October 17, 2023

Extreme Precipitation Variations this Autumn

 The Pacific Northwest is an area of meteorological extremes, particularly for precipitation.

To illustrate, check out the regional precipitation totals for September 1 to October 17 for this year (see below).    Quite amazing.

On the windward (southwest) slopes of the Olympics there was OVER TWENTY inches of rain.  In fact, nearly two feet at one location.  But a few miles away on the leeward (northeast) side of the barrier near Sequim only 0.83 inches.

Puget Sound received about 4 inches (lower over the north Sound than the south Sound), with precipitation increasing into the Cascades.   Large areas of the Columbia Basin, in the rain shadow of the Cascades, had less than a quarter inch.

Bottom line: there was a much as HUNDRED FOLD difference of rainfall between the Columbia Basin and the slopes of the Olympics.  

That is impressive.

The craziest contrasts have to be over the northeast side of the Olympics (see a plot of precipitation for the same period below).

One location just south of Port Townsend had only about 0.37 inches and and lighthouse near Sequim only received 0.56 inches.    About 30 miles away to the southwest, there was 23.90 inches. 

As I have discussed in previous blogs, such precipitation contrasts are mainly forced by terrain, with strong upslope flow on the windward (southwest) side of the Olympics producing bountiful precipitation, while downslope flow on the northeast leeward side results in extreme drying (see schematic)

No wonder folks like retiring around Sequim, from Port Townsend to Port Angeles.


  1. I'm curious what the cause(s) of the "nanoclimate" so-to-speak might be around Chimacum and Port Ludlow area (.37in vs 3-5x that within just a few miles)? There seems to be a more localized version of the bigger Olympic rainshadow, perhaps from the series of small ridges that run north/south between Chimacum and Port Ludlow? Or does it have to do with the sound? We live in the midst of it and the stark variation has always baffled me to the point of questioning if there is indeed a trend, but this seems to confirm that something is at play. Any thoughts?

  2. Being a north sound (Lake Stevens) resident has its perks on rain shadow days but I've learned that convergence zone days give weather.com's near real-time rain forecast lots of trouble. Convergence zone showers often appear to "blossom" out of nowhere and the radar can't spot them approaching like a frontal band does. The wettest bike ride I've done this autumn was on a stretch of the centennial trail between Getchell and Snohomish and the only reason why I picked that stretch of trail was because the forecast only showed spotty light rain in the few hours I needed. When I got back home, I took a peek at the past radar and it was much drier in every single direction except the one I took, which appeared to get trapped under one narrow band of steady precipitation that kept on popping up about five miles to the west of where I was at.


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