Profound Rainshadowing over the Western Washington Lowlands

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.

The next 24 hours, ending 5 AM Sunday?  Virtually the same story. Wet Olympics and the North Cascades, with generally dry lowlands (see below).   I plan to work in my garden tomorrow and go for a long run.

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).

That pattern of windward enhancement and leeward rain shadowing explains the complex precipitation pattern shown above.

You can see the precipitation/cloud pattern setting up this afternoon.

Less cloudy areas are developing over the lowlands (see visible satellite image at 3 PM, arrows show the areas)

And the rainshadow is clearly evident in the regional weather radar imagery.

So stay out of the mountains and you will be relatively dry.