Friday, January 13, 2012

A Clearer Picture

Nowcast at 3 PM Saturday.   I am going to try doing some nowcasts for this event....first this afternoon.... Just finishing a paper on this subject if anyone is interested.  Found here.

We are going into a period of very interesting and active weather.

I have just finished looking over the latest forecast model output from the U.S. and foreign centers, as well as the UW high-resolution simulations, and the conclusion one draws is that most of you are going to see some snow during the next few days--ranging from a dusting to several inches.  If anything the threat of snow is greater for the latest series of runs, but they are also emphatic that we are not facing a major, widespread snow event.  Instead we are talking about snow showers for many, with a some lowland areas that might get several inches (such as in the convergence zone). 

Right now the air above us is quite warm, except for a veneer of cool air near the surface (see plot for Seattle below).  A strong inversion caps the cool air, resulting in poor air quality and the current burn ban. No chance of lowland snow with this temperature structure!

But things are going to change rapidly tomorrow as a strong front with very cold air moves through (see figure), with a switch to northwesterly winds behind (see figure).  Relatively

steady rain from the front should be over around lunchtime and then with the cold air behind, we will enter the normal post-frontal showers.  You could see the front and following showers lurking out there during the afternoon (see image).  Interested fact--in the eastern U.S. when a cold front goes by there are few

showers...it dries up.  Why?  We have relatively warm water out there and cold air over warm water is unstable and produces cumulonimbus clouds.  Over most of the continent the ground is cold, and cold air over cold ground does not initiate convection.

The atmosphere will progressively cool later in the afternoon and early evening, with 850 mb (roughly 5000 ft) temperatures dropping below -6C by Saturday evening and -8C on Sunday.  Such temperatures are cool enough for snow even with onshore flow...which we will have...Certainly snow above a few hundred feet ASL  and down to the surface when there is any real precipitation intensity.  By midday Sunday it will be all snow no matter what. North of the border there could be some light snow with the initial frontal passage on Saturday morning, according to the WRF model.

The issue is that most of the showers over lower elevations will be random, hit or miss affairs, except for a few locations where they will be more persistent and stronger.  One such location could be the Puget Sound convergence zone that could well develop under northwesterly flow.  The CZ could set up anywhere from Seattle north to Everett.  Or could be in a band radiating from the Olympics over  to Whidbey Island.  The exact location is difficult to predict.  Here is the latest UW WRF prediction of 24-h snowfall ending 4 PM on Sunday:

And here is for the next 24-h:

This is not snowmagedon, but several inches from Seattle northward, snow over SW Washington, and a foot or more in the mountains.  Appreciable snow over NW Washington. You notice a lack of snow over the Kitsap and south of Seattle...that is the Olympic rainshadow with NW flow.  The higher elevations of eastern Washington pick up some light snow.  Snow comes and goes as a series of disturbances move through during the period through Tuesday.  If one of the them is stronger than predicted and sets up a low center over SW WA, we could get much more snow.

But the best is yet to come.  In the NW we often get snow going into cold AND going out of cold.  The models indicate the mother of all warm fronts to approach early Wednesday (see below--sea level pressure, near surface winds, and temperature in the lower atmosphere--shading).  Wow...that is quite a warm front!  As it comes in, warm air and lifting will come in aloft.  The low level cold air will hold for a short while, giving a burst of snow...perhaps several inches...before it all turns to rain.  Just in time for the commute!

And did I mention that the strong winds and moist air is predicted to cause VERY heavy rainfall later in the week.  Here is the 48h rainfall ending 4 AM on Friday. Some places...like in the Siskiyous near the Oregon/CA border will get more than 10 inches if this is true, and virtually all the coastal mountains and Cascades will get 5-10 inches.  Flooding on some rivers is quite possible if not probable.

Well, after one of the most boring winters I can remember in a while, we are about to start experiencing some interesting times.  And with the new Langley Hill radar we will have a new view of the action and should know whether the forecasts are going wrong in time to provide some warnings and updates.

39 comments:

Kelly said...

Thanks Cliff!
But a bit bummed about Kitsap not getting any AGAIN! :-(

Steve said...

Thanks so much, Cliff! We all appreciate your insight!!

Unknown said...

Are we going to have thawing during the day and refreezing at night that will take a small amount of snow and turn it into a sheet of ice?

ILoveWinter! said...

Wow! You're going to be really busy trying to keep up with what's going on...or not going on! Will be interesting to see how this all turns out! Thanks for the updates Cliff!

Scott said...

Just like the local meteorologists say every year: after, what is it, November 30?...and definitely by December 31, we're totally past the worst of the winters storms.

lhsouthern said...

well this is finally going to interesting for chehalis! Just in time for me to drive to Olympia for my RN nursing orientation at St Peters! It might freak out some people that we have snow, but I lived in Pullman long enough to learn how to drive in this stuff. I just wish others would!

Colleen said...

"Be careful what you wish for", I keep telling myself. Sure, I wanted some interesting weather ~ and would've loved a white Christmas season. But this series of events could be a potential mess ~ especially for those of us who work outside. (And prefer to run outside, for that matter!)

Re the comment above about driving in snow. I have problem driving (or running) in genuinely snowy conditions. That's rarely the reality in Western Washington, where slushy snow showers, freezing rain, and black ice are more common.

Colleen said...

Oh, and by the way, Friday night is already proving up what I said above. Light blankets of snow keep giving way to freezing rain, so the world north of Bellingham at this midnight hour is somewhat white and definitely icy.

bxb said...

Thanks, Cliff!
Right now NOAA is calling for freezing levels to stay below 3500 late next week. With 8+ inches of precip. forecasted, thats a helluva lot of snow up high! Thats over 2+ inches an hour for two days. Do you see the freezing levels staying that low?

Drang said...

Well, after one of the most boring winters I can remember in a while, we are about to start experiencing some interesting times.
Not a phrase one wants to hear from a meteorologist...

Patti said...

What about us down South in Normandy Park and Des Moines. Do you think we will see any accumulation of snow on Sunday and Monday?

Jeff said...

This morning's GFS runs aren't completed, but it would seem that they are changing their minds a little in terms of snow for the lowlands.

Wonder if the GFS accounts for the convergence zone setup or not?

Scott K. said...

The rain forecast for the mountains next week is impressive. I thought I read at some point that 1 inch of rain = 10 inches of snow. Is that correct? Would that mean they could be expecting 80 inches of snow next week? That seems a bit too much.

cornbread said...

Light dusting of snow in south Bellingham at about 200' ASL this morning. Must hace been mixed with rain, as it was raining pretty hard around 5AM.

Unknown said...

Piers Corbyn, the controversial UK physicist, said that the PNW would receive an "extreme wind event" around January 16-17. He made that prediction back in October I think and has probably updated it since, but I just thought I would throw that into the mix.

Super said...

It is snowing HARD just south of Mukilteo. We had ice pellets for about 5 minutes, then it switched to snow. Flower beds and roofs are white and it's starting to stick to the roads. Curious how long this will go for.

Scott K. said...

I don't think it was predicted, but here in Spanaway/Frederickson we are getting some pretty serious winds and a rain/snow mix. The trees are leaning in the wind!

snapdragon said...

It's windy and raining in Hazel Dell. Hoping for snow...

Unknown said...

Snowing steadily near Maltby Road and Hwy 9; started about 11am.

mle_ii said...

Just started snowing here in Redmond, WA.

Travis said...

Snowing hard in Bothell / Alderwood Manor area. 1/4" accumulation so far. CZ formation?

Benjamin said...

Steady, heavy and wet snow in Greenwood, Seattle. It started almost an hour ago and shows little sign of letting up. Light accumulations on foliage/grass. Temps dropped from 41 to 36 within minutes.

Kevin Purcell said...

Sleeting on Capitol Hill, Seattle at noon.

There may be ice pellets in there too (that slightly more crisp sound as it falls).

Unknown said...

I'm still at work here in Downtown Seattle, about 1/4 of a mile from the space needle and I can clearly see snow flurries mixed in with the frozen rain. ...might want to rethink how late I stay out so I can get home before conditions get too slick. *is trying to contain child-like excitement, but fails miserably*

Super said...

Taking a breather from the snow here. KOMO's radar remains my favorite (not as "speckly" as others), but looks like it's having a LOT of glitches today. Bad day for that.

Unknown said...

Steady snow rate in Shoreline.

Steve Snyder said...

Snowing pretty good here up in the hills around Woodinville!

JewelyaZ said...

Snowing hard here in Bellevue at 210 ASL... It started as a rain/graupel/snow mix and is not graupel & snow... Sticking to cars and road... is it going to let up, or should I go to the grocery store through it?

teri said...

Snowing steadily for an hour now near Magnuson Park at Sand Point.

teri said...

Snowing steadily at Magnuson Park at Sand Point in NE Seattle. Time to cancel evening plans downtown?

Unknown said...

Precipitation switched from rain to heavy snow about 11:30am in the redmond ridge area. Minor accumulation on the roads and sidewalks. Probably 1/4" or so thus far.

Jarv said...

36 degrees and snowing since noon in Duvall.

KC said...

Ridiculously light snow in Bonney Lake at 600ft!Great Paper, love all the Model References!

Jim said...

Really windy and sunny on the Key Peninsula/Case Inlet. 39 deg., SW winds @15 with higher gusts. No snow, no precip. since around 9:00 am.

Jeff said...

Can see that a convergence zone is setting up in the north. Looks like it stretches from about Everett north up to Stanwood. It also looks like the winds have made the northerly shift as well.

Take a look at the CZ setting up at:

http://www.atmos.washington.edu/weather/radar.shtml

glassfloat said...

Hello, I was just reading the forecast discussion on the Seattle NWS site and the author said "SURPRISE SNOWFALL ON THE UPSIDE CAN HAPPEN WHENEVER IT
GETS CONVECTIVE LIKE IT SHOULD LATER TODAY." How exactly does he know that it will get convective today? Is it an issue of the air mass types? Is there a certain model or plot to look at that will tell you this? Just wondering, thanks!

Unknown said...

So accuweather.com says an historic Seattle snowstorm is possible by midweek. Has anyone else seen it? http://www.accuweather.com/en/weather-news/ingredients-on-deck-for-histor/60270

Colleen said...

I like reading the comments ~ interesting to have "up to the minute" reports from various regions! What really interests me is that it apparently started snowing in the Seattle regions just when things cleared up here in Whatcom County.

Early this morning my son & I "enjoyed" (not so much!) a run in Lynden, squishing through 9 miles of slush. Conditions worsened as we ran, dealing with sleet & icy wind. We assured ourselves that we were doing the right thing, getting our run out of the way early.

Ahem. In downtown Bellingham at the moment (2 p.m.) it's a lovely, mild winter day. Sun shining, temps up in the low 40s, and only a slight breeze. Couldn't be farther from what I expected. Go figure!

James said...

So here in south Bellevue (Newport Hills/Newcastle) we are at about 300ft and the rains stopped around noon and the sky cleared up for us all afternoon through sunset. However we could see dark clouds to the north through most of the afternoon. I guess we are just outside the convergence zone?