If you like extreme differences in precipitation, the Pacific Northwest is the place for you!
A moderate atmospheric river is heading into our region: some locations will get huge amounts of precipitation, while others will enjoy only a few sprinkles.
At lower elevations in western Washington, you will hardly need to use an umbrella.
Let's start by looking at the predicted 24-h precipitation totals ending at 5 AM Saturday morning (below).
Parts of the Cascades will get 2-7 inches, while southwest Vancouver Island will be drenched with nearly a foot of rain. In contrast, many western Washington locations will receive 0.05 inches or less. Near Sequim, almost nothing.
All this will occur as a moderate atmospheric river slams into our region. To illustrate, below is a plot of the key measure of atmospheric river activities, integrated water vapor transport, a measure of how quickly the wind is moving water vapor horizontally. The magnitude is indicated by the shading (purple is the most) and the direction and magnitude by the arrows.
A strong plume of moisture (and wind) is moving from the southwest to the northeast at the time shown (8 PM tonight, Friday). Most of the weekend will have the same pattern.
With winds from the southwest, air rises on the western side of the regional terrain (producing more precipitation) and sinks on the eastern slopes (where this is a rainshadow).
And the rainshadow is clearly evident in the regional weather radar imagery.
So stay out of the mountains and you will be relatively dry.
The current NWS forecast discussion makes it sound like everybody is getting a soaking. The lack of nuance is puzzling.
ReplyDeleteVery true with some of the Twitter weather accounts. I am astonished at the power of the rainshadow. I live in North Seattle, and I rarely see what I call heavy rain, and even moderate rain seems uncommon. When I hear there is an atmospheric river coming, I wonder if we will get any rain at all in North Seattle.
DeleteI never heard that such a thing as a "moderate atmospheric river" Could exist!...I always had thought that such a river meant full-tilt, long-lasting rain....here in S. Everett, sparse rain, here and there.
ReplyDeleteCliff, why do some systems create a rain shadow while others that come in from same direction don't? Same with the convergence zone.. systems come in from the same direction, sometimes the convergence zone forms and other times it doesn't. There is more going on than just the winds. Can you explain more?
ReplyDeleteMore than direction can be significant. Strength of the winds are important, vertical stability of the air can be important, moisture content of the air can vary. Winds are key, of course, because the direction and speed of the winds control the flow up and down the mountains.
Delete