'False fall' in St. Louis: What is it? And why 90-degree days could come back

ST. LOUIS - After several weeks of sweltering heat in July and August, September arrives on a slightly milder note for St. Louis. But don't get too comfortable just yet. The late-summer sun may still have one last hurrah.

This seasonal phenomenon, often known as "false fall," teases St. Louis with cooler temperatures and hints of autumn on a near-annual basis in September, only to let soaring highs sneak back in, a reminder that summer heat isn't ready to let go just yet.

"False fall," according to FOX 2 Meteorologist Chris Higgins, is "the first cool stretch of the [summer] season that typically breaks the back of a long stretch of hot temperatures." Daily highs that have lingered near the 80s and 90s could dip to the 70s, and some leaves may even start to change colors.

That said, what St. Louis perceives as "false fall" is generally short-lived.

"It is long enough to give everybody a break, and it does signal the early stages of the change of season. However, the cool air never stays locked in for long and a rebound back to near to above normal temperature typically follows within a week to ten days," said Higgins.

The rebound often brings highs back into the 80s and 90s for consecutive days, sometimes even into October. And in rarer cases, 100-degree high temperatures are even possible after Labor Day.

According to data compiled by FOX 2's Chris Higgins:

• In recorded history (152 years), St. Louis has seen 804 days with highs of at least 90 degrees in September and October, showing that 90+ days can still be common well into the fall.
• The latest 90-degree day on record is Oct. 30, 1950 when the high was 90.
• Only 30 days have reached 100 degrees in September, with none on record in October, November or December.
• The last 100-degree September day in St. Louis was Sept. 3, 2011 with a high of 101.
• The latest 100-degree September day in St. Louis is Sept. 29, 1953, when the high hit 102.

How "false fall" plays out into St. Louis' September weather remains to be seen. Highs aren't expected to top 85 degrees over the next seven days (starting Tuesday), and highs may settle in the 70s for several days (starting Thursday), signs that "false fall" could already be settling into the region.

The 2025 calendar year officially shifts from the summer season to the fall season on Monday, Sept. 22, 2025.