FrostList

When to Winterize Your Boat in Seattle, WA

ON TRACK144 days until first 28°F freezeDec 2

Freeze protection for a Seattle boat keys off the first 28°F freeze, which averages December 2, so haul out and winterize by November 25 to beat it. Plan for the early end: the one-in-ten date lands about 20 days before the median.

OUTLOOK

Typical first first 28°F freeze near Dec 2; local deadline about Nov 25. The live 10-day outlook loads here.

Local freeze dates for Seattle

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Nov 1Nov 17Dec 6
28°F (hard freeze)Nov 12Dec 2Dec 30
24°F (severe)Nov 21Dec 14Jan 29

NOAA station: Seattle Boeing Fld · 5.4 mi away · 20 ft elevation.

Numbers for Seattle come from Seattle Boeing Fld, 5.4 miles away at 20 feet, where the medians fall 32°F by Nov 17, 28°F by Dec 2, 24°F by Dec 14. Year to year the 28°F date has ranged from Nov 12 to Dec 30 — about 48 days apart. Spring's last 32°F freeze clears around Mar 13. Snowfall averages 4 inches a year, first reaching an inch near December.

Seattle usually sees its first 32°F night about Nov 17, with the first 28°F hard freeze close behind near Dec 2. That first freezing night has ranged from Nov 1 to Dec 6, roughly a 35-day spread. On the spring side, the last 32°F freeze clears around Mar 13 and as late as Apr 1 — the green light for reopening water and de-winterizing. Only about 4 inches of snow falls in a typical year, so cold protection outranks snow removal.

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, Seattle's first-freeze date near Dec 2 sits about a week ahead of Bellevue (Dec 11) and about a week ahead of Kirkland (Dec 11). Washington's deadlines span Oct 9 to Dec 4 statewide — one date for all of Washington would be off by weeks for Seattle. Once you know Seattle's freeze date, use it to winterize an RV and store a motorcycle too.

Other winter jobs in Seattle

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

Sprinklers → Nov 22Frozen pipes → Nov 1Ice dams → Nov 15Car battery → Dec 2Snow blower → Nov 24Sump pump → Mar 13RV → Nov 22Motorcycle → Nov 17

See the full Seattle 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 Dec 2 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 WA 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 Seattle Boeing Fld, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.