November 17, 2010

Cold Wave Over the Lowlands

Other than a good snowfall in the mountains and I90 eastbound being temporarily closed due to spun out cars--things are quieting down right now--for a while.

It is now become clear--with consistency among model runs and between models--that we will be entering a cold period starting late this weekend, with Monday and Tuesday bringing the coldest air since early December 2009. And it is sure that there will be strong northeasterly flow flowing through the Fraser River Valley, across Bellingham, and out across the San Juans and that this flow will strike the N. Olympics peninsula with a vengeance.

What is less sure is the snow over the lowlands, from Seattle southwards. And I don't want to overhype things...the cold will be brief, the snow light if it comes at all, and normality will return well before people have to travel for Thanksgiving (I have just lost the interest of several TV stations around here!)

The big Kahuna goes first..the cold wave. Arctic air will first start leaking into the area on Friday through the Fraser and other gaps and then push southward over the region late Sunday. Here are a few images of the model output for Friday afternoon and Saturday am showing near surface winds, and the surging air from the interior of BC. They are going to be strong! Heading over the northern San Juans, into Vancouver Is and then southward towards the Strait.

Initially (Friday and Saturday), the cold air will be mainly over the gap-wind areas.

Here are pressures, winds, and temperatures at low levels at 7 PM Friday. Blue colors are roughly cold enough for snow. You will also notice a substantial low pressure center over the SW part of the state.

Saturday afternoon...still too warm for snow over most of the western lowlands.

But on Sunday the low goes south and the cold air follows. We are then at the temperature transition point and there will be some light precipitation left. Yes, a chance of snow showers in the lowlands on Sunday, but nothing major and lots of uncertainty.

And Monday afternoon, cool air has made it over us. But there will be little precipitation and thus little snow.

Monday and Tuesday might see highs limited to the 30s! and Monday and Tuesday morning temps could easily fall into the 20s...and even the teens in outlying cold locations. Good time to remove hoses from faucets so they won't freeze up and other protective actions. Protect vulnerable plants. But I don't expect really damaging temps--and keep in mind this is early so the soils are relatively warm.

What about lowland snow? On Saturday it will be limited to NW WA and the Fraser outflow region, so Bellingham, San Juans, Vancouver Island, and the north Olympic peninsula. You will undoubtedly find some TV station remote crews there. Here are the 24-h forecast amounts ending 4 PM Saturday. Also some snow in the mountains.

Sunday we have a battle going on...the cold air is moving in, but the moisture is moving out. And you need both for snow. Here is what the model thinks for the 24-h ending 4 AM on Monday. Seattle and snow lowland locations do get some...but folks, I would not bet on any of this. The next series of runs will undoubtedly have the snow in a different pattern. And after that we go dry and cold. Sorry.

So be prepared for some light snow on Sunday, as I am sure the Seattle DOT and WSDOT will be. The model runs on Saturday will be a far more dependable guide than what we have four days out. If you want to read more about what it takes to get snow around here and read about some of the great NW snowstorms, check out Chapter 4 of my book.


  1. I just want to tell you how much I appreciate your helpful posts. We live North of Bellingham smack dab in the path of the Artic Outflows. We'll be checking the generator, hauling in wood, and battening down the hatches.

  2. Sleet plastering my windows in the Central District right now.

  3. As a Bellingham resident, I'm happy that we have the highest chance of snow. Not looking forward to the Fraser Valley winds (it's never a fun walk to the bus stop), but if we can get some snow it'd be all worth it.

  4. Cliff-

    What are your thoughts on possible convergence forming along the arctic boundary? If the low stays stationary off the coast for any substantial period of time, it seems almost inevitable to me that some sort of convergence with enhanced snowfall will occur with Southerly flow rotating around the low and strong NE winds coming out of the Fraser. The big question is where this would be and at this point that is impossible to say. We've seen this situation a number of times in recent years(i.e. 11/27/06, 1/10/07, 12/17-18/08) What are your thoughts on this?

  5. Well...this is only the first semi-cold spell of the year...and technically it's not officially winter until December 21st....But we at least had a couple of days of some decent wind! Hopefully this is just the beginnig of things to come for this season!

  6. Mosquito Lake Road, Deming, elevation 700': about an inch of snow at 4:30 a.m. Clouds breaking up at dawn.

  7. The 2010111812 UTC (i.e. Nov 18 12Z) Extended WRF-GFS run seems to take all the snow out of the lowland Puget Sound. Precip that happens is rain.

    What are the odds of fog at SeaTac on Monday? That runs seems to be showing a clear sky including surface to 3K cloud). Does it always get the radiative cooling right?

  8. How exactly is the "snow level" calculated?

  9. Whatcom County got its big blast of wind on Wednesday morn after it was spared Monday night. It looks like we may get a bigger, earlier hit from the freeze on the way. I understand the Seattle bias of a UW based blog but you have a loyal following up here at the outflow of the Fraser River Valley. Why not follow the more interesting weather of your near neighbors? It seemed that the chance of snow was deemed of no interest even when the early forecasting indicated Whatcom County would probably take a hit. We are here! We are here!

  10. Holly said... "How exactly is the "snow level" calculated?"

    It comes from the numerical weather model outputs.

    It's the height at which the air temperature is 0 Celsius (32F). Obviously this varies with location depending up the air masses in place so it should be qualified with location.

    I suspect for most WWA forecasts the most important "snow level" is in the Cascade passes on I-90 and US2.

  11. Cliff points out on a different blog entry my definition is incorrect: waht I describe is the freezing or melting level.

    Cliff said: "Kevin...this is not correct. The snow level and the freezing level are not the same. Typically, the snow level is about 300 meters (roughly 1000 ft) lower than the freezing level...cliff mass"

    I've been deluding myself!

    And it just started raining on Capitol Hill ...

    A little reading in Microphysics of clouds and precipitation explains the logic behind the difference. The snow takes time to melt falling through the melting level and doesn't melt fully until it reaches the +5C level several hundred meters lower. Though it's wet snow at that point.

    I suspect the microphysics in the models gives accurate level for each block in the model as plots are provided for snow level.

    Well I learnt something!


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