FrostList

When to Winterize Your RV in Longmont, CO

ON TRACK93 days until first 28°F freezeOct 12

Aim to winterize your RV in Longmont by October 2, roughly a week and a half before the median first 28°F freeze of October 12, which one fall in ten shows up by September 30. The early-to-late range spans roughly 25 days, so treat the median as a midpoint, not a promise.

OUTLOOK

Typical first first 28°F freeze near Oct 12; local deadline about Oct 2. The live 10-day outlook loads here.

Local freeze dates for Longmont

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Sep 23Oct 5Oct 16
28°F (hard freeze)Sep 30Oct 12Oct 25
24°F (severe)Oct 6Oct 21Nov 5

NOAA station: Longmont 2 Ese · 1.3 mi away · 4,950 ft elevation.

Longmont draws its numbers from Longmont 2 Ese, 4,950 feet up and 1.3 miles away. Its median first-freeze dates are 32°F by Oct 5, 28°F by Oct 12, 24°F by Oct 21. The 28°F freeze has come as early as Sep 30 and as late as Oct 25, a 25-day spread. Spring's last 32°F freeze clears around May 7.

Longmont usually sees its first 32°F night about Oct 5, with the first 28°F hard freeze close behind near Oct 12. That first freezing night has ranged from Sep 23 to Oct 16, roughly a 23-day spread. On the spring side, the last 32°F freeze clears around May 7 and as late as May 22 — 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 May 7.

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, Longmont (first freeze Oct 12) runs close to Boulder (Oct 15) and close to Loveland (Oct 10). Across Colorado, local prep deadlines in our data range from Sep 30 to Oct 23, so a statewide rule of thumb would miss Longmont by weeks. In Longmont, that same cold is your cue to shut down your sprinklers and winterize a boat.

Other winter jobs in Longmont

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

Sprinklers → Oct 2Frozen pipes → Sep 23Car battery → Oct 12Sump pump → May 7Boat → Oct 5Motorcycle → Oct 5

See the full Longmont 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 Longmont?
Wait until the last spring freeze has passed, which averages near May 7 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 Longmont 2 Ese, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.