May 12, 2025

No Mid-May Heatwave This Year

 It is almost a tradition.

A significant heatwave during mid-May, where temperatures rise into the 80s.


A heatwave followed by cool "June Gloom" conditions during late May and June.

A mid-May heatwave is standard fare around here.   For example, in Seattle, there have been 70 days since 1946 in which the daily high temperature reached 80°F or more during the period May 7-May 21 (see below).   

This month, no day has reached 80°F, and we will be lucky to reach 70°F during the upcoming week.  

The figure below shows the daily extreme highs (red color) and lows (light blue) at Seattle Tacoma Airport from May 7 to May 21, with the blue colors showing the observed highs and lows.  Temperatures have been moderate so far...with no day approaching record highs, which are in the mid-80s. 


Consider the cool forecast from the highly skillful European Center (below).  High temperatures dropping into the 50s on Thursday, and no day reaching the 70s.


Turning to the forecast regional temperature anomalies from climatology (normal) for the next week. Blue and green are below normal.  Cold week ahead!


The following week is also colder than normal.

This cool weather will greatly slow the melting of the current snowpack, which is good for water resources. Also good will be the expected rain during the next week (see below)


My professional advice? Don't put your sweaters away yet.





12 comments:

  1. Well that's bad news for my tomatoes and my patio.

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  2. I always look forward to sunshine this time of year, and in a warm spring, I even start swimming outside before the end of this month. Last May most of the weekends were cloudy and it looks like we might be heading that way again. I hope Memorial Day weekend will be sunny. If not, let's hope June makes up for it! I have always felt cheated when the sunny weather waits until after the solstice- a uniquely northwest problem.

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  3. May and June, always interesting in the Northwest. Did some farming when younger, and many years it was possible to get the first cutting of hay done in May, then have a wet June (oh, so many wet and cloudy Junes). Second cutting of hay then, in July or August - prime. Then maybe a third cut of grass, but by then a thinner crop. So it goes for the farmers with four-footed livestock. Not sure that city temps are the most accurate, regionally. But variation has always been normal, that's for certain.

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  4. Some models were hitting at a late may warm/hot spell but now appears to have back off a bit.

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  5. But if one really gets desperate, 2-3 hours on I-90 eastbound usually solves the problem!

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  6. If we are stuck in a cloudy period why are the predicted rainfall totals so small?

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  7. I wonder whether we need to start watering sooner this season. Normally, we start the sprinkler systems in July!

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  8. Precipitation has been pitiful over Western Washington for at least the past month and shows no particular sign of making up for the increasing deficit. The cool, cloudy weather is in fact a boon given the antecedent conditions and how rapidly vegetation would begin to dry out in hot weather.

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    Replies
    1. I'd rather have more rain AND more sun. West Coast weather has trouble doing that. Not so the Eastern half of the country.

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  9. Cliff- could you comment on whether the last month is abnormally dry? I don't usually have to water my vegetable starts so often. I know it has been cool, but also seems dry.

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  10. Well it now looking like we could have a heatwave by late May according the 12z Gfs and even the 00z Euro is showing that too.

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  11. Its good to hear that the melting of the snowpack will slow down a bit. Its important that our region's water storage systems fill up as much as they can so we'll have enough water to go around when the weather does warm up.

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