February 22, 2021

Amazing Precipitation Differences and Avalanche Threat

 The super rain shadow event of the past day resulted in startling precipitation contrasts over the region.

A picture worth one thousand words

The map below shows the 24h amounts ending at 11 AM this morning (click to enlarge, and a blow up is below).  A few hundredths of an inch over north Seattle and northern Kitsap, but as much as 6 inches in the Cascade foothills, tens of miles away.




The forecast of 24-h precipitation ending at 4 PM today, shows the rain shadow and the immense precipitation on the lower western slopes of the Cascades.  And look at eastern Washington: an immense dry zone in the lee of the Cascades.


This huge precipitation gradient was supercharged by strong westerly flow associated with a narrow, but potent atmospheric river.   And the air warmed considerably yesterday, resulting in rain hitting the lower passes.   Such heavy precipitation in the Cascades, on top of substantial snow of the past few weeks, has led to a serious avalanche risk, with the Northwest Avalanche Center going for high to EXTREME.   So serious that WSDOT closed down Stevens and Snoqualmie passes overnight.

The local snowpack currently is well above normal, as shown by the SNOTEL network this morning:


And the snow forecast this week is very aggressive, with 2-5 feet in the Cascades and Olympic by Friday evening.  Classic La Nina.


With the 8-14 day forecast from the Climate Prediction Center being colder than normal, the snow is not going anywhere soon (see below).   So both summer water resources and March snow fun look to be in good shape at this point.



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7 comments:

  1. This was a pretty disappointing storm. I generally get very excited about AR events. But this rain shadow for the Seattle metro was extremely bad. I've only lived in Seattle for about a year, and this is one of the worst I have seen. So sad... Pretty bad when an extremely strong storm is impacting the area, but the mountains only get it. The lowlands got nothing (I saw a lot of sun)... Strong winds, but blah.. :(

    ReplyDelete
    Replies
    1. If you want storms find a different place to live.

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  2. PSCZ and Rain Shadow. I remember they are related to Froude number (mountain waves) !!

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  3. Exactly as you said would happen. The winds were strong where I am as it started up (lost power for a very short time), then when it ended, we got a downpour (again, for a very short time).

    I like reading about what's going to happen on your blog. It prepares me.

    Looks like plenty of sun and low overnight temps for the next couple of weeks, then spring forward for ... wait for it ... spring. Yay!

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  4. We live in Port Ludlow which is partly in the rain shadow. Last night (2/22) we had a heckuva hailstorm around 9:30 or so. Enough accumulation that I can still see small amounts on my neighbors roof at 12:30 today. Crazy!

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  5. 4pm tuesday. Double convergence zones...

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  6. Just out of the rain shadow on the other hand, Driving down I5 from Mount Vernon at approximately 1 PM was no less than one of the most torrential downpours I’ve ever driven through in decades of driving in the PNW, which is saying something. Some people even had their hazards on while we crept along, it was coming down so hard. Could only see a couple car lengths ahead through the rain.

    This all preceded by heavy hail.

    ReplyDelete

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