March 4, 2021

Weather News – Road Conditions – weather forecast

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

2 min read

 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.

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