March 06, 2019

Potential for Light Snow at Lower Elevations

It is getting very late season for significant lowland snow west of the Cascade crest, although light snow accumulations have occurred infrequently into mid-April.   But an upper level low approaching from the west has the potential to produce some light, non-accumulating snow to near sea level, with perhaps some light accumulations about 500 ft.

The difficulty, as I have discussed many times in this blog, are temperatures that are on-the-edge for snow to reach the ground.

On the other hand, the mountains will get significant snow, as will sections of eastern WA.

As I write this, the shield of clouds has now spread across Oregon into Washington State (see infrared satellite image at 6 AM) as a low center approaches from the southwest.   Our blue skies are now history.


The current freezing level is way high--around 2250 ft according to the graphic from the City of Seattle's SnowWatch website (see below).   The means a snow level around 1250 ft.  Thus, as this system spreads precipitation northward, there will be rain over the lowlands of western WA, while snow will occur over the Cascades and eastern WA.


The accumulated snow forecast through 7 PM tonight from the UW WRF high-resolution modeling system shows this distribution.  Portland, exposed to the cold air exiting the Columbia Gorge, may get a few flakes, with the more serious snow over the lower eastern slopes of the Cascades into the Columbia Basin.


But then there is the second act of our meteorological play.  As the weather system exits to the northeast, cooler air comes in behind.  Not the primo arctic cold air that we experienced last month, jetting into our region through the Fraser River Gap, but cold air from off the Pacific, warmed at low levels by its passage over the relatively warm eastern Pacific (see forecast map of sea level pressure and low-level temperatures at 1 AM Thursday).


A trough of surface low pressure develops off the northwest coast (see above), associated with an approaching upper level low (see forecast upper level map below at 4 AM Thursday), which brings some upward motion and thus some precipitation.


Our latest model forecast suggest the potential for some light snow that could whiten some higher hills and produce some flakes down to near sea level.

Going back to the snow accumulation maps, by 7AM Thursday, the UW WRF has some very light snowfall over central Puget Sound (this is snowfall, NOT accumulation)...perhaps a few tenths of an inch, with a bit more over Portland.

Snow showers could occur into Thursday, with the total snowfall getting to perhaps a half-inch over areas away from the water. The biggest initial threat is in the north Sound, where a Puget Sound Convergence Zone may form tonight.  Accumulations on the ground will be very light, particularly over warmer roadway surfaces.

The very latest NOAA HRRR (High Resolution Rapid Refresh) model run, accentuates this "snow threat" with less than an inch over Snohomish County through 10 PM tonight, but much more at high elevations.  This is no Snowmageddon. 


And more general snow showers on Thursday could spread some non-accumulating snow over the region on Thursday (see totals through 4 PM Thursday below).


So don't rush to the supermarket or buy another bag of salt.   Near sea level, you may see some occasional flurries, while they could be a light accumulation on the grass for the higher hills.

20 comments:

  1. Pretty decent snowfall started at 1130 Wednesday at 1000' near Sequim. This is from the moisture that spread northward through the Puget Sound this morning and has now invaded the eastern end of the Strait.

    Not sure if the moisture is upsloping or convergence-zoning yet over the south shore of the Strait yet -- radar will reveal over the next hour -- but some of the moisture has drifted far enough west into the Strait to get to us.

    Ground ambient temp is 36F with dewpoint that rose from 25F last night to 32F now. After the recent dry spell, I'll take moisture any way I can get it.

    But if I hadn't read you new blog entry, Cliff, this would have surprised me.

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  2. Yass... It's snow time! PNCZ doing what it do. Consistent snow near Paine Field in Everett; snow is wanting to stick and I'm enjoying it.

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  3. Here in Gig Harbor, light, but steady snow fall started around 11am, and is coming down harder since the last half hour or so. Snow is accumulating on the ground.

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  4. It's been snowing in Mukilteo for 2+ hours and sticking to the grass. We're at about 450 ft. Usually I get excited about this but not anymore... Ready for spring now!

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  5. Just started snowing at 300ft next to the water in Bellingham. Nothing sticking yet here, but there are still parts of town with several inches left over from the Feb snows.

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  6. It has been snowing consistently on Orcas Island (at sea level and above) for at least two hours now. It is beginning to accumulate on the grass, cars, etc. Does not appear likely to stop soon, based on the Aldergrove, NWS Seattle, and NEPAC radar.

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  7. Steady light snow began in Friday Harbor about 12:30 PM Wednesday, still snowing at 2 PM, melting on he streets but beginning to accumulate on the grass where I am at 125 ft. elevation.

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  8. It started snowing in Bellingham about 1 pm. This is more than flurries. Grass is turning white.

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  9. The Bow WA. weather conditions reporting rock-on-a-string here has nearly 1/2 " of white coating with moderate snowfall at 2:15 PM Wed. Rock caretaker says bah-humbug!

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  10. Snowing whatcom county. Everybody parked and making snow angels in the slush. 🤨

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  11. Mid-afternoon in north Whatcom County and it's been snowing here for a couple of hours. So over it.

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  12. Sleeting all afternoon on Bainbridge

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  13. A solid inch+ on the ground and still coming down here in S Bellingham at 5pm.

    The NWS kinda whiffed on this forecast. They should have thrown up a warning this afternoon when it started to stick.

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  14. I live in Edgewood. It has just within the 5 O’Clock hour finally stopped snowing. It began closely after 0900. What a beautiful, if unexpected weather day!

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  15. Kevin's right. Forecasts were off and should have been revised in real time.

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  16. The last three weeks of NWS forecasts for Whatcom County have been somewhat inaccurate, except when sunshine is forecast. With all the threats of 1/2"-3'' on several days during different weeks in later February, we'd accumulated 6-12" more minumum. On every occasion snow was forecast, the morning dawned sunshine and zero moisture.

    Now today most the forecasts called for rain/snow mix, with little or no accumulation starting after 4pm, yet it started snowing around 1:30pm till (7:00pm) now where it let up. Maybe an inch + at 80' above sea level. Weird thing was the temp was a claimed 39 when it started. Some snow is sticking to our neighborhood street, but has turned to slush.

    Not complaining, merely observing daily the forecasts closely since the significant dump (for us) at the beginning of February.

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  17. 2 inches of wet snow in Mount Vernon / Burlington. Melted on most roads. Accumulated on north facing roads and driveways (shoveled again ready for early start).

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  18. Black Ice on back road, hills and bridges in Skagit County.

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  19. Woke up to 4-5" on the ground. I'm in Matlock, 23 miles west of Shelton. I think we are around 900ft. It's still snowing. 31°.

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  20. Coming down fairly hard and starting to stick at 6:00 AM in Northeast Seattle at about 200ft elevation.

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