Pool season · Missouri
O'Fallon Pool Season: the Year on One Bar
Last updated: July 25, 2026 · dates · water temp
This is the one-page view of O'Fallon's pool year: both model windows, the estimated water-temperature curve, and today's live position on the season bar. The detailed checklists live on the opening and closing guides; this page is the calendar you check between them.
O'Fallon, MO — 106 swim days a yearThe bar compresses O'Fallon's whole pool year into one lane rope: gray floats for the off-season, hollow teal for the April 11–May 2 opening window, solid teal for swim weeks, hollow amber for the closing stretch that ends October 14. Wherever the today marker sits, the nearest color change is your next appointment.
The O'Fallon pool calendar
Opening — target April 18
Target April 18; the practical window runs April 11–May 2, and the 7-day mean crosses the 61°F threshold around May 2. Cold-water starts are the cheap ones.
Closing — deadline October 14
Hold the cover until October 4, then winterize by October 14, a week ahead of the October 26 freeze normal. Late and cold beats early and warm.
Estimated water temperature by month
Monthly averages of the site's water model run on O'Fallon's own normals — the shape of a typical year, not a forecast. The widget above shows where the current year actually sits.
| Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec |
|---|---|---|---|---|---|---|---|---|---|---|---|
| 32° | 32° | 40° | 50° | 61° | 70° | 76° | 77° | 73° | 62° | 51° | 39° |
The curve peaks near 77°F — comfortable-water months here usually involve a heater or a solar cover.
Every station tells its own story: compare this curve with Lee's Summit's pool season and Springfield's — neighboring normals stations can shift the windows by a week or more. The full ranking lives on the Missouri season index.
O'Fallon pool season FAQ
When should I open my pool in O'Fallon, MO?
Target April 18. That date backs off two weeks from the day O'Fallon's 7-day mean temperature reaches 61°F, so you start up in cold, algae-resistant water. Work the full checklist in the O'Fallon opening guide.
When should I close my pool in O'Fallon?
The closing deadline is October 14 — late enough that the water has cooled out of the algae zone, early enough to beat O'Fallon's first-freeze normal by about a week. See the O'Fallon closing guide for the winterizing steps.
How long is the swim season in O'Fallon?
O'Fallon averages 106 days a year with normal highs of 80°F or better — right on the statewide median. The Missouri season index ranks all 8 covered cities by the same metric.
How warm does pool water get in O'Fallon?
An unheated pool near O'Fallon typically peaks around 77°F in high summer and bottoms out near 30°F in winter, never quite reaching the 78°F comfort line without a heater or solar cover. The month-by-month curve above shows the whole cycle; the methodology page explains the ±5°F error bars.
Email me when O'Fallon hits the closing window
Data: NOAA 1991–2020 climate normals via Weldon Spring NWS (6.2 mi); live outlook by Open-Meteo. Attribution: sources · assumptions: methodology.