FrostList

When to Winterize Your Boat in Sunnyvale, CA

ON TRACK173 days until first 28°F freezeDec 31

In Sunnyvale, target December 24 to drain, antifreeze, and lay up the boat, about a week before the median first 28°F freeze of December 31; a surprise cold snap is exactly how freeze claims happen. 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 31; local deadline about Dec 24. The live 10-day outlook loads here.

Local freeze dates for Sunnyvale

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Dec 3Dec 23Jan 16
28°F (hard freeze)Dec 13Dec 31Jan 22

NOAA station: Moffett Federal Airfield · 2.6 mi away · 39 ft elevation.

For Sunnyvale, the nearest NOAA station with freeze data is Moffett Federal Airfield, 2.6 miles out at 39 feet. Median first-freeze dates there run 32°F by Dec 23, 28°F by Dec 31. That hard freeze has landed anywhere from Dec 13 to Jan 22, a swing of roughly 40 days. Spring's last 32°F freeze clears around Jan 6.

The freeze arrives in two steps in Sunnyvale: 32°F around Dec 23, then a hard 28°F near Dec 31. The 32°F date swings from Dec 3 at its earliest to Jan 16 at its latest, near 44 days. The last spring freeze averages Jan 6 and as late as Feb 4, which sets the safe window for reopening outdoor water and de-winterizing gear. Snow barely registers here; the freeze date, around Jan 6, 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

Against its neighbors, Sunnyvale (first freeze Dec 31) runs close to Mountain View (Dec 31) and close to Cupertino (Dec 31). Across California, local prep deadlines in our data range from Jan 9 to Dec 30, so a statewide rule of thumb would miss Sunnyvale by weeks. In Sunnyvale, that same cold is your cue to winterize an RV and store a motorcycle.

Other winter jobs in Sunnyvale

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

Sprinklers → Dec 21Frozen pipes → Dec 3Car battery → Dec 31Sump pump → Jan 6RV → Dec 21Motorcycle → Dec 23

See the full Sunnyvale 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 31 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 Moffett Federal Airfield, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.