March 16, 2024

Most Northwest Heatwaves are Local

 Northwest heatwaves are funny things.  

It is rare to have a heatwave over the entire region.   Instead, real warmth is generally restricted to a distinct subset of the region.

Why is this the case, you ask?  

Because of the complex regional terrain and land/water contrasts.


Since the regional waters are cold, onshore flow is cool, but offshore flow (particularly during the warm portion of the year) is warm.

Downslope warm is warm, since the air warms by compression as it sinks, and sinking kills clouds (and thus more sun).  Upslope flow is cooled by expansion in addition to associated clouds and precipitation.

Consider the "heatwave" of yesterday (Friday) and today (Saturday).  Let's start with the high temperatures on Friday around the Olympic peninsula.  Temperatures were in the fifties around Sequim, Port Angeles, and Port Townsend, but in the 70s south and southwest of the Olympics. 


TWENTY DEGREE DIFFERENCE!

Why?  Because the winds were generally northerly (from the north) over the lower atmosphere, so the northern Peninsula had cool air from off the Strait, plus upslope flow.  In contrast, the air sank down the southern slopes and warmed before it reached the warm zone to the southwest and south.

Today, Saturday, the regional flow turned more easterly, a direction that strengthens the downslope to the western slopes of the Cascades and Olympics.  With easterly flow, the warmest temperatures are often near the western foothills of mountain barriers.

The local high temperatures near and east of Seattle today  illustrate this (see below).

Temperature role to 80F in Carnation.  Mid-70s on the east side, but 50s and 60s near Puget Sound.  


There has been some talk in the media and some climate activist groups that the warmest temperatures are in south Seattle in the urban core. This is generally NOT true during western Washington heatwaves, which generally occur with easterly, downslope flow. The eastern side of the Metro area, away from the water and near the slopes, is generally warmest.  

Those poor folks in Duvall, Carnation, Monroe, North Bend, Issaquah, and other foothills towns.  If you really want to see this, below is a high-resolution simulation of the temperatures today at 2 PM around the central Sound area:

Finally, let's consider the southern portion of our region, from central Oregon into northern California, on Friday, a period of moderate easterly flow (see below).  Mid-70s on the coast, but only in the 50s over the desert area of eastern Oregon.   The offshore, easterly flow sank of two mountain ranges (Cascades and the coastal terrain) and the cool marine air was pushed offshore.


The simulated surface temperatures on Friday at 5 PM illustrate this temperature pattern even better:


Enjoy this warm interlude.  It won't last, by Wednesday the cool/wetness will return.  But it sure was wonderful today.  I even got to work on my vegetable garden.  😁



4 comments:

  1. With the strong easterly flow, yesterday I saw something very unusual.. a lenticular cloud over the west side of Mt. Rainier. Common for them to form east of the mountain, but very odd to see one on the west side.

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  2. I find this topic timely. It's spring, and I also garden. Before reading this new post this morning, while jotting down my typical "highs and lows" for the 24 hr period, out of sheer curiosity I took time to add-up the number of hours that the air temp was in the 60's, 50's, 40's and 30's here. Yesterday was pretty far from "a sixty degree day." The distribution was roughly 11.5 hrs in the 30's, 5 hrs in the 40's, 3.25 hrs in the 50's, and 4.25 hrs in the 60's. Pretty hard to call it "a 67 degree day." I enjoyed the bit of warmth, but it was pretty hard to call yesterday a heat-wave. The point I'm trying to make is that any day's "high" can be a very misleading statistic.

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    1. This comment has been removed by the author.

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    2. Yes John, all true. What makes headlines is some number that might be near, match, or exceed some "record" for the calendar date, sans context. Most "news worthy" temperatures don't last long. Our 67.1 high yesterday was two brief spikes, less than five minutes. Numbers 'in the records' that some statisticians (and news-folk) can lead to very wrong conclusions. Near-term weather forecasting models have improved tremendously, but I have serious doubts about some climate models; I doubt many of those are as "data rich" as they should be. That concern goes to 'number of locations' as well as the siting of those instruments (and calibration).

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