Monday, January 10, 2011

Brief Snow, Then Rain and Wind

It is clear now what is about to happen. Yes, there are always uncertainties, but they are much, much smaller than a few days ago. How do I know that? Of course, forecast errors decline as the verification time comes closer, but there are other reasons:

(1) Ensemble of predictions starting with different initializations (different starting points from major operational centers around the world) are producing similar forecasts (that was not true over the weekend).

(2) The forecasts from the National Weather Service models have settled down and are not changing much, cycle to cycle. There is only a slight drift north with the latest run (which makes it warmer by the way)

(3) A comparison of satellite imagery and observations with a model short-term (3-h) forecast looks good. No major errors in initialization or features. Here is the graphic for 7 PM (yellow are 3-h forecast pressures):


Don't be deceived, the low pressure center is NOT in the middle of the swirling clouds.

I think we have a solid forecast to work with, based on all the tools of the trade.
So, what will happen?

Today we had air that was just cool enough for snow over us and there was some light snow in a few showers that moved through this morning. No big deal.

Tomorrow the low will move northward towards Vancouver Island, with the attendant warm front moving up the coast with it. Here is the forecast surface chart at 7 PM:

Warm air is already sneaking in (greens and yellows) at this time . And there is another problem for snow lovers on the east side of Puget Sound and Seattle--easterly downslope flow off the Cascades--the snoweater! Here are the winds for 7 PM tomorrow:

See the easterly flow...this is bad news for snow..the descending air dries and warms. With low pressure offshore flow through the Fraser, relatively weak to start with, is moving our the Strait. Strong winds over the Pacific!

As the low and front reach us and move northward, there will only be a short period of possible snow. Cold air will be in place, and initially evaporation and melting of the falling precipitation will help keep it cool enough to snow. But eventually the profound warming aloft will win.

Here is the forecast 3-h snow at 7 PM. Some light snow along the north coast and NW Washington as well as the Kitsap region SE of the Olympics. Seattle will see a dusting. Certainly possible to be as high as 1 inch or two. And that of course would be slippery.


Next, take a look at the surface chart at 4 AM. The low is making landfall on N. Vancouver Island and warm air has spread across western Washington. No more snow. Rain will pick up and so will the winds. The winds will strength further as the low loves north of us--20-30 mph gusts are reasonable over central Puget Sound.

Here is the 24-h snowfall ending 4 AM Wednesday. Lots of snow over the N. Cascades and the Olympics. NW Washington gets a few inches, but not much south of Everett over the lowlands. Sorry.

Freezing levels will rise quickly and temperatures in the lowlands will return to the 40s. The Olympics and Cascades will get crushed with snow...Mt. Baker could easily get THREE feet. And two other Pacific lows are lined up to follow this one.

Finally, I really appreciate all the feedback today on the value of long-term forecasts. I agree that it is important for people to have a "heads up" on potential high-impact events, but my profession needs to give better quantitative guidance on the uncertainty of the forecasts. We are working on it and some tools already exist.

PS: The National Weather Service is going for a bigger event tomorrow: 1-3 inches over the lowlands, which would melt during the AM hours. This is certainly within the range of possibility, although it looks high to me. So there is a range of possibilities from nothing to a few inches in Seattle. Everyone agrees it will be warm and raining by daybreak. Precipitation will hit around the commute home tomorrow. Anyway, lets see how things look tomorrow AM. Want to be sure about the situation?....leave work or school by 4 PM!

15 comments:

Avalanche said...

Ok Cliff, we appreciate your discussion days ago about a possible snowstorm that isn't going to happen.

Will you please go work for the NWS for these situations though???

Seattle 1-3, 6-9 hood canal, heavier in NW interior, 6-10 inches from Wenatchee to Spokane.

4 to 7 days forgivable, but to be so off within 24hrs, just a joke.

Scrapycandy said...

very interesting!

kjt said...

bummer-- I was hoping for snow in the city.

Weatherfreak said...

I have seen this happen way to many times in the past! I should know better by now. You have got to have thick skin and almost enjoy being disappointed, to be a snow lover in Seattle. It's really to bad this epic event never unfolded. Hopefully, this strong La Nina will show it's cold side once again before this winter is over. Now, bring on the rain and wind!!!

Alison said...

Cliff, thank you for your post. Is there any chance you can do a post on La Nina in the near future? It seems that the La Nina forecast of colder and wetter than normal conditions is not turing out to be the case. It sure has seemed warm this winter except for a handful of days in Nov. And Dec. Meanwhile the south is getting record cold and snowfall. When I look back to last summer the early prediction was a warmer and drier summer than normal. As we all know that did not turn out to be true, could this also be happening to the winter long range forecast of general trends?

Damon said...

If this were a El Nino year, I think I would have been more skeptical of the GFS. I sort of figured (in a very non-scientific way) that La Nina would be our "ace" card. We don't really have a lot of winter's cold heart left to get things going in the "white" direction...at least for our lowlands.

dahlia said...

Thanks so much! I've been following your forecasts diligently the last few days. We have a flight out Wednesday morning and have been debating whether to go stay by the airport or if we'll be able to get out to drive there ourselves. Your predictions are helping fuel that decision. Will check back in tomorrow!

Teresa said...

You wrote "Anyway, lets see how things look tomorrow AM. Want to be sure about the situation?....leave work or school by 4 PM!"

Would you please tell this to UW administrators? I've never in my life had a problem with employers allowing me to leave work early or come in late to avoid a snowstorm....but school? Always a test on a snow day and you could never delay it. It is irresponsible to make students put their safety on the line for something as arbitrary as a professor's schedule. Thankfully I'm done at UW, but I speak for fellow students who still have to deal with this. It's an ongoing horrible problem.

Also, BTW, NWS was the most accurate of all the forecasters before the last snowstorm...

FadedFedor said...

SOOOOOOOOO disappointed!

I'm born and raised here, I love the Puget Sound and I love living in Seattle, but the one thing I can't stand is the serious lack of snow and cold.

I'm a snowboarder too, but it's just not the same. All winter long I'm fiendin for white stuff and all I get is cold rain. Way too much cold rain. I need urban snow.

Liz said...

I, for one, am relieved at the possibility of no snow. I'm 9 months pregnant and I live what is normally a 20-30 minute drive from the hospital (going north on 99 or 1-5). If there is going to be snow, my family is going to need to go stay with my parents who live 5 blocks from the hospital, just in case. It sounds to me like it would be safe to stay home, but would you advise there is enough of a possibility of snow to mess up traffic enough that I should stay with my parents?

Ugg, it was so much easier having a kid in the summer.

Lee said...

Cliff. Only recently became aware of your blog and enjoy it a lot, especially since I was a wx observer in the old Air Corps and a year after discharge, in Kelso WA. This was in 1947!
We are spending this winter in Mexico and really like getting to know what's going on up there.
Thank you,
Lee Lange
handilee@gmail.com

Colleen said...

Why oh why, as a native Western Washingtonian, don't I learn my lesson? Why do I get my hopes up? I should've known better. Rather than heaps of lovely snow, we'll deal once again with freezing rain, black ice, and of course, the northeaster ~ which for those of us in north Whatcom County is nothing new.

I'm another who would like a post solely devoted to La Nina. Is this it? Sure wasn't what it was cracked up to be!

LMeyers said...

I have noticied in the past, living in Edgewdood (Right about Sumner), that Central Pierce and North Pierce will get less snow to...downslope or warming of off cascades or rainier...depending on the track of the low. I look forward to the next month of waiting for a big storm (flood, wind, snow or ice!)

Ginger said...

I'm sad about no snow, but I've gotten used to it here. I have a totally unrelated question, though. Do windfarms show up on radar? There is a permanent red spot on the radar just south of Kennewick, and if you zoom in on google satellite, there is a line of windmills along a ridge in that location.

natchrl8r said...

The winter wonderland has persisted for a change up in Whatcom County. Here the thick, heavy snow still clings to all the tree branches and everything else. Now it looks like we will add 3-6 inches. Hooray! Yesterday, I headed south for a dry forest hike and actually picked fresh Matsutake mushrooms at a sunny, southern exposed forest edge!

It looks like we are expecting a significant snow event tonight and OUR transition will take place during the Wednesday morning commute:


http://forecast.weather.gov/showsigwx.php?warnzone=WAZ503&warncounty=WAC073&firewxzone=WAZ653&local_place1=Bellingham+WA&product1=Winter+Storm+Warning

Winter Storm Warning

URGENT - WINTER WEATHER MESSAGE
NATIONAL WEATHER SERVICE SEATTLE WA
336 AM PST TUE JAN 11 2011

WAZ503-511-111945-
/O.UPG.KSEW.WS.A.0001.110111T2100Z-110112T1800Z/
/O.NEW.KSEW.WS.W.0001.110112T0000Z-110112T1500Z/
WESTERN WHATCOM COUNTY-HOOD CANAL AREA-
336 AM PST TUE JAN 11 2011

...WINTER STORM WARNING IN EFFECT FROM 4 PM THIS AFTERNOON TO
7 AM PST WEDNESDAY FOR THE HOOD CANAL AREA AND WESTERN WHATCOM
COUNTY...

THE NATIONAL WEATHER SERVICE IN SEATTLE HAS ISSUED A WINTER STORM
WARNING FOR THE HOOD CANAL AREA AND WESTERN WHATCOM COUNTY FOR HEAVY
SNOW...WHICH IS IN EFFECT FROM 4 PM THIS AFTERNOON TO 7 AM PST
WEDNESDAY. THE WINTER STORM WATCH FOR THIS AREA IS NO LONGER IN EFFECT.

* ACCUMULATIONS...THREE TO SIX INCHES ARE LIKELY.

* TIMING...SNOW WILL BEGIN LATE THIS AFTERNOON AND CONTINUE INTO
WEDNESDAY MORNING BEFORE TURNING TO RAIN.

* IMPACTS...TRAVEL WILL BE SIGNIFICANTLY IMPEDED.

* FREEZING RAIN...THERE IS A POSSIBILITY OF FREEZING RAIN DURING
THE CHANGE FROM SNOW TO RAIN. FREEZING RAIN IS EVEN WORSE FOR
DRIVERS THAN SNOW.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

A WINTER STORM WARNINGS MEANS SEVERE WINTER WEATHER IS OCCURRING
OR IMMINENT. THOSE PLANNING TRAVEL IN THE WARNED AREA SHOULD BE
PREPARED FOR HAZARDOUS...WINTER DRIVING CONDITIONS AND PLAN
ACCORDINGLY.

Let's be careful! Consider staying home or waiting for conditions to settle.