Finally! Significant Snowfall and Heavy Precipitation in Northwest Mountains

We have waited a long time for this and finally it is close at hand:  significant mountain precipitation and snowfall over the Pacific Northwest.

To warm up, let me show you the forecast of 72-h accumulated snowfall ending 5 AM Monday. Several feet in the Rockies and in the higher elevations of the BC Cascades. The WA North Cascades could get as much of a foot, with modest snow along the Cascade ridge into Oregon.

But most of the precipitation will be rain, so here is the total precipitation for the same 72-h period.  The Washington Cascades gets hammered, with extensive areas on the windward side of the barrier getting 5-10 inches.  Even eastern Washington will get soaked with an inch or two.  Water-refill season will begin in earnest with this event.

This will actually be a two-step event.  During the first part, strong, moist westerly flow will develop over the eastern Pacific between high pressure off California and a trough over the Gulf of Alaska (see upper level map for Friday at 8 PM).

The forecast for 2 AM Saturday morning of atmospheric water vapor, shows a tongue of high values extending towards our region....this is an atmospheric river.

With moist, strong westerly flow hitting and ascending our mountains,  large upslope precipitation will occur.  This event will be significant enough that some local rivers will raise rapidly, some to just below flood stage (see example for the Snoqualmie River).

On Sunday stage 2 will begin, as a strong upper level trough approaches our region.  By that time, temperatures will cool, and with the upward motion associated with the trough, moderate to heavy snow will fall at higher elevations of our mountains.  A tease for our skier friends.

After the trough moves through the Northwest, it pushes into California and a huge ridge of high pressure builds over the eastern Pacific (see upper level map for 5 AM Wednesday).  The Northwest will dry out, but California will receive some welcome moisture.