Saturday, January 11, 2020

The Detailed Snow Forecast for Sunday and Monday

We are now close enough in time that we can apply some of our most powerful prediction tools to the snow forecast problem for Sunday/Monday.  Plus, we are close enough so that uncertainties have lessened.

Before I talk about that, let me note that as expected the Cascades are being hit very hard with a large snowfall, with Snoqualmie Pass receiving about 2 feet of snowfall during the past day.  Snoqualmie was closed for a while, and both Snoqualmie and Stevens require chains at this time.


But let's talk about the situation in the lowlands.   It is clear that the Arctic Front will push southward across western Washington Sunday afternoon and Monday morning, with snow associated with its leading edge and when cold air is forced up terrain.  The snow action should be over by 4 AM Monday morning.

Below is the latest super high resolution forecast of total snowfall for the 48-h period ending 4 AM Monday.   There is, of course, a huge amount of snow in the mountains, particularly over the western slopes of the Cascades and Olympics, reaching 2-3 feet.

Another focus of snow is from Bellingham northward, across the San Juans and Victoria and then to the northern slopes of the Olympics.  This band is associated with the cold outflow of the Fraser River Valley, providing cold air for snow when Sunday's weather system moves through.  More snow occurs as the cold air is forced to rise on the northern slopes of the Olympics.   Another region of snow (.5 to 3 inches) is found from Seattle southwards, where the cold northerly flow converges with westerly/southwesterly flow from off the Pacific.  So at this point, south Seattle looks more heavily effected.


Another very powerful tool is the large high-resolution ensemble (many forecasts) system run at the UW.  This is the best such system in the country...and something the National Weather Service should be doing. 

Below is the ensemble snow forecast for Seattle Tacoma Airport.  The black line shows the average of the many (25) forecasts.  So what can you conclude?  Virtually no chance of snow in Seattle before 3 PM (00Z/13), but considerable uncertainty for amounts after that.   The average is about .6 inches and no member had more than 1.5 inches. Most of the action is between 4 PM and 11 PM.

So no big snowfall, but enough to whiten things up. 

An important issue is road temps.  The City of Seattle and WSDOT collects such information and is available on the City of Seattle SnowWatch site.

The latest values (11AM Sunday) show road temperatures (in the boxes), in the low to mid-40s.  This is good-- the light snow will tend to melt.  If Seattle's Department of Transportation (SDOT) does a  bit of pretreatment with salt in vulnerable locations (like bridges and elevated roadways), then everything should be fine for the commute on Monday.  The timing of the snow will give them several hours to make sure all the roads are fine. And salt will help prevent refreeze as the temperatures cool.


And talking of temperatures, here is the ensemble prediction for Sea-Tac.  Wow.  Rapid cooling Sunday evening (about 15-20 degrees!), with subfreezing temperatures occurring for several days.  This is going to be quite a shock.  And as I noted before, the City must get all the homeless folks off the street.

48 comments:

  1. Love your Blog, Cliff. Please make it snow !!

    ReplyDelete
  2. Nothing about Port Angeles or Sequim specifically?

    ReplyDelete
    Replies
    1. Snow is expected for Port Angeles and sequim. NWS says 2-6 inches possible, but with upslope flow in that area, could be locally higher. Will need to be watched closely.

      Delete
    2. Pretty much anytime the North Slopes of the Olympics are meantioned in snow forecasts it includes the likes of Port Angeles and Sequim. The snow totals then depend upon the set up and intensity of the bands of snow. Also there is huge differences as elevation changes and you get closer to the Olympic slopes. You may end up with a couple inches in downtown Port Angeles with nearly a foot a couple miles to the South and few hundred feet in elevation.

      Delete
  3. Is there still potential for significant lowland snow Wednesday/Thursday?

    ReplyDelete
  4. How are Wednesday and Thursday looking for lowland snow at this point?

    ReplyDelete
  5. Thankee Professor! (read carefully while sitting in fortress of hardtack and fatback, snow shovels at the ready) Look forward with bated breath to the update for tuesday and wednesday.

    ReplyDelete
  6. Thank u again for the detailed information as always!
    Much appreciated!

    ReplyDelete
  7. What is you opinion of ventusky.com? I really like the upper air flows visualization. They do have a page with snow fall areas and depths. I'm unclear how they predict future snow depths, and their forecast accuracy. Post-Wednesday depths align with the possible combination of the very cold air (a certainty) and the incoming low-pressure system Wednesday. The big uncertainty here is how far north/south will that low be when it arrives to the WA/OR coast. Accurate prediction of that today is still "crystal ball" territory.

    ReplyDelete
    Replies
    1. I was wondering the same thing. ventusky and weather.com are showing a very different story for Bellingham and north. They are saying no snow until Monday. ����‍♀️

      Delete
  8. How about snow later in the week?

    ReplyDelete
  9. Darn it. No snowmageddon in Seattle area, what a disappointment.

    ReplyDelete
    Replies
    1. He's only talking about Sunday and Monday. Seattle snow is predicted for later in the week.

      Delete
  10. The rest of the week looks cold with just light snow chances and slowly moderating temps

    ReplyDelete
  11. What about Wednesday? I'm supposed to fly out of Seatac at 1130?

    ReplyDelete
  12. hopefully only a small amt of snow the less the better

    ReplyDelete
  13. What are your thoughts on the Winter Storm Watch issued by NWS calling for 3-4 inches north of Seattle, 1-2 in Seattle, and less South. Interesting that it's exact opposite of your forecast! Makes me confused since you are a brilliant weather mind, but the NWS is paid to be the best to make a forecast for us! Not sure!

    ReplyDelete
    Replies
    1. I noticed that too. KomoNews is saying 4 to 6 inches from pierce county north between sunday and monday, then more snow wednesday into thursday that could bring a lot more accumulation.

      Delete
    2. One of the differences is the NWS meteorologists are actually hired and paid by someone to issue forecasts. This is all they do for a living.

      Delete
    3. @MAC you know Cliff is also a paid professional. A prof at UW for atmospheric science, he very well could have taught some of the folks at NWS.

      Delete
  14. Nothing to see here. I still don’t understand all this build up for what seems to be just a typical snow event for Puget Sound, i.e., 1-2” of snow in the grass and highs in the low 30’s, quickly followed by rain and 40’s.

    ReplyDelete
    Replies
    1. Actually, it appears to be 1-2" of snow, followed by a precipitous drop in temperatures that do not rise above freezing for a few days. Did you read the post?

      Delete
    2. Yes, 1-2 inches like I said and a couple days with highs in the low 30’s. Nothing unusual for the greater Seattle area. We are saying the same thing.

      Delete
  15. Cliff, thank you for your very accurate forecasts. Most helpful for my business. Still to early to expound on your snow predictions for later in the week?

    ReplyDelete
  16. What about Hoodsport on the eastern slopes of the Olympics? I’m at about 700 feet.

    ReplyDelete
  17. What about snow in Hoodsport, The Eastern slopes of the Olympics? I’m at 700 feet.

    ReplyDelete
  18. thanks Cliff! you are the best!

    ReplyDelete
  19. Keep up the updates. You're the only source for weather that we enjoy reading.

    ReplyDelete
  20. You will do well in this situation.

    ReplyDelete
  21. My prediction, for the Puget Sound starting late Sunday -tomorrow-: With a big 360° in and around Everett I predict 2-4+ inches of snow, which includes Whidbey Island, Marysville, Lake Stevens, Mill Creek and Snohomish. South, up I-5, at the Snowhomish (See what I did there?) and King County line, inches of snow will decrease by 1- 2" and so on. Ofcourse the further North and NE, before Everett and further up Highway 2, will add 1+ inches. A determining factor is also what cold air gets thrown back or sucked up by the Olympics and even the Cascades... Could be a snow sandwich of sorts. Mostly, I know this due to the constant directional wind we've been
    experiencing for the last few days, the coldness of the floor and the fluff in the squirels tails.
    Go SNOW-HAWKS!

    ReplyDelete
    Replies
    1. Called it!
      And, much obliged to the Seahawks... Keep making us proud! Win or loose, the 12's got you.

      Delete
  22. Ba humbug!
    Snow is for little kids with runny noses and clip on mittens...

    I say snow and cold should stay where it belongs. (At elevation and east of the Cascades.)

    Seriously. Think about the folks that work outside.. makes for harder working days.

    All you indoor office folks should just use some vacation and drive to the snow instead of wishing it on the lowlands.

    Snow costs me money.. Lost lots of week infustructure with that 18+ inches of wet heavy low quality snow we got at the admiralty inlet last year!

    According to Cliffs map. It Looks like we are not forecast to get any this time around at the admiralty inlet.

    Hope it don't snow nor the cold stick around long. It burns fossil fuels!!!

    Think about all those poorly insulated homes with their heaters cranked contributing to atmospheric CO2....

    Been such a nice winter up until now.

    ReplyDelete
  23. Convinced that Cliff controls the weather. Who’s with me?

    ReplyDelete
  24. Any idea what we are looking at in Chehalis around 500ft?

    ReplyDelete
  25. Cliff, Your blog snow updates are so popular!

    I appreciate the time and energy you invest in the community's well being--especially the reminders for peoples' safety (slowing down on roads vulnerable to freezing temps, getting homeless people inside and ensuring pets are warm).

    What would we do without you!

    ReplyDelete
  26. If the weather track persists from due West, there will definitely be clear winners and losers in the snow lottery over the next week. Kitsap might even eck out the entire week with more sun than clouds. Never mind actual precipitation. Sequim and PA in the meantime will be snowy all week. Areas downtown up to Shoreline might also remain dry, or get a wild card snow dump from a CZ. Its 50/50 odds most places get an inch or more but Sequim/PA/Interior are almost 100% odds while Kitsap is less than 30% with central Kitsap approaching zero.

    If your weather is usually interesting with this pattern, it will continue to be. If its usually dull...well...expect dull. All that snow, rain and wind is going to happen elsewhere for you. Since the majority of adults prefer dull weather during the work week, then perhaps Kitsap is the winner. Shoreline a runner up. This is NOT to downplay the plight of the homeless when its cold or to say places have no chance of having an opportunity for kids to play in the snow. More an emphasis that with this week ahead certain areas will have a pronounced bonus and others a pronounced handicap as far as the winter wonderland chances go.

    ReplyDelete
  27. Google "ksew zone forecast". This is the official NWS zone forecast for the Seattle area. Sunday 231AM forecast is predicting 1-4 inches of snow for this zone (Shoreline-Seattle-Kent-Federal Way). Microclimates, upslope effects and other local terrain will affect how much snow is where. In short, no real way to know how much to expect where you are. My area, Canyon Park, is just south of the Lynnwood/Everett convergence zone, and we have historically seen huge snow falls, over 2 ft, for these types of events. Use good judgement, and stay off the roads unless you absolutely must travel Monday into Seattle.

    ReplyDelete
  28. Hi Dr. Mass. I am a student in your Atmospheric Sciences 101 class this quarter, as well as an avid reader of your blog. The majority of forecasts I am reading, including yours, indicate about an inch of snow for the Seattle/Puget Sound Region. However, when I look at the National Weather Service forecasts, they are predicting almost four inches for the University of Washington campus. What are your comments on this?

    ReplyDelete
  29. So far, the most interesting part of all of this from a prediction standpoint is the big miss on the intensity of the NE winds in Whatcom County. Most forecasts, including the predictions based on the models that Dr. Mass relies upon, had 50-60 mph gusts. That did not come close to happening. At the Bellingham airport, the maximum gust was in the mid-30's. Certainly, the winds were strong enough to bring in the cold air, but they were not the intense winds that I have grown too accustomed to experiencing that blow down trees. The brief NE outflow that we had about a month ago did not bring us cold air, but it was much more intense. The southerlies on January 7 and January 10, were quite a bit stronger as well.

    The NWS also missed on this as well. But all of this illustrates that being a professional weather forecaster involves a lot more than playing with some computer models. It also requires tempering judgment and the weighing of a number of factors. It is my observation that the NWS relies a fair amount on the judgment of well-established forecasters who have a great deal of experience with the numerous factors affecting Northwest weather. In order to do this well, it really does require the full-time attention of an experienced forecaster rather than the occasional part-time foray into distributing weather predictions.

    ReplyDelete