There are two things Minnesotans associate the state fair with: the end of summer and heat. August typically brings a couple of ‘cooler blasts’ mid month as we come off the peak of heat in July and anecdotally I always have heard people say “well, I suppose it’ll get hot in time for the fair.”
Maybe it’s because we like to the think of the ‘Great Minnesota Get-Together’ as summer’s last hurrah both in terms of activities but also weather? To be sure, your chance of hitting 90 after the first 10 days of September drops off precipitously but is there any statistical truth to an end of August/start of September bump up in temperatures?
It turns out my grandparents and observant Minnesota farmers are on to something perhaps. When we look at August temperature date for the currently used ‘normal’ period (1991-2020), you do see a typical mid August slump followed by a ‘last hurrah’ spike end of the month.
Picture above are averaged high temperatures for each date in the 1991-2020 period. When we report the ‘normal high’ or low, we smooth out the numbers so it’s a straight line downward but the truth is that there are actual variations. Sure enough, there’s some statistical backing to some cooler weather mid month followed by a rebound of some sort.
That’s looking at averaged high temperatures. If we break it down into the hottest temperature by date, we see a similar but perhaps more pronounced pattern.
While the hottest temperatures of each day of August are mainly in the 90s, you do see a mid month slump. Note that August 19 -23, the hottest temperatures range from ‘just’ 88 to 91, while from August 24 to 30 the highest readings range from 92 to 97!
If we look at the full record for the Twin Cities, which encompasses 1873 to 2021, you won’t see quite as dramatic of patterns because there’s a lot more data, so you expect it to be ‘smoothed’ out some. However, there is still discernable trends of mid month cooler temps, and end of month heat.
In the above averaged daily high temperature for August, the trend is more of a smoother downward trend but you do see measurable spikes in the August 25 to 30th period. When you keep in mind this is averaged over 148 years that’s a noticeable signal.
Again, it’s not as pronounced because there’s a lot more data to ‘smooth out’ the noise, but there are some measurable ups and downs.
All of this is to suggest that, without even looking at forecast models, it appears more often than not we can expect our first, albeit brief, cool temps in mid August followed by a ‘last gasp’ of heat sometime toward the end of August.
What about this year?
The computer models are kind of mixed as to at least the first several days of the State Fair.
The European and Canadian models both have some warmer temperatures (near 90 by Friday or Saturday, August 26-27) by next weekend but the ensemble models, which are the multiple variations of the models averaged together, show a slight cooling trend.
The computer models have had a tough time forecasting temperatures accurately past 5 days or so lately, maybe due to the subtle shifts in the end of the summer season, but if history is our guide, we can expect some hotter days at some point in the August 25 – September 5 period.
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