May 06, 2024

Heatwave Ahead!

Almost every May we enjoy a "heatwave" during the middle of the month, a few days in which temperatures surge into the upper 70s or low 80s west of the Cascade Crest.

This year will be no exception.

For those gardeners frustrated by the cool, showery weather, get your plants and implements ready!

To "warm up" the conversation, consider the daily forecasts of temperature at SeaTac Airport from the National Weather Service's National Blend of Models, a statistical combination of major weather forecast models (see below).  Usually highly skillful.

This forecasting system predicts a steady warm-up this week, with high temperatures on both Friday and Saturday near 80F, followed by a cooling to around 70F.

East of the Cascades in the Columbia Basin?   Same story, but add about 10°F and delay one day (see temperatures in the Tri-Cities below). Roughly 90°F on Saturday and Sunday.

The Northwest heatwave is associated with the development of an upper-level high-pressure center (also known as a ridge) over southwest British Columbia (see map below for 2PM Thursday at 500 hPa--about 18,000 ft).

The shading indicates the difference from the climatological normal, with red indicating above-normal heights/pressures. Although significant, this high is not in the same league as those producing record-breaking warmth, such as in June 2021.  

A ridge of high pressure is associated with sinking air, which warms by compression.  It is also associated with low-level offshore-directed (easterly) flow, that produces further compression as it sinks on the slopes of local terrain.  And easterly flow keeps the cooling influence of the Pacific at bay.

The mid-May heatwave is a local favorite

Remember last May?  A heatwave around May 13th (see a plot below for Seattle, blue are the May 2-23 temperatures, red indicates record highs).   

This year will not be as warm.

Or consider the highest temperatures each year between May 7 and May 21 in Seattle since the late 1940s (shown below).  Two-thirds of the years have at least one day above 75F.  There is only a slight upward trend in the highest temperatures during mid-May.

Why do we often get mid-May warm spells, followed by cooler weather?

First, the sun is very strong and the days are quite long at this time of the year.  Pretty much the same as late July.  So solar heating is potentially powerful.

Second, weather disturbances such as lows and highs are still fairly energetic (they are strongest during mid-winter) and these disturbances are crucial to get a big heat wave. To get a strong heat wave in the Northwest one needs a well-defined high-pressure area over coastal British Columbia.

Third, during mid-summer, low-level high pressure builds to the west of our coast, resulting in strong, cool onshore flow.  Often associated with low clouds as well, such onshore flow keeps temperatures down from late May into mid-summer.

Anyway, I am really looking forward to the warmth.   And I know others that feel the same way:


  1. We could use a warm break up in the northwest corner. I've been trying to get grass to sprout, "as-if" with overnight temps still in the 30's in Glacier (and snow within reach, just above us, every morning). I DO golf I (a golf junkie), in Bellingham, so "fingers crossed." 'Watching and waiting. BTW - minor note re celestial mechanics for those into it: we're zooming away from the Sun this time of year (perhelion, "closest to" was Jan 2, and we'll be at aphelion "farthest from" on July 5). Here's to "tilt" or "wobble." Such a lucky planet, Mother Earth.

  2. Over on the weather channel this morning (May 8th), they are predicting today to be mid 60's, sunny, though puffy clouds are present, sun currently hidden by some clouds at the moment.

    Tomorrow, mid 70's, sunny, Friday and Saturday, mid 80's (85), then back to mid 70's Sunday, then after that, back to mostly dry, but seasonal temps of upper 60's to around 70, with most days being either mostly to partly cloudy.

    Now, if that holds, it'll be almost summer like the next couple of days. Don't recall last May being warmer but I would not be surprised.

    Back in 1985, I DO recall one Sunday the temps did reach the 80's, I think mid 80's more like the middle of May and my parents and I, and inlaws drove to Bainbridge Island from Tacoma to meet up with my middle sister and her first husband to look at a house there near Winslow on Oddfellows Rd. The house was not what they had in mind, but it was already very, very pleasant that morning when we left, mid to late morning and was more than warm, bordering on hot on Bainbridge that afternoon. There may have been 2-3 other days that nice that May as well.

  3. Professor Mass, if my memory is accurate, in May of 1964, the end of my freshman year of high school, we had a scorcher of a heat wave. Maybe it was a "heat-few-days," but it was hotter than my parents, who had been around since the 1920s, and my grandparents, who were in the NW since the 1890s, remembered.


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