FrostList

When to Winterize Sprinklers in Napa, CA

ON TRACK161 days until first 28°F freezeDec 19

In a typical year, winterize your sprinkler system in Napa by December 9. The median first 28°F hard freeze at Napa's NOAA station is December 19 (1991–2020 normals); one year in ten it arrives as early as December 1. 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 9. 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 sprinklers checklist

  1. Shut off the irrigation water supply at the main valve and, if you have one, the dedicated sprinkler shutoff inside the house.
  2. Turn off the controller or set it to the "rain" mode so valves do not open while the system is dry.
  3. Drain the mainline using the manual, automatic, or blow-out method your system was built for; most pros prefer a blow-out.
  4. Connect a compressor to the blow-out port through a proper adapter and run 40–80 psi, one zone at a time, until the heads mist and clear.
    Helpful gear: Air compressor blow-out adapter — Recommended pick
  5. Insulate the backflow preventer and any above-ground valves; this brass assembly is usually the first part to crack.
    Helpful gear: Insulated backflow preventer cover — Recommended pick
  6. Cap outdoor hose bibs with foam covers after the hoses come off so the last exposed fittings stay protected.
    Helpful gear: Foam outdoor faucet covers — Recommended pick
  7. Open the backflow test cocks a quarter turn so any trapped water has room to expand.
  8. Log the date and the psi you used; you will want the reference next fall.

What to have on hand

Air compressor blow-out adapter
Quick-connect fitting that mates a shop compressor to your irrigation line.
Recommended pick
Insulated backflow preventer cover
Pouch that shields the above-ground backflow valve from early frost.
Recommended pick
Foam outdoor faucet covers
Slip-on domes that protect hose bibs after you drain them.
Recommended pick
Portable air compressor
A modest pancake unit moves enough air to clear most home zones.
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 6 to Dec 27 statewide — one date for all of California would be off by weeks for Napa. Once you know Napa's freeze date, use it to protect your indoor pipes and winterize an RV if you own one too.

Other winter jobs in Napa

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

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

See the full Napa winter checklist, in order →

Frequently asked questions

What temperature freezes sprinkler pipes?
Standing water in shallow lines and the backflow assembly can start to freeze once air temperatures drop into the mid-20s for several hours. Buried mains are slower, but the exposed brass backflow preventer and above-ground valves are the parts that crack first, which is why the deadline is tied to the first 28°F freeze, not the first frost.
Do I need to blow out my sprinklers or just drain them?
It depends on how the system was installed. Manual or automatic drains work if the lines were pitched to low points. Most irrigation pros recommend a compressed-air blow-out because it clears water that drains miss, especially in flat yards. If you are unsure how your system drains, a blow-out is the safer choice.
What happens if I don't winterize my sprinkler system?
Water left in the lines expands as it freezes and can split the backflow preventer, crack valves, or break fittings. Repairs to a backflow assembly commonly run more than the cost of a blow-out. The damage often is not visible until you turn the system on in spring and find leaks.
How much does a sprinkler blowout cost?
A professional blow-out typically costs somewhere in the range of a modest service call, depending on the number of zones and your region. Buying an adapter and using your own compressor costs less over time, but you need enough air volume to clear each zone. Prices rise once the first freeze warnings appear, so book early.
When should I turn my sprinklers back on in Napa?
Wait until the last spring freeze has passed, which averages around Feb 12 here in the 1991–2020 normals. Turning the system on too early risks refreezing a charged line. Open the main slowly to avoid a pressure surge, then walk each zone to check for winter damage before you rely on the schedule.
Can I winterize sprinklers myself?
Draining and insulating are within reach for most homeowners. A blow-out is doable if you own a compressor that moves enough air and you use a proper adapter, working one zone at a time at safe pressure. If your system lacks a clear blow-out port or you are not confident, hiring a pro once is a reasonable call.

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