Can you imagine a region in which rainfall can vary by a factor of 100, with 50 miles or less?
No need to imagine if you are living in Western Washington!
The map below shows the 24-h precipitation totals ending at noon today (Wednesday) over western Washington.
Over two inches over the southern and southwest sides of the Olympic Mountains, but only 0.01 inches around Sequim and Camano Island. A factor of 200 between the rainfall at the two locations!
High-resolution computer model forecasts of rain (the total precipitation for the 36h ending 5 AM this morning) accurately predicted the huge variation (see below), Two inches or more on one side, virtually nothing to the NE of the barrier.
There is a reason that the zone from Port Angeles to Sequim to Port Townsend to Whidbey Island is popular among retirees.
So what happened? Moist southwesterly (from the southwest) air flow was forced upwards by the southern Olympics, producing lots of rain, but north of the mountains, the air descends, causing drying and little rain. (See graphic below).
Rising air cools by expansion as air goes from high pressure at low levels to lower pressure aloft. That causes cooling--like you note when air escapes from a car or bicycle tire. Cool air can't hold as much moisture as warm air, so water vapor condenses out as rain.
Sinking air warms by compression, causing clouds to evaporate and precipitation to cease.
You could see the drying yesterday on the northwest side of the Olympics in high-resolution visible satellite imagery, with a big hole in the clouds to the NE of the barrier (see visible picture).
With all the precipitation, all of the Olympic Peninsula rivers are now running at normal levels (green dots) or above normal levels (light blue and blue).
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