FrostList

When to Winterize Your Boat in Napa, CA

ON TRACK161 days until first 28°F freezeDec 19

Plan to haul out and winterize your boat in Napa by December 12. The first 28°F freeze near Napa averages December 19 in the 1991–2020 normals, and freeze damage often surprises owners in milder states. The early-odds date runs roughly 18 days ahead of the median, so build in that buffer.

OUTLOOK

Typical first first 28°F freeze near Dec 19; local deadline about Dec 12. The live 10-day outlook loads here.

Local freeze dates for Napa

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Nov 15Dec 4Jan 4
28°F (hard freeze)Dec 1Dec 19Jan 23
24°F (severe)Dec 5Dec 25Jan 19

NOAA station: Napa State Hospital · 1.8 mi away · 35 ft elevation.

Napa draws its numbers from Napa State Hospital, 35 feet up and 1.8 miles away. Its median first-freeze dates are 32°F by Dec 4, 28°F by Dec 19, 24°F by Dec 25. That hard freeze has landed anywhere from Dec 1 to Jan 23, a swing of roughly 53 days. Spring's last 32°F freeze clears around Feb 12.

The freeze arrives in two steps in Napa: 32°F around Dec 4, then a hard 28°F near Dec 19. The 32°F date swings from Nov 15 at its earliest to Jan 4 at its latest, near 50 days. The last spring freeze averages Feb 12 and as late as Mar 13, which sets the safe window for reopening outdoor water and de-winterizing gear. Snow barely registers here; the freeze date, around Feb 12, is the one that matters.

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, Napa's first-freeze date near Dec 19 sits later than Vallejo (Jan 2) and about a week ahead of Fairfield (Dec 24). California's deadlines span Jan 9 to Dec 30 statewide — one date for all of California would be off by weeks for Napa. Once you know Napa's freeze date, use it to winterize an RV and store a motorcycle too.

Other winter jobs in Napa

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

Sprinklers → Dec 9Frozen pipes → Nov 15Car battery → Dec 19Sump pump → Feb 12RV → Dec 9Motorcycle → Dec 4

See the full Napa 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 19 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 CA 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 Napa State Hospital, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.