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September was also a VERY dry month, with the Twin Cities 2.6 inches behind the monthly norm.

hrap-all-minnesota-30day_anom-4366400

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In fact, in the Twin Cities we’ve only had 0.23 inches of rain the entire month, making it the driest September on record.

Driest SEPT

It’s also the fourth driest June 1 through Sept. 30 on record in the Twin Cities.

June thru SEPT

The recent drought monitor shows 4% of Minnesota in severe drought, which includes most of the Twin Cities and an area around Marshall in southwest Minnesota. The abnormally dry part of Minnesota expanded from 42% to 54% in just the past week, with actual drought increasing from 15% to 22% of the state.

DROUGHT monitor

The prolonged drought in the Twin Cities is having its impact. Take a look at the lake level of Lake Minnetonka since May:

Minnetonka

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:

Minnehaha discharge

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 only look at temperature and precipitation charts, but the real world impacts are visible all around us – especially at a transitional time of year.

September warmth and impact on fall colors

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 was 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 in 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 Aug. 1. 

AUG-SP AVG T

From Sept. 1-21, 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 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.

AUG-SEP Max Min

I discussed this a few times in the last couple weeks, but we had to wait a long 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 Sept. 22, which is 12 days beyond even the modern average for the first night below 50° F. The pre-modern history first sub-50° F was Aug. 25.

All of this is keeping with the theme of climate signals on our summers and September. Summers are getting longer, with warmth is starting earlier and lasting later. July and August, our two hottest months,are seeing some of the slowest warming, but the length of summer is rapidly increasing.

The late arrival of chilly weather, particularly at night, has an impact. This year, we could really see it in the changing of fall color. When we look at the last three years there’s a big difference in the fall color map for Sept. 26th. 

Credit: Minnesota DNR

Credit: Minnesota DNR

You’ll note, it’s not just southern Minnesota but also northern Minnesota that’s been behind. That’s because temperatures have been above normal this month, particularly at night.

tmin30dydev

Fall color is a particularly interesting variable because it’s more dependent on overnight temperatures and precipitation (in addition to of course its primary driver: decreasing sunlight). September 2020 was chilly statewide. It’s one of the few recent years with September temperatures cooler than normal and you can see the impact in the colors.

In the Twin Cities, we had a chilly blast of overnight lows Sept. 8-11, 2020 between 38 and 45° F. That's two weeks earlier than anything close to that this year. 

Those low temperatures shock the trees into 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 brief chilly spell in between.

If we look at the number of nights below 50° F in September this year, there have only been four. We had ten such nights in 2020. The normal is eight. 

Take a look at the average high and low for the first three weeks of September the last three years. 

  • 2022: 80.2° F / 59.9° F
  • 2021: 76.9° F / 58.0° F
  • 2020: 69.5° F / 51.2° F

Looking at northern Minnesota, we see a similar pattern. 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 at 37° F. 

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