September Weather Update

September Weather Update:

Since last week, a number of Minnesota climate stations have reported overnight low temperatures in the 30s F. At least 20 climate stations have reported a frost, and a few others have seen overnight temperatures drop into the upper 20s F. It was just 26°F last Sunday morning (September 7) at Brownton (McLeod County). Average temperatures this month are ranging from 4°F to 8°F below normal so far, but that is about to change with a long run of above normal temperatures coming up and several days expected to bring high temperatures in the 80s F, may a few 90s F, and overnight lows in the 60s F. A few southern Minnesota communities saw 80°F or higher on Thursday of this week. Dew points will also creep upward into the 60s F and low 70s F. Outlook models continue to favor above normal temperatures for most of the balance of September.

Rainfall this month has been above normal to near normal in most places around Minnesota. The range across the state has been from just 0.08 inches at Wolverton (Wilkin County) to over 3.60 inches in the Duluth area. Most places have received an inch or more. A few observers reported new daily record rainfall amounts on September 9th, including:
1.30 inches at Pokegama Dam
1.42 inches at Two Harbors
1.97 inches at Brimson

With the added surface moisture from rainfall this week, a number of consecutive mornings produced dense fog, especially in eastern areas of the state. 

Comment on fall weather in Minnesota:

During September and October in Minnesota we tend to experience some of our finest weather. There are simple reasons for this: (1) There is a decline in convectively induced thunderstorm activity (shorter days, reduced surface heating); (2) the polar frontal boundary and jet stream which are conducive to the formation of mid latitude cyclonic storms remain to our north in Canada; and (3) vegetation in the landscape tends to die (mature) or go into dormancy and therefore transpiration declines which reduces the amount of water vapor released to the lower layers of the atmosphere. These and other factors help to make the weather more pleasant in the early fall season. In fact, statistics show that the last few days of September and first few days of October have a higher frequency of clear skies than any other period of the year. 

Weekly Weather Potpourri:

Today is Cloud Appreciation Day (September 12th) around the world. The Cloud Appreciation Society is advocating for everyone to consider taking a photo of clouds today and posting on their Memory Cloud Atlas page. Also, they encourage people to share what clouds mean to them, and stories about cloud forms they have witnessed. The atlas is full of wonderful pictures, so make time to take a look.

The Copernicus Program in Europe released an update on the thirst 8 months of 2025 from a global climate perspective. The first 8 months of this year are second warmest in the global temperature record, trailing only 2024. A number of countries reported their warmest summer in history (June-August) including the U.K., Ireland, China, Japan, and South Korea, and of course it was a bad year for wildfires. You can read more detail on their web site.

New research published in the journal Nature Communications reveals that East Antarctica is warming faster than expected, mostly due to increased advection of warm air from the Southern Indian Ocean, where sea surface temperatures have been increasing. Thus the pace of climate change in the Southern Indian Ocean is having direct consequences on the melting of ice in East Antarctica.

MPR listener question:

Lots of early morning fog this week for us commuters. Is this a bit unusual for September? 

Answer:

According to historical climatology for the Twin Cities it is not uncommon to have at least one day with heavy fog during September. Multiple days is a bit unusual. More frequent heavy fogs tend to occur in the months of December, February, and March. The least number of days with heavy fog tends to be in July. 

Twin Cities Almanac for September 12th:

The average MSP high temperature for this date is 75 degrees F (plus or minus 10 degrees F standard deviation), while the average low is 56 degrees F (plus or minus 7 degrees F standard deviation).

MSP Local Records for September 12th:

MSP records for this date: highest daily maximum temperature of 94 degrees F in 1948; lowest daily maximum temperature of 55 degrees F in 1974; lowest daily minimum temperature of 36 degrees F in 1940; highest daily minimum temperature of 73 degrees F in 1931; and record precipitation of 4.96 inches in 1903. There has not been any snowfall on this date.

Average dew point for September 12th is 52°F; the maximum dew point on this date is 72°F in 1909 and the minimum dew point on this date is 26 degrees F in 1923. 

All-time state records for September 12th:

The state record high temperature for this date is 102 degrees F at Beardsley (Big Stone County) in 1931. The state record low temperature for this date is 17 degrees F at Kelliher (Beltrami County) in 1917. The state record precipitation for this date is 7.42 inches at Edgerton 0.5 W (Pipestone County) in 2019. A trace of snowfall was reported on this date in 1923 at Two Harbors, Duluth, Roseau, Virginia, and Bemidji. 

Past Weather:

On September 12 of 1923 observers in Beltrami, Roseau, Lake, and St Louis Counties reported snow flurries! Some of the earliest autumn reports of snow.

Record-setting hot temperatures gripped the state on September 12 of 1931. Most areas reported afternoon high temperatures in the 90s F. September of 1931 was the 5th warmest in state history with many 90°F days.

Record-setting cold temperatures prevailed on the morning of September 12 in 2000 across northern Minnesota communities. It was 28°F at Embarrass, 22°F at Black Duck, and just 17°F at Kelliher, where the daytime high temperature only reached 57°F.

A slow-moving low pressure system brought repeated rounds of thunderstorms to southern Minnesota counties over September 10-12 of 2019. Many observers reported 4 to 9 inches of rainfall. Rivers in southwestern Minnesota reached flood stage and many roads were closed for a time. Some farm fields were flooded for days. 

Outlook:

Significant warm-up over the weekend with many areas of the state seeing temperatures of 80°F or higher. Some southern and western areas may see 90°F or higher and Heat Advisories may be issued by the National Weather Service. Increasing cloudiness on Sunday night with a chance for showers and thunderstorms, carrying over into Monday. Warm temperatures will continue through most of next week with a chance for showers and thunderstorms Tuesday through Thursday. Temperatures will cool somewhat towards the end of next week.