January 25, 2021

The Wednesday Snow Event Will Be On the "Wrong" Side of Our Mountains Plus the Upcoming California Deluge

 More snow is coming to our region late Tuesday and Wednesday, but Seattle snow lovers will be disappointed if they hope for snow in their backyards.  

Substantial snow will be very close, but on the eastern sides of local terrain.  And the upcoming storm system will rapidly push southward, bringing welcome, heavy precipitation to drought-stricken California.

The key feature is an approaching low pressure system, as shown by the forecast for Tuesday morning at 10 AM


This offshore low, combined with high pressure inland, will create a large east-west pressure difference that will drive strong easterly (from the east) winds at low levels. And instead of moving eastward into Washington, the low will push southward towards California.

The result of the easterly flow will be upslope flow and bountiful precipitation on the eastern sides of the Cascades, Olympics, and other region terrain features--the opposite of normal.   In contrast, the normally wet western slopes will be in the rainshadow (or snow shadow) of local mountains.

Consider the latest forecast of the UW super-high resolution WRF model, with the accumulated snowfall through 10 AM Thursday shown below.  The southeast side of the Olympics and parts of Kitsap will see snow, as will the eastern slopes of the Cascades.  But look how dry the western slopes of the Cascades will be as well as the lowlands from Everett to Olympia.  Same for Portland.



Looking at the big big picture, a very deep trough of low pressure will develop over the eastern Pacific with storm systems heading southward, deep over California (see upper level map for 10 PM Thursday).  


The precipitation totals in California will be extraordinary, with the amounts through Friday morning shown below.  We are talking about 6-7 inches along the central coast and in the Sierra Nevada, with substantial amounts from San Diego to the northern border.


Considering the substantial precipitation deficit of this water year so far (see the figure below that shows how far behind they are), California will still need more water, but this one period will go a long way in mitigating the drought, particularly for the southern half of the state.


And he snowfall will be bountiful in the Sierra Nevada, with an extensive area getting as much as 4 feet.  Good skiing is ahead and a snowpack that will help provide water this spring and summer.











11 comments:

  1. Very informative. What are your thoughts for the rest of the PNW winter? It seems like a low snow year from where I'm at in north bend, any chance of a wetter/cooler pattern?

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  2. What happened to the "La Nina is going to bring lowland snow" talk from months ago? So far, this feels like a normal winter...

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    1. Actually most la Nina years means less snow for the lowlands unless la Nina is two years in a row then the second year for some reason gives us more snow. More snow events seem to happen in neutral and el nino years.

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    2. This is what I was seeing looking back to 1950'ish, that it's year 2 where we get "The Big One", but I wasn't smart enough to grasp the why of it. It's consistent though in the ONI records.

      Any thoughts on why?

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  3. Will the extra snowpack and rain help with wildfires this Summer?

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  4. Do we have any chances of legit snow coming...anything on the horizon...

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  5. So much for the La Nina year. I feel like its been way warmer than usual. Good for California though.

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  6. When will we get significant she in the mountains...3 weeks now with only a few inches.

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  7. Why does the Hoodsport/Seabeck area usually get more snow than the rest of the Puget Sound basin? If anything it ought to be slightly rain-shadowed by the mountains...

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    Replies
    1. Fraiser outflows stack up cold air against the eastern side of the Olympics and can cause major snow storms in this area. Creates a lot of uplift for precipitation to fall.

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  8. Interesting... lots of precipitation on the radar but completely dry outside... Why are the radars picking up precipitation when there is none?

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