Weather in St. Louis in September

Prime time. If you get to pick your dates, September is the one. Highs average 27.6 °C, lows 15.1 °C, and rain falls on 9 days — nothing in the month is working against you.

September ranks 2 of 12 in St. Louis. Only October scores clearly higher, and the difference is small enough to be settled by price rather than weather.

Rain on 9 days, 2 of them heavy. Enough to need a plan B, not enough to need a plan around it. Nothing specialised needed — this is the month you pack for the trip rather than for the weather.

What the averages hide

Averages flatten a month, and this one hides a few things: 9 days top 30 °C — hot spells arrive, but they break. Across the twenty years measured, the hottest September day reached 38.5 °C and the coldest night fell to 3.3 °C, and the wettest single day dropped 67 mm — roughly 83% of the month's rain in one day.

September is moderately variable: 8 mm in the driest year against 168 mm in the wettest, around an average of 81 mm. Roughly two Septembers in three land near that average, and the third does something else entirely.

Wind is light at 6.2 km/h, so the temperature on the page is close to the temperature on your skin, and humidity is unremarkable at 70%.

How September sits in the year

The month is on the way down: 3.4 °C cooler than August, with October dropping another 7 °C. The first week and the last are not the same trip.

Daylight in September

12.5 hours of daylight, losing 1.2 hours across the month. That is between St. Louis's extremes of 14.8 and 9.5 hours, so days are long enough not to constrain the plan.

An inland climate

With no coast nearby, the year swings a fair way — 36.6 °C between the warmest afternoons and the coldest nights, wider than any coastal city at this latitude would manage.

How busy it gets

Peak season. Expect the highest prices of the year and to be booking well ahead.

Crowd levels are estimated from climate desirability and school-holiday calendars, not from visitor numbers.

September in St. Louis versus nearby

If St. Louis is one option among several, the nearby comparison is worth making. Indianapolis is close enough to be a coin toss — within a degree on temperature, 1 day less rain. Kansas City is close enough to be a coin toss — within a degree on temperature, the same number of rain days.

So: September is effectively as good as it gets here — 1 points behind October, which is inside the margin where the weather stops being the deciding factor and price starts being it.

Every month in St. Louis

See all twelve months in St. Louis, or where else to go in September.