Tuesday, January 7, 2020

The Lowland Snow Threat

I would not run to the food store yet.   But there is a significant chance for some light lowland snow over portions of western Washington during the week.

Uncertainty is still high, but sufficiently cold air will be in place to allow snow to reach sea level for substantial portions of time.  The issue is getting precipitation during those cold periods.

The large-scale flow pattern that will dominate during the next week is a classic for cold and snow in the Northwest (see upper level map below for 1 PM on Saturday---500 hPa level, about 18,000 ft). A big ridge/high pressure area will be positioned over the central Pacific, with northwest flow over the West Coast--which brings in colder air from the north.

Snowstorms are generally associated with a trough moving southward on the eastern side of the ridge/high.   Such a trough causes upward motion, resulting in clouds and precipitation, and helps produce a low pressure area near the SW WA coast, a feature associated with many Puget Sound snow events.


The first potential for lowland snow will occur late Thursday and early Friday, but it is only a minor threat (see sea level pressure and low-level temperature map for 4 AM Friday).  Marginally cold enough air will be in place and an approaching front will bring the precipitation.


The latest UW model run showing the snowfall (not snow accumulation) for the 24-h ending 4 AM Friday is presented below. Some light snow north of Seattle (.5 to1.5 inches), particularly at higher elevations (above 400 ft).  More in the mountains.  Some of you will see some flakes.  Fortunately, temperatures hav been much warmer than normal and light snow will melt on most roadways.


The next, major snow threat is on Monday.  On Sunday, much colder air will push south into WA state, with a low center developing just offshore of the Columbia bars (see below).  The question is how much precipitation will be associated with the low center and the associated upper level trough.  And how far it will push northward.


The UW model forecast for the 24-h snowfall ending 4 PM Monday shows significant (2-4 inch) snow spreading across Portland and southwest Washington, with Seattle on the edge.  A lot of uncertainty where the edge will be.


As described in the blog many times, for situations like this, with lots of uncertainty, one must look at ensembles of many different forecasts to determine the probabilities of snow.  So let's look at the best...the European Center ensemble forecast of total snowfall (not accumulation on the ground, which would be less) at Seattle-Tacoma Airport.

Each line is an individual snowfall forecast, color coded as noted on the right.  The average of the ensemble is shown by the green bars on the bottom and the solid blue line on the bottom is their high-resolution forecast.

Although many forecasts have a dusting here on late Thursday/Friday AM, significant snow waits until late Sunday/Monday/Tuesday (several inches).  But considerable differences are apparent between the runs in amount and timing of the snow.  This is important.


So don't run to the market right away (particularly since I need to go shopping for groceries), but keep your eye on the forecasts, which could change as the details of the approach systems clarify.  But there is a real threat of lowland snow in western Washington and the probability that cold air will move in is very high.
















28 comments:

  1. The model for afternoon temps featured in this blog post has central WA (Wenatchee area) at -9°F on Monday afternoon, while the Weather Channel (usually pretty accurate) has the temp at +30F. Why the huge discrepancy?

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    Replies
    1. The temp is in Celsius and at 825mb

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    2. Asked & answered in the prior post. The low temp is at 2500' altitude.

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  2. Dr. Mass
    Mt. Baker ski area is encouraging its faithful shredders to check your blog for updates on up coming snow falls in the coming week. Pretty smart!!

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  3. Dr. Mass, I love this blog and have been following you for years, but the new ads are just over the top with clickbait style and misinformation. Perhaps a small subscription fee? A donate button like Wikipedia? I just can’t stomach these ads!

    ReplyDelete
    Replies
    1. Not sure whether it helps. You can click at the top right corner of the Ad. Then in "Ad Choices", you can turn off "Ad personalization" or "Ad personalization across the web".

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    2. Get an Adblocker. I have the same distaste for these obnoxious ads - but with my Adblocker I don't have to see them on most sites.

      Delete
  4. Here's the deal: you gotta get atleast couple moistures around for these snows to occur. Look, its gonna be snow. See, what you have to understand here is that cold always gets in the realm of possibility ofsnow

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  5. Here's the deal its gonna snow

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    Replies
    1. Of course... in the Cascades. At best it would lightly dust in lowland but that's extremely unlikely given the ground temp not being low enough to stick

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    2. temps will be cold enough on monday and enough moisture for a few inches, and especially later that week

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  6. Hey Cliff thank you as always for the excellent post...you showed 24hr snow ending 4am Friday, I assume you saw 24hr snow ending 4PM Friday. Classic hood canal snow event at least as shown the last few runs. Good southeasterly gradients to trap that cool air...decent recent snows in the Olympics...looks like a classic ! Rain for Seattle, some snow/rain Snohomish northward, and a DUMP for western kitsap

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  7. Sorry Cliff
    Californians have already cleared the shelves at your store.

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    Replies
    1. Californians have already moved back to California!

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  8. Just had a mix of rain/snow in Issaquah, now back to rain.

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  9. Ok, so I analysed the data and came up with some of my own predictions(For Seattle and area): Possibly light, wet snow early Friday, and turning to rain as the morning continues. Then, on Sunday, a better chance comes as the clock reaches 7 - 10pm. A few showers of snow until sometime Tuesday, and Possibly more on Wednesday, but with high uncertainty. We will see! I'm hoping for some good, nice snow. Except, it might be powdery because of the cold temperatures, as wetter stickier snow happens at temperatures just above freezing. The forecast for Tuesday shows the temps getting to even the teens!

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  10. Hello there, how the heck do you read the above chart for snowfall probabilities? I get dizzy just looking at it! 😵😁

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  11. How do weather.com and weather.gov (NOAA) usually compare to each other? weather.com forecasts much colder temperatures for next week than NOAA. For example, right now weather.com forecasts 24/21 and partly cloudy for Tuesday 01/14. NOAA forecasts the high temperature in the 30s and "a slight chance of snow".

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  12. Hey, we just had a bit of snow falling in Auburn, WA. Elevation about 480ft.

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  13. Wet snow falling near 526 in S. (Central) Everett, near Paine Field.

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  14. Snowing in Bellingham at 5:30 pm. Just started and not sticking but exciting none the less!

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  15. Snowing again, but now it's moderate snow, here in east Auburn. 480ft

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  16. It is beginning to snow in Lakewood at 9:40 pm Wednesday evening....

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  17. Snowraining for the past hour or so in Redmond (10:05 a.m. right now). Nothing sticking.

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