FrostList

When to Winterize Your RV in Flagstaff, AZ

ON TRACK90 days until first 28°F freezeOct 9

Aim to winterize your RV in Flagstaff by September 29, roughly a week and a half before the median first 28°F freeze of October 9, which one fall in ten shows up by September 28. The early-odds date runs roughly 11 days ahead of the median, so build in that buffer.

OUTLOOK

Typical first first 28°F freeze near Oct 9; local deadline about Sep 29. The live 10-day outlook loads here.

Local freeze dates for Flagstaff

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Sep 16Sep 28Oct 9
28°F (hard freeze)Sep 28Oct 9Oct 22
24°F (severe)Oct 7Oct 20Nov 3

NOAA station: Flagstaff Pulliam AP · 3.8 mi away · 7,003 ft elevation.

Flagstaff draws its numbers from Flagstaff Pulliam AP, 7,003 feet up and 3.8 miles away. Its median first-freeze dates are 32°F by Sep 28, 28°F by Oct 9, 24°F by Oct 20. That hard freeze has landed anywhere from Sep 28 to Oct 22, a swing of roughly 24 days. Spring's last 32°F freeze clears around Jun 4. Snowfall averages 90 inches a year, first reaching an inch near October.

The freeze arrives in two steps in Flagstaff: 32°F around Sep 28, then a hard 28°F near Oct 9. That first freezing night has ranged from Sep 16 to Oct 9, roughly a 23-day spread. On the spring side, the last 32°F freeze clears around Jun 4 and as late as Jun 19 — the green light for reopening water and de-winterizing. Snow totals near 90 inches a year mean plowing and ice-dam control share the winter to-do list here.

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

Against its neighbors, Flagstaff (first freeze Oct 9) runs about a week ahead of Prescott Valley (Nov 8) and about a week ahead of Prescott (Oct 31). Across Arizona, local prep deadlines in our data range from Sep 29 to Dec 25, so a statewide rule of thumb would miss Flagstaff by weeks. In Flagstaff, that same cold is your cue to shut down your sprinklers and winterize a boat.

Other winter jobs in Flagstaff

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

Sprinklers → Sep 29Frozen pipes → Sep 16Ice dams → Sep 15Car battery → Oct 9Snow blower → Sep 24Sump pump → Jun 4Boat → Oct 2Motorcycle → Sep 28

See the full Flagstaff 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 Flagstaff?
Wait until the last spring freeze has passed, which averages near Jun 4 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 Flagstaff Pulliam AP, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.