September is ending much the way it began and persisted: warm and dry. If not for the recent cool blast, we’d have had an even more ridiculously warm month in the books. September’s ending up about +2.6° F above the 30-year normal (1991-2020 average) and +4.3° F above the historic average, 1.3 standard deviations above normal in the Twin Cities. Statewide the numbers are similar ranging from 2° F to 4° F above normal.
September 2022 will go down as the 20th warmest out of 149 years of records for the Twin Cities. The remarkable aspect of the warmth this year was how long into the month it lingered. Essentially August was extended into the first 3 weeks of the month. We can see that clearly in the average temperature (highs & lows) compared to normal since August 1:
From September 1 - September 21, the average high was 80.2° F and the average low was 59.9° F. Compare that to August, where the normal high is 80.8° F and low is 62.4° F.
In particular this month, the overnights were quite consistently mild. If we break down the chart above into the actual highs and lows compared to the normal highs and lows, you can see that distinction.
I discussed this a few times in the last couple weeks, but we had to wait an unusually long period of time for the first real ‘chilly’ night. I define that as a low temperature below 50° F. This year we didn’t see that first chilly night until September 22nd, which is 12 days beyond even the modern average first night below 50° F. Historically, the first night slipping below 50° F came around August 25th.
All of this is keeping with the theme of climate signals on our summers and September: summers are getting longer. Summer warmth is starting earlier and lingering later. July and August, our two hottest months are seeing some of the slowest warming, but the length of summer really is rapidly increasing.
The late arrival of chilly weather, particularly at night has an impact. This year, we could really see it in the annual changing of fall color. When we look at the last three years there’s a big difference in the fall color map for September 26th (several days ago):
You’ll note, it’s not just southern Minnesota but also northern Minnesota that’s been behind. That’s because pretty much statewide temperatures have been above normal this month and particularly at night.
Fall color is a particularly interesting variable to look at because it’s more dependent on night time temperatures and precipitation (in addition to of course its primary driver: decreasing sunlight). 2020 was a chilly September, statewide. It’s one of the few recent years were September temperatures cooler than normal and you can see the impact.
In the Twin Cities we had a chilly blast of overnight lows September 8-11, 2020 of 45° F, 41° F, 38° F, 44° F! A full two weeks earlier than anything close to that this year. Those low temperatures ‘shock’ the trees into really getting ready for Autumn. Last year (2021) the first 3 weeks of Autumn were cooler than the first three weeks this year (but much warmer than 2020). Last year however, it stayed warm in the last week of September with a more brief chilly spell in between.
If we look at the number of nights below 50° F in September, this year it’s only been 4 which is the same as last year and in 2020 we had 10! Normal is 8. So, in 2020 we were a bit above normal in chilly nights but this year and last we’ve had half the normal number of chilly nights.
Take a look at the average highs and low for the first 3 weeks (the period most impacting fall colors on Sept. 26) of September the last 3 years and in parentheses, the final September averaged high and low (which includes the last 9 days of the month):
2022: 80.2° F / 59.9° F (76.0° F / 56.3° F)
2021: 76.9° F / 58.0° F (76.6° F / 56.3° F)
2020: 69.5° F / 51.2° F (70.3° F / 52.2° F)
Looking at northern Minnesota we see a similar pattern (relative to their normal). For Hibbing, the average low this year has been about 42° F to 43° F, but in 2021 it was 41° F and in 2020 it was much chillier 37° F! Hence, the fall color map of years past.
Driest September on record for the Twin Cities
September was also a VERY dry month. This time, unlike much of the summer, almost everywhere was below normal for precipitation. But, like the summer, it was most extremely dry in the Twin Cities, while northeastern Minnesota was behind by less. Brainerd for example is only behind by about 0.42 inches for September, International Falls 0.46 inches, but in the Twin Cities we’re behind by 2.60 inches!
In fact, in the Twin Cities we’ve only had 0.23 inches of rain this whole month, making it the driest September on record.
It’s also the fourth driest period since June 1 through September:
The recent drought monitor continues to keep 4% of Minnesota in severe drought which includes most of the Twin Cities and also an area around Marshall in southwest Minnesota.
The abnormally dry part of Minnesota expanded though from 42% to now 54% of Minnesota in just the past week with actual drought increasing from 15% to 22% of the state.
The prolonged drought in the Twin Cities is having its impact. Take a look at the lake level of Lake Minnetonka since May:
It’s fallen a foot and a half since June 1. Minnetonka is the water source for the Minnehaha Creek and eventually the beloved Minnehaha Falls which are basically not running at all right now with just a few stagnant pools.
Below is a chart showing the discharge of Minnehaha Creek since May:
When we had some rain in August, the discharge was about ten times what it is now. Of course a small creek varies greatly but the flow is very low or zero presently. In the spring, when rains were abundant, the flow was 10 to 40 times more than it’s been recently.
Some of the fascinating aspects of studying weather and climate is that we don’t need to just look at temperature and precipitation charts but the real world impacts are visible all around us, especially at a time of year of transition.
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