FrostList

When to Winterize Your RV in Cupertino, CA

ON TRACK173 days until first 28°F freezeDec 31

Cupertino's deadline to winterize your RV is December 21: the local first 28°F freeze runs December 31 on average and December 13 at its earliest (1991–2020 normals). Cold deepens quickly here — only about 8 days separate the first frost from that hard freeze.

OUTLOOK

Typical first first 28°F freeze near Dec 31; local deadline about Dec 21. The live 10-day outlook loads here.

Local freeze dates for Cupertino

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 · 5.8 mi away · 39 ft elevation.

Cupertino draws its numbers from Moffett Federal Airfield, 39 feet up and 5.8 miles away. Its median first-freeze dates are 32°F by Dec 23, 28°F by Dec 31. Year to year the 28°F date has ranged from Dec 13 to Jan 22 — about 40 days apart. Spring's last 32°F freeze clears around Jan 6.

The freeze arrives in two steps in Cupertino: 32°F around Dec 23, then a hard 28°F near Dec 31. That first freezing night has ranged from Dec 3 to Jan 16, roughly a 44-day spread. On the spring side, the last 32°F freeze clears around Jan 6 and as late as Feb 4 — the green light for reopening water and de-winterizing. With almost no snow in a normal year, cold — not plowing — sets the calendar, and it centers on Jan 6.

Your rv checklist

  1. Drain the fresh, gray, and black tanks, then drain the water heater only after it has cooled and lost pressure.
  2. Bypass the water heater so you do not waste six gallons of antifreeze filling the tank.
  3. Blow out the lines with a compressor at about 30–40 psi, then pump non-toxic RV antifreeze through every faucet until it runs pink.
    Helpful gear: RV antifreeze (6-pack) — Recommended pick
  4. Use a water-pump converter kit to draw antifreeze straight from the jug through the plumbing.
    Helpful gear: Water pump converter kit — Recommended pick
  5. Pour a cup of antifreeze into each drain trap and the toilet to protect the seals.
  6. Fit a blow-out plug at the city-water inlet so you can clear that line too.
    Helpful gear: Blow-out plug — Recommended pick
  7. Remove batteries or put them on a maintainer, and store them off a cold concrete floor.
  8. Cover roof vents, close the propane, and set out traps or deterrents so mice do not move in.

What to have on hand

RV antifreeze (6-pack)
Non-toxic pink fluid for the fresh-water plumbing and traps.
Recommended pick
Water pump converter kit
Siphon kit that draws antifreeze straight through the pump.
Recommended pick
Blow-out plug
Adapter that lets a compressor clear the lines before antifreeze.
Recommended pick
Heated fresh-water hose
For anyone staying hooked up through a cold snap.
Recommended pick

What this means locally

Compared with nearby cities, Cupertino's first-freeze date near Dec 31 sits close to Sunnyvale (Dec 31) and close to Santa Clara (Dec 28). California's deadlines span Jan 6 to Dec 27 statewide — one date for all of California would be off by weeks for Cupertino. Once you know Cupertino's freeze date, use it to shut down your sprinklers and winterize a boat too.

Other winter jobs in Cupertino

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

Sprinklers → Dec 21Frozen pipes → Dec 3Car battery → Dec 31Sump pump → Jan 6Boat → Dec 24Motorcycle → Dec 23

See the full Cupertino winter checklist, in order →

Frequently asked questions

At what temperature do RV pipes freeze?
An RV's exposed plumbing and thin-walled tanks can begin to freeze once temperatures sit in the mid-20s for a few hours, sooner than a house because there is little insulation around the lines. That is why the winterizing deadline tracks the first 28°F freeze rather than the first light frost.
How much RV antifreeze do I need?
Most travel trailers and motorhomes take about two to three gallons of non-toxic RV antifreeze, and larger rigs more. Bypassing the water heater saves roughly six gallons you would otherwise waste filling the tank. Buy an extra jug; stores sell out once the first freeze warnings hit.
Can I just use my RV furnace instead of winterizing?
Relying on the furnace is risky. A propane run-out, a tripped breaker, or a dead battery during a cold night leaves the plumbing unprotected, and the furnace may not keep the underbelly and bays warm enough anyway. Draining and adding antifreeze is the dependable way to protect the system.
How do I winterize an RV water heater?
Let the tank cool and lose pressure, then drain it and, if fitted, pull the anode rod. Do not add antifreeze to the water heater tank; instead, set the bypass valves so antifreeze skips the tank and fills only the lines. Fresh antifreeze in an unbypassed heater just wastes several gallons.
When is it safe to de-winterize an RV in Cupertino?
Wait until the last spring freeze has passed, which averages near Jan 6 here. De-winterize by flushing the antifreeze from the lines, sanitizing the fresh tank, and checking for leaks before a trip. Reopening too early risks a surprise late freeze on a charged system.
Do I need to winterize an RV stored indoors?
A reliably heated indoor space that stays above freezing protects the plumbing, so full winterizing may not be needed. But an unheated garage or barn can still drop below freezing on a cold night. If you cannot guarantee the space stays warm, winterize to be safe.

Data: NOAA 1991–2020 normals via Moffett Federal Airfield, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.