Amazing the micro climates and topography and geography around here influencing weather patterns. Had that band set-up shop 5 miles north of the Airport and there would
Have been little problem with all the whiners at the airport. I do think many airlines should have known the inherent uncertainties in the forecast as the band formed between 4am and 8am dumping near 6 inches in a very narrow band-area. This post was spot-on. At that point all you can do is NOWCAST and that's hard for airlines who seemed to have majorly been caught of guard. Well done Cliff.

Richard said...

And not as cold, either. Typical.

Sulla said...

Still going 5 years since the last real snowfall around the region. It doesn't look like this cold spell (I won't say snap since we'll be over freezing through most of it) is going to do it either.

Thecatguy93 said...

Cliff, can you please please do a blog on how or why Mission Ridge received 39 inches of snow in 24 hours? That is almost unheard of for that area, especially when only a few inches were forecast, and surrounding areas like Stevens, Snoqualmie, Leavenworth and Ellensburg got virtually nothing. Looks like Blewett Pass also got about 15 inches, so was this just confined to the Wenatchee mountains? I'm baffled by this and would to hear what caused it. Thanks!

Richard said...

Yep. I don't count it as "cold" unless the high is below freezing nearly everywhere and certainly at Sea-Tac. This is a cool spell, not really cold. Disappointing.