We hit 100 occasionally around here, but in June or July typically IF we do, especially in the most recent modern decades. August is much less likely, despite it being our second warmest month of the year on an average basis.
As I’ve discussed previously, an extreme forecast, many days out is a less likely outcome, BUT probabilities in a forecast are a function both of their extreme nature but also how much time into the future they are. As we get closer and closer to a potential weather event and models still forecast it, the chance of it occurring is more likely because there’s less chaos/error in the models the less forecast time there is.
Several models are still producing 100 degree high temperatures coming up Tuesday or Wednesday (August 22 or 23). Some, are models that were not doing so a few days ago but now are as we get closer, theoretically in a more accurate window of time. It’s still slightly MORE likely we do NOT hit 100, but the odds are almost even now. I would put it at about a 41% chance we hit 100 either Tuesday or Wednesday. I discount the extreme American (GFS) models in this calculation because they have a known issue with forecasting extreme high temperatures in hot scenarios due to a soil moisture problem (think: ‘dust bowl’ when this was a real thing). Here's an example with the European model, a model that has occasionally gone 100 but mostly in the upper 90s:
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What will make the heat excessive and dangerous is that we’ll also have high dew points- something that has actually been relatively rare this year due to the drought. What’s changed? Moisture. Yeah, we’ve finally had some rain. Not just here but through much of the central U.S. down to the Gulf of Mexico, this means humid air is moving over moist soils so it can retain much of its content rather than lose it to a desert like air mass in the Plains like much of May, June and July this year.
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How rare is this kind of heat in LATE August?
Okay, so it may or may not be 100 Tuesday or Wednesday or very close. I’ve discussed that this is rare. How rare? Well, consider that in the past 75 years, we’ve only hit 100 degrees in August ONCE, that was in 1988- a notorious summer (the summer with the most 90s we’ve ever recorded in the Twin Cities: 44).
If we look at a blend of the full historical record (1873 to 2022) and the modern normal (1991-2022 data) we can see some patterns. The reason I’m using the two sets is because 100 degree days are not many, so we need some bigger sample sizes over larger time periods. If we look at the months themselves (rather than specific periods within the months, which we’ll do in a moment), it’s about three times more likely we hit 100 in June than in August and about 10 times as likely we do so in July compared to August. Even a more conservative view gives June a 40% higher likelihood of hitting 100 compared to August and July a 7 times higher likelihood.
BUT, you may have already wondered: well gosh, we’re talking LATE August here. That’s gotta be different than early August, right? Yes. Despite what everyone thinks, the Minnesota State Fair is not magically hotter than July or early August. Yes, we do have some hot days during that period but they’re less common than we think and typically summer’s last gasp. AND, guess what, we’ve never hit 100 anywhere in the August 20 to September 7 time period.
Let’s compare two time periods: July 15-17 (statistically the mid point of our hottest two weeks of the year) and August 21-23 (the center of the hottest days ahead in the forecast).
The probability based on climatology of hitting 100 degrees or hotter in mid July versus mid to late August is 10 times greater!
What about climate change?
Almost certainly, the fact that we’ve been warming the planet is making these occurrences more likely. In Minnesota summers it is complicated. June and September are our fastest warming months in the warm season while July and August have seen more modest warming. Late August warming is likely the beginning of this late summer/September heat signal we’re seeing with more heat lingering in a warming world. In fact, the trend is pretty obvious looking at the late August time period over 60 years:
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We’ve warmed this period of the summer’s average temperatures by almost a whole standard deviation, which is certainly enough to make an extreme temperature more likely than it used to be. This upcoming heat wave is the perfect combination of a rare heat dome, probable El Nino influence, drought then rain, and human-caused climate change. Either way, it’s something we will have to deal with in this week ahead whether it’s 95 or 103 degrees.
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