August 23, 2025

3D Northwest Heat

 We are now in a short warm spell with temperatures getting into the mid 80s to lower 90s in western Washington.  Cooler weather (and some rain) will arrive next week.

 But for me, one of the most fascinating aspects of our warmth is how three-dimensional it is.  How spatially varying the heat can be.

Let me show you, by starting with yesterday's highs (below). Mid-80s around Seattle, 70s and low 80s over Northwest Washington, around 100F in the Willamette Valley, and upper 90s in the Columbia Basin.   An interesting feature is the near 100°F in the higher terrain between Olympia and the coast,.


In western Washington, the influence of the cool Pacific and inland waters results in cold air near the surface, but warmer temperatures right above.  This is particularly true when (like yesterday) there is easterly  (from the east) flow aloft moving warmer air from eastern Washington towards the west.

Yesterday (Friday) morning, this situation was so profound that there was an inversion (temperature increasing with height) above Seattle, something illustrated by the temperatures above SeaTac Airport at 8 AM (below).  Temperatures in the mid-60s near the surface, but 70s above 2200 ft.  

A profound inversion from 1000 to 2500 ft (more than 10 degrees).  I bet morning hikers noticed this warming with height!

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This morning at 7 AM, the inversion was even stronger  (see below), going from 67°F to 79°F above 1500 ft.


The inversion weakened during the day as solar heating warmed the surface.

A temperature inversion can act as an atmospheric lens for weather radar beams, causing  the radar beam to bend downward to the surface, which acts as a target.

This inversion bending of radar explains the huge number of false radar echoes off our coast right now (see below).  The radar beam hits the ocean surface and then is reflected back. 

Looks like a lot of rain out there...in reality there is nothing.


When we have inversion conditions, maps of temperature can get jarring, such as this morning at 7 AM in the Cascade foothills near North Bend.  Around 70°F on ridgetops but lower 50s and 40s in valleys.


According to the latest National Weather Service forecast, the warmth will continue over the weekend, with cooling by mid-week.  Possible rain next weekend.


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2 comments:

  1. FriendAugust 23, 2025 at 2:56 PM

    In Whatcom County, the summit of Sumas Mountain, elevation ~3400', had an overnight minimum temperature of 69F and afternoon high (thus far) of 86F, while the top of the Mount Baker Ski Area (Panorama Dome), at an elevation of ~5000’, had an overnight minimum temperature of 68F.

    Remarkably, the overnight minimum temperature at Camp Muir, at ~10000’ elevation on Mount Rainier, had an overnight minimum temperature of 51F.

    With the very low relative humidity (<20%) being reported from some high elevation stations, there’s no wonder NWS Seattle has issued a Red Flag Warning for such areas.

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  2. KMDAugust 23, 2025 at 3:13 PM

    I find it fascinating that Darrington (of all places) is a hot spot during these events. Darrington is far from the water, but it's also set up on the highest and easternmost part of the Stilly valley, so it seems like the downslope heat effect shouldn't be at its strongest. Yet, Darrington is 5-7 degrees warmer than the other Snohomish County communities that are east of I-5.

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