FrostList

When to Winterize Your Boat in Great Falls, MT

ON TRACK92 days until first 28°F freezeOct 11

In Great Falls, target October 4 to drain, antifreeze, and lay up the boat, about a week before the median first 28°F freeze of October 11; a surprise cold snap is exactly how freeze claims happen. Year to year the date swings about 34 days, which is why the live outlook beats the calendar.

OUTLOOK

Typical first first 28°F freeze near Oct 11; local deadline about Oct 4. The live 10-day outlook loads here.

Local freeze dates for Great Falls

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Sep 17Oct 1Oct 16
28°F (hard freeze)Sep 25Oct 11Oct 29
24°F (severe)Oct 2Oct 24Nov 8

NOAA station: Great Falls 16St · 0.5 mi away · 3,505 ft elevation.

Numbers for Great Falls come from Great Falls 16St, 0.5 miles away at 3,505 feet, where the medians fall 32°F by Oct 1, 28°F by Oct 11, 24°F by Oct 24. The 28°F freeze has come as early as Sep 25 and as late as Oct 29, a 34-day spread. Spring's last 32°F freeze clears around May 6. Snowfall averages 66 inches a year, first reaching an inch near October.

The freeze arrives in two steps in Great Falls: 32°F around Oct 1, then a hard 28°F near Oct 11. Year to year, the first 32°F night has fallen anywhere from Sep 17 to Oct 16 — about 29 days apart. Spring's final freeze lands near May 6 and as late as May 19, so that is when outdoor water and stored gear can safely come back online. With about 66 inches of snow a year, roof, snow-blower, and ice-dam prep all belong on the same calendar.

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

Compared with nearby cities, Great Falls's first-freeze date near Oct 11 sits later than Helena (Oct 7) and later than Bozeman (Oct 3). Montana's deadlines span Sep 26 to Oct 5 statewide — one date for all of Montana would be off by weeks for Great Falls. Once you know Great Falls's freeze date, use it to winterize an RV and store a motorcycle too.

Other winter jobs in Great Falls

Every task below is dated to Great Falls's own freeze and snow normals.

Sprinklers → Oct 1Frozen pipes → Sep 17Ice dams → Sep 15Car battery → Oct 11Snow blower → Sep 24Sump pump → May 6RV → Oct 1Motorcycle → Oct 1

See the full Great Falls 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 Oct 11 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 MT 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 Great Falls 16St, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.