A Strong Atmospheric River Should Hit the Northwest Next Weekend

A series of moderate atmospheric rivers will hit the Northwest this week, followed by a major one over the weekend.  Serious rain in the mountains.

Earlier this fall, the National Weather Service was going for a dry autumn for our region (see below).  That is NOT going to happen.

The last 72-h has been wet enough, with 4-5 inches along the western slopes of the Cascades, 3-4 inches in the Olympics and 1-2 inches over Puget Sound, which is rainshadowed by the Olympics,  Eastern Washington had much less of course (click on image below to enlarge).

But hold on to your hats....Pacific moisture has big plans for us.  Plumes of moisture will move around a region of high pressure off of California and then produce substantial precipitation as is it forced to rise by our region terrain.

You know that name of these currents of high moisture values:  atmospheric rivers. 

Below I will show you a series of forecast column-integrated water vapor, basically summing up the water vapor in a vertical column.  Red is high, white and blue are very high.

The first one hits on Wednesday morning, with moderately high values moving in from the west.

Thursday night and Friday morning, another weak one.

But Saturday morning is another thing...much stronger and heading directly from the west.

Now moisture is important, but so is wind.  A strong wind pushes more moisture up the terrain, producing more precipitation.  And, of course, we can quantify this, using something called IVT:  Integrated Vapor Transport.  Throw that term around and you will either impress your friends or be classified as a hopeless nerd.

Well, in any case, here is the IVT for Saturday morning.  OMG.  This is serious.

With lots of moisture pushing westward, the 48h precipitation ending 5 AM Monday is substantial, with as much as 5-10 inches in the Cascades.

But if you really want to be impressed, here is the total precipitation forecast for the next 7 days. Wow.  Most of the higher terrain gets 5-10 inches, with several inches in the lowlands.  Because the winds are from the west, the interior lowland (e.g., Puget Sound) will be partially rainshadowed by the Olympics.

 Rivers will rise, reservoirs will fill.  And unfortunately, the long-term forecast will prove unskillful.

from Cliff Mass Weather and Climate Blog https://ift.tt/2DcExAb