Warm, Wet, and WEIRD September

temperature and precipitation graph for Duluth
Graph showing relationship between monthly temperature and precipitation for all Septembers on record at Duluth. September 2023's lonely position in the upper-right corner of the graph exemplifies its unique combination of warmth and wetness. Graph produced by Minnesota State Climatology Office

What a strange month!

September 2023 began hot and dry, ended hot and stormy, and in its course took Minnesotans on a climatic roller coaster that included record-high temperatures, the worst levels of drought possible, multiple heavy thunderstorm events, flash-flooding, and near-record precipitation in Duluth. In fact, Duluth experienced a rare combination of extreme wetness and extreme warmth, such that no other September was even close to having so much of both.

As the month began, Minnesota was in a dire drought situation, having just experienced four straight months of statewide precipitation deficits, leading to average shortages of about 42%, or about 6.5 inches during that time. Although a relatively wet period had set in from late July through mid-August, many areas had not gotten a drop of rain for two weeks or more as of September 1st.

It did not help, then, that the state was thrown into a historic, rare, and extreme heat wave for the first five days of September, with multiple 100-degree readings recorded across southern, western, and central Minnesota. The State Climatology Office measured over 1.6 inches of evaporation during the long weekend at the University of Minnesota- St. Paul campus; this removal of water from Minnesota landscapes only worsened drought conditions.

When the US Drought Monitor update was released on September 7th, the "Extreme" drought category had expanded to cover 16% of Minnesota, and one small area near Albert Lea and another near Cloquet had been degraded into "D4" or "Exceptional" drought, which is the worst level possible. By the September 19 release, the Extreme Drought area had grown to cover 26% of the state, the Exceptional Drought area had doubled in size, and the hardest-hit areas were seeing the worst drought conditions in decades.

Meanwhile, however,parts of northeastern Minnesota near Lake Superior began experiencing heavy rains, with 2.77 inches falling in Duluth on September 5th. Another heavy rainfall event on the 11th caused flooding on Duluth's steep hills, but generally missed areas outside of northeastern Minnesota.

By later in the month, the heavy rain bug caught more of Minnesota. On September 23rd and 24th, a spiraling area of thunderstorms with steady to heavy rainfall moved slowly across the state, producing a badly-needed soaking across many areas, but aggravating conditions near Lake Superior, where Duluth recorded another 4.5 inches in two days. Additional heavy thunderstorms on Monday the 25th made this the most widespread heavy precipitation event of the year for Minnesota.

After a few dry days, more thunderstorms with heavy rain, arriving in three or four separate waves, affected parts of central, and southern Minnesota from September 29th into the 30th. On the final day of the month, however, the rains were followed by what would become yet another historic heat wave--which reached its peak on October 1st.

At virtually every climate station in the state, September 2023 finished among the 10 warmest on record, and in the top-five at the majority of stations. It was the warmest September on record at the Twin Cities station, which has 151 years of September temperature observations. 

Precipitation was less clear-cut, because the abundant rains favored eastern and northeastern parts of the state, whereas southwestern Minnesota remained quite dry. The preliminary September 2023 precipitation map shows that southwestern Minnesota was 2-3 inches short of normal, while areas from the near the Twin Cities, northeastward through the Duluth, were often 2-5 inches above normal, with small areas reaching surpluses of six inches or more.

The Outstanding Climatic Feat of the month, almost certainly goes to Duluth. With 10.36 inches of rain, it was the second-wettest September on record, going back to 1872. It was also Duluth's third-warmest September on record, back to 1874. The rare combination of high-ranking wetness and warmth together is highly unusual, and sets September 2023 apart from all other Septembers in Duluth. No other September in Duluth's top-five for warmth was within 3.5 inches of 2023's precipitation, and no September in Duluth's top-five for precipitation was within three degrees of 2023's temperature.


Updated November 1



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