December 05, 2023

Subtropical Warmth, Heavy Rain, and Filling Reservoirs

You did not have to travel to Hawaii this morning to experience subtropical warmth or tropical-intensity showers.  It was here in the Pacific Northwest.

As a substantial atmospheric river interacts with our terrain, large amounts of precipitation have fallen during the last 36 h, with extreme contrasts over a few miles.

Before are the 36-h totals.  Some locations over NW Oregon and SW Washington got to around 8 inches...and more is coming.  6-7 inches in the Cascades.

But as impressive as the large totals are (breaking daily records at some locations), the LACK of rain in some amazing rainshadows is also notable.  

Consider the rain shadow northeast of the Olympics.  Over the higher terrain, there were over 6 inches in 36h. In contrast, a station west of Port Townsend only had 0.12 inches, and portions of Whidbey Island were only about a quarter of an inch (see below).

To put it another way, there was about a fifty-fold difference between the rain at higher elevations and downstream in the rainshadow.   Amazing.

The air over us has felt subtropical.  The temperatures this morning have gotten into low-60s  (see proof below).  The dewpoints (a measure of water vapor content) rose into the mid-50s.   

The freezing level (the level of the atmosphere where temperature declines to 32 F) was as high as 11,000 ft this morning based on the Salem, OR radiosonde (see below).   All rain in the mountains!

All this warm rain is rapidly filling our reservoirs, which is a good thing.  The water level in the big Chester Morse Lake that supplies a large proportion of Seattle's water is going up fast (see plot below).

Finally, let's get back to the subtropical origins of the air over us.  Below are 72h trajectories.....3D paths over time... of the air that was over Seattle this morning ( I used the wonderful NOAA Hysplit website to create these).

Our air started around 30N//.the latitude of northern Baja California!  The air did not come from Hawaii, so let's not call it a Pineapple Express. 😀

Things will dry out a bit for the remainder of this week before ANOTHER atmospheric river reaches our shores on Saturday (see below)--so keep your umbrellas and rain jackets handy!


  1. Can't wait for El Nino, saving money on fire wood is great.

  2. The one on the way for Sat/Sun appears to be colder with snow at the higher elevations .
    I've had rain/mist on 5" of snow (north of EBRG) and 700 feet higher.
    Photos of flooding on KOMO web news -- not pretty.

  3. The past few late Decembers, we've had temps in the 20s. With the change to an El Nino pattern, I wonder whether that will happen again anytime soon. It's not so much that I'm looking for a White Christmas, but I use three continuous days of reliable subfreezing temperatures to empty my downstairs freezer of food and defrost it. Thawed burritos are no bueno.

  4. My grapefruit tree will appreciate this. Helps just a bit with the heating bill too. Now when will they update the drought monitor?


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