October 24, 2023

Early Heavy Snow for the North Cascades

 The mountains of Washington State will get an early blast of heavy snowfall during the next few days, with a particularly heavy snow dump (1-2 feet) over the eastern slopes of the  Cascades.  

If this was a few months later, I would be warning about snowfall over the lowlands of western Washington, but this IS mid-October.

Let me start by showing you the snowfall prediction from the NOAA/NWS National Blend of Models, the guidance heavily used by local National Weather Service forecasters.

The snowfall total through Wednesday at 5 PM is impressive and crazy high over and to the east of the North Cascades---as high as 1.5-2 feet on some of the highest terrain.  Virtually all of Cascades and Olympics above 2500 ft will get significant snow.  Stevens Pass would be treacherous.


What about the UW WRF weather prediction model?  The high-resolution (4-km grid spacing) domain also produces heavy snow, but shifts the snow epicenter down to the central Cascades. (see below).  Snoqualmie and Stevens passes would both get significant snow.  Leavenworth as well.


So why such heavy early-season snow?   First, much cooler air will be moving into the region as Arctic air shifts southward on Tuesday and Wednesday.   

But there is more:  a potent low-pressure system will be moving to the Washington coast as well.   To understand the complex geometry of the predicted weather, below is a forecast of sea level pressure, surface winds, and low-level temperatures (color shading) for 2 AM Wednesday morning.

The low center is centered near Hoquiam.  Very cold air has moved into southern BC, Alberta, and western Montana.  And with the cold air is high pressure (cold air is more dense/heavy than warm air).


But why is the snowfall so large on the eastern side of the Cascades?

Because of strong winds from the east that are forced to rise by the eastern slopes of the Cascades.  And rising air is associated with precipitation.  

To illustrate the situation, consider the winds and heights (like pressure) at around 5000 ft (850 hPa pressure) at 2 AM Wednesday (see below).  Strong easterly flow is predicted... flow that is headed straight up the Cascades.


The result?  Lots of snow.

Both the Seattle and Spokane offices have put out special snow warnings (check below).

Seattle

Spokane

Be prepared for winter driving if you are crossing the Cascades on Wednesday!
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10 comments:

  1. As predicted - YES, here it comes. This morning it was 39.9 in Glacier, and I see snow at just under 3,000 on Church Mountain. And this is just the start, "so they say." Classic fall so far (early and wet - very well recovered, very green).

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  2. Could be fun! I'll believe it when I see it...........

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  3. Cliff, please review this salmon white paper. It claims warmer ocean waters has lead to the Chinook decline. The appendix has the studies. Would love to hear your take on it. Thanks
    https://savefamilyfarming.org/is-chinook-recovery-harmed-by-a-great-misdiagnosis/

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  4. The cpc is forecasting a warmer and dryer then normal spring and summer next year, so we really need the Mt snow pack to build up as soon as possible.

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    1. This early in the year, only the highest elevation snowfall doesn't melt shortly after falling, resulting in typically minimal contribution to the snowpack for October and early November snows.

      You can see a hint of this by looking at the Snotel basin maps. As of today, the Puget Sound basin snowpack, for example is at an impressive sounding 300% of average.

      However, the average for this time of year is very close to 0, and the current snow-water equivalent for the basin is only 0.4 inches. It's about 3 more weeks before persistent snow typically accumulates. Even in the snowiest October in recent decades, the snowpack was only about 8% of the average season peak snowpack.

      Snow season starts in earnest around November 15.

      https://www.nrcs.usda.gov/Internet/WCIS/AWS_PLOTS/basinCharts/POR/WTEQ/assocHUC6/171100_Puget_Sound.html

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  5. @CliffMass - I know the low is literally right over us now and we will get more precipitation and colder air through the day, but it seems like a big miss by the forecasting models in multiple facets: 1) snow level is still above snoqualmie pass and barely below Stevens pass 2) precipitation amounts at snoqualimie pass (and most everywhere else in the NW forecast area) are MUCH lower than forecasted. Would be interesting to get an after-action review to understand why / how our multiple models missed so badly?

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  6. Early season snow plus much lower than usual temps...must be Climate Change. Yeah, that's the ticket.

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  7. Hello Cliff. I'm hoping you'll post an article about the "Super Fog" that was in the news this week.

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  8. BLI was the big rainfall winner among the more important interior Western Washington climate stations, smashing its previous daily record (0.90" in 2001) with 1.58" on 10/25 and supplanting 7/24 as the rainiest day of the year as well as the rainiest day since 11/4/22. Even so, it will most probably end the month with below normal precipitation making this the 12th consecutive such month.

    After recording its driest water year on record during 2022-2023, and with nearly 4" less YTD precipitation than the 2nd driest YTD on record, it will need >90% of normal precipitation during November-December for this not to be the driest calendar year on record. Additionally, despite the week or so of forecast below normal temperatures, it's still on track to log a warmer than normal October due to the persistent anomalous warmth of the first 3 weeks therein.

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  9. Hey Cliff - would be good to understand why the forecasts so badly missed on both precipitation amount and temperature. Never got below freezing at Snoqualmie pass, and looks like less than half forecasted precipitation in many areas. Thoughts?

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