The upcoming, potentially record-breaking September heat wave may just be a sign of things to come. We are looking at a likely 4 or 5 consecutive days in the 90s starting Friday through Tuesday. That hasn’t happened in 45 years. The last year that even comes close to that is 2017 we had three consecutive 90s in September. The record high for Labor Day is 97 degrees and while we might fall short, we have a few chances to get close or break records in that five day stretch as each day has record highs of either 97 or 98 degrees.
What’s remarkable here is that this is the SECOND RARE late season heat wave just 10 days apart. The culprit is the incredible heat dome that developed this spring and has been contracting, expanding and bouncing around pretty much all of western and central North America sparking a record Canadian wildfire season which in turn gave us a record awful summer for air quality alerts in Minnesota.
Summer 2023 finished as the 5th warmest in the Twin Cities beating out every Dust Bowl year except 1933. Again, this is made more remarkable in that we had the #1 hottest summer just two years ago in 2021 (also a bad air quality summer). How did 2021 and 2023 beat 1936 for example where we had our hottest July on record and a score of days in the 100s? Because the modern heat is consistent and it’s that which is big evidence of climate change. The summer of 1936 had a cool June and August, our summers now are almost consistently warmer than normal starting in May through September. We don’t get the cool breaks of Canadian air like we used to get because, well Canada and the Arctic are way warmer than they used to be.
August is finishing as the 13th warmest for the Twin Cities at a whopping 3 degrees above normal (the standard deviation for August average temperatures is just about 2.7 degrees). Only far northern Minnesota in a little green spec around International Falls was *slightly* cooler than normal.
This brings me back to September. It’s one of our fastest warming months in Minnesota. Summer is lingering as winters are shrinking. In fact, we’re one of just three hot spots in the country for September warming over recent decades.
September average temperature in the Twin Cities has warmed more than 4 degrees over just the past five decades.
This has resulted in a staggering 20 additional days of above normal fall temperatures. Keep in mind meteorological fall is just 91 days. In an ‘average’ fall, we should have (in theory) 45 above normal days and 45 below normal days. Recent trends suggest we now have 65 above normal days and maybe 26 below normal days (as a rough example), that’s a more than 40% increase in warmer fall days.
The fact that we’re starting the first 5 days of September nearly 20 degrees above normal means that we’d have to counter it with another 5 days that are 20 degrees colder than normal. That scenario isn’t likely to play out like it used to decades ago. For an example, if we were to be 20 degrees cooler than normal at the end of September, let’s say, we’d be in the 40s for highs for 5 days! Yikes! Where’s that cold going to come from? The cold is warmer than it used to be after all.
This is why I find the Climate Prediction Center’s September forecast puzzling. They’re forecasting ‘equal chances’ of a below or above normal September. They usually do exemplary work and this should not be interpreted as major criticism, but…
To be fair they’re saying it could go either way, but there’s just not a lot to support that. For one, my tried and trusted technique for forecasting any month or season is look at the climate change trends FIRST.
14 out of the last 20 years, Septembers have been above normal. You better see some major blocking pattern anomalies in terms of temperature and precipitation to think September will be cool in advance of the month. That’s close to summers where 19 of the last 24 have been above normal. Compare that to winters which 12 of the last 24 have been above and 12 have been slightly below. That’s because there’s much more variability in winter. In summer, our weather is overall driven by the sun and climate change now. Other patterns can exacerbate them or limit them but to get cool summers now it must result from wet summers (i.e. block the sun).
Don’t look at that winter figure and think ‘how are winters warming if it’s half and half?’ Remember, ‘normal’ is based on the modern 30 year average which is MUCH warmer than the historical averages.
The Climate Prediction Center kind of botched August for us. This was their forecast:
And remember, we had our 13th hottest August (of 150 years). WELL above normal.
For summer, again, they missed the fact that we’d have a top 10 summer. This was their forecast for summer (June, July, August 2023):
So, what say the models?
Well, even if you excluded the paragraphs of my dissertation on the climate change trends, three of four long range models also forecast a HOT September.
To be fair, these models (especially the European long range) tend to over do it a bit, but either way this, combined with climate trends and just looking ahead at our first 5-10 days of the month, all point toward a much warmer than normal September.
But, don’t expect September 90s all month. It is a month of transition after all. A hot September is still cooler than a summer month. No matter all the damage we’re doing to the atmosphere and the giant heat dome lurking, the arctic is getting rapidly darker. That means colder air is developing and will start plunging south, even if that air is much warmer than it used to be.
Winter. Is always coming!
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