For most, summer doesn’t officially end until the autumn equinox, but for meteorologists and climatologists, fall begins on September 1! The meteorological seasons slice the calendar year into four, three-month chunks based on the annual temperature cycle. This makes it easier to calculate seasonal statistics from year to year. That also means our office will be drinking pumpkin spice all month long!
Temperatures
The Outlook: To Be Determined
NOAA’s Climate Prediction Center is not seeing any significant signal in the temperature outlook in Wisconsin for the upcoming month. They are giving the state equal chances of experiencing above, below, or near average temperatures in September.
The forecast highlights parts of the Central Plains and Rocky Mountain regions as having a chance of above-average temperatures in September.
The Averages
Wisconsin’s average temperature in September is just below 60 degrees (Fahrenheit). Temperatures change significantly throughout the month: the beginning of September often feels more summer-like, while the end of the month can take on a brisk, autumn feel.
The warmest September was in 2015, when the average temperature across the state was 64.7 degrees. September of 2024 was also exceptionally warm: the statewide average temperature was 63.9 degrees, and the average high temperature was a hot 77 degrees!
Many locations in northern Wisconsin experience the first freeze of the season before the start of October. Forest County in the northeast region typically sees the earliest first fall freeze in the state, with an average date of September 20.
September 1918 brought the coldest conditions to the state — the statewide average low temperature was in the upper 30s. The coldest day of that month was September 30, when the central part of the state reported temperatures in the upper 20s while the northwest region woke up to temperatures in the teens! The first cases of Spanish Influenza were reported in Wisconsin around this time, and it’s possible that the unusually cold conditions that month aided in the rapid spread of the virus.
Precipitation
The Outlook: To Be Determined
Similar to the temperature outlook, the Climate Prediction Center does not see any significant signal in the precipitation outlook for Wisconsin, giving the state equal chances of experiencing above, below, or near average precipitation in September.
The Florida peninsula is highlighted as having the highest chance of the lower 48 states of experiencing a wetter-than-average September.
The Averages
Thunderstorm activity typically decreases through September, and the state begins to dry out. Drier stretches during this month are crucial for farmers looking to harvest, as both the crop and soil need to be sufficiently dry. However, too little precipitation during the month can hurt the storage of moisture in the soil going into the winter season. Wisconsin’s average precipitation during the month is 3.59 inches.
The last five Septembers have been drier than average; 2024 ranked as the eighth driest September since 1895. The driest September on record was in 1952, when the average precipitation across the state was 0.83 inches. Milwaukee, Madison, La Crosse, Wausau, and Green Bay received less than a half inch of rain that month.
By contrast, the soggiest September in Wisconsin was in 1986 when many locations in the central region of the state measured over a foot of rain! Beaver Dam in Dodge County reported the highest monthly total of 15.05 inches. The statewide average precipitation was 8.38 inches.
September Snow??
It’s not unheard of for a few flurries to fall across the northern counties in late September, but most of the state has until October before the white stuff begins to appear.
However, an unusual snowstorm in 1942 brought inches of snow to the west and central parts of the state from September 26 to 27. The highest total was 6.3 inches, reported north of Eagle River in Vilas County — that’s the largest amount of snow measured in Wisconsin in September. I think most of us can agree that we don’t want to give that record a run for its money any time soon!