The October 2024 heat wave was the warmest early-October weather in the Bay Area since 1980. Above average temperatures began for San Geronimo Valley just before the autumnal equinox, which was the beginning of 18 days over 80 degrees F, 10 days over 90 F, and 3 days over 100 F. Two of the days in the 90s occurred in September, then there was one cool day--September 25th, where the high only reached 69 degrees. After that there were 13 days in a row exceeding 80 F and 8 days in a row exceeding 90 F. In October, it had never been over 95 F before, but 3 days over 100 F and 3 days over 95 F set new October monthly records (since I started recordkeeping in 2012). October has now become a month when it can not just get hot, but really hot.
As with many heat waves, the remarkable thing was how warm it was at night above the inversion layer. In the graph below, you can see red and blue lines for the temperature on the ridges, and a green square for the highs and lows on the valley floor (with blank spots for occasional missing data). Starting September 30th, for 9 nights in a row, valley floor temperatures were below 60 F while ridgetop temperatures were in the upper 60s or warmer. And some nights were much warmer, with the ridges staying near 80 F all night! Who would have guessed that Marin County would have low temperatures in the 80s in October, rivaling Death Valley temperatures!
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| Death Valley-like nighttime temperatures in the 80s in Marin County |
The third remarkable thing (after the extreme October heat and the heat above the inversion layer) was the short duration of each day's heat on the valley floor. Because the October days are shorter, the 50-degree temperature uptick was packed into fewer daytime hours. On multiple days I would be cold all morning, then go outside in the afternoon to warm up, then get too hot. Inside our house, temperatures never got above the upper 70s, with some mornings starting at a cool 62 F. Another year (like 2017 or 2019, when nights were in the 30s and highs were in the 70s) we might have turned on our heat to keep our house warm, but this year the heat wave provided our house heating!
So while this October heatwave was extremely anomalous and concerning, the impacts were lessened on the valley floor due to the lateness of the season and the nighttime cooling. But the concerns are big ones--warmest October high temperatures ever, warmest October water temperatures ever, driest soils and vegetation ever, and the impacts to plants and animals on the ridges of seven days in-a-row of low temperatures in the 70s and 80s.
