FrostList

When to Winterize Your Boat in O'Fallon, MO

ON TRACK117 days until first 28°F freezeNov 5

The first 28°F freeze near O'Fallon averages November 5 (1991–2020 normals) — enough to crack an unprotected block even where owners assume they're safe — so winterize the boat by October 29. Year to year the date swings about 28 days, which is why the live outlook beats the calendar.

OUTLOOK

Typical first first 28°F freeze near Nov 5; local deadline about Oct 29. The live 10-day outlook loads here.

Local freeze dates for O'Fallon

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Oct 14Oct 26Nov 7
28°F (hard freeze)Oct 25Nov 5Nov 22
24°F (severe)Nov 3Nov 18Dec 6

NOAA station: St Peters 2 Se · 6.6 mi away · 575 ft elevation.

The reference station for O'Fallon is St Peters 2 Se (6.6 mi, 575 ft). First freeze there: 32°F by Oct 26, 28°F by Nov 5, 24°F by Nov 18. That hard freeze has landed anywhere from Oct 25 to Nov 22, a swing of roughly 28 days. Spring's last 32°F freeze clears around Apr 9. Snowfall averages 15 inches a year, first reaching an inch near December.

In O'Fallon, freezing nights (32°F) typically begin around Oct 26 and the first hard freeze (28°F) follows near Nov 5. Year to year, the first 32°F night has fallen anywhere from Oct 14 to Nov 7 — about 24 days apart. Spring's final freeze lands near Apr 9 and as late as Apr 25, so that is when outdoor water and stored gear can safely come back online. Snow is light here, near 15 inches a year, so pipe and battery cold usually matters more than plowing.

Your boat checklist

  1. Change the engine oil and filter while the motor is warm so contaminants leave with the old oil.
  2. Run the engine on antifreeze until pink fluid reaches the outlet, protecting the block and manifolds.
    Helpful gear: Marine antifreeze — Recommended pick
  3. Fog the cylinders with fogging oil as the engine runs down to coat internal parts against corrosion.
    Helpful gear: Fogging oil — Recommended pick
  4. Top off the fuel tank and add stabilizer to limit condensation and protect ethanol-blend fuel.
    Helpful gear: Marine fuel stabilizer — Recommended pick
  5. Drain the raw-water system, the livewell, and any washdown lines that can trap water.
  6. Remove the battery, charge it fully, and keep it on a maintainer over the layup.
    Helpful gear: Battery maintainer — Recommended pick
  7. Support the cover so snow and rain shed off, and leave vents so the interior can breathe.
  8. For an inboard or sterndrive, follow the manual closely — those blocks are the ones freeze claims hit hardest.

What to have on hand

Marine antifreeze
Propylene-glycol fluid rated for engine blocks and systems.
Recommended pick
Marine fuel stabilizer
Additive that protects ethanol-blend fuel over the layup.
Recommended pick
Fogging oil
Spray that coats cylinders against corrosion in storage.
Recommended pick
Battery maintainer
Keeps the cranking battery topped off on the trailer.
Recommended pick

What this means locally

Against its neighbors, O'Fallon (first freeze Nov 5) runs close to St. Peters (Nov 5) and about a week ahead of St. Charles (Nov 10). Across Missouri, local prep deadlines in our data range from Oct 19 to Nov 8, so a statewide rule of thumb would miss O'Fallon by weeks. In O'Fallon, that same cold is your cue to winterize an RV and store a motorcycle.

Other winter jobs in O'Fallon

Every task below is dated to O'Fallon's own freeze and snow normals.

Sprinklers → Oct 26Frozen pipes → Oct 14Ice dams → Nov 15Car battery → Nov 5Snow blower → Nov 24Sump pump → Apr 9RV → Oct 26Motorcycle → Oct 26

See the full O'Fallon winter checklist, in order →

Frequently asked questions

At what temperature does a boat engine block crack?
Trapped water in an engine block or manifold can freeze and crack the casting once temperatures fall below 32°F and stay there long enough. Inboard and sterndrive blocks hold the most water and are the most vulnerable. Draining fully and adding antifreeze before the first hard freeze near Nov 5 is the protection.
Do outboards need winterizing?
Yes. An outboard should be drained so no water sits in the powerhead or lower unit, and the fuel system stabilized. Outboards self-drain better than inboards when tilted down, but fogging the cylinders and changing the gear oil are still part of a proper layup.
Fogging oil: is it necessary?
Fogging coats the cylinder walls and internal parts with a film that resists corrosion during months of storage. On engines that sit all winter it is cheap insurance against rust, especially in humid or coastal storage. Follow your engine maker's guidance on how to apply it.
How much does boat winterization cost?
A professional winterization varies with engine type and boat size, and inboards cost more than a simple outboard. Doing it yourself costs mainly antifreeze, oil, and fogging oil. Either way, the bill is far smaller than repairing a cracked block, which is the failure winterizing prevents.
Do boats in MO really need winterizing?
If temperatures reach freezing where the boat is stored, yes. Freeze claims are common in milder states precisely because owners assume they are safe and skip the step, then a rare cold snap cracks a block. Check the local freeze dates above and protect the engine before the first hard freeze.
When should I de-winterize my boat?
De-winterize in spring once the risk of a hard freeze has passed for your area. Reconnect and charge the battery, replace drain plugs, check hoses and clamps, and inspect the fuel system before the first launch. Reopening too early leaves the block exposed to a late cold snap.

Data: NOAA 1991–2020 normals via St Peters 2 Se, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.