Pool closing · Montana
When to Close Your Pool in Missoula, MT: Deadline, Window & Checklist
Last updated: July 15, 2026 · Model window year: 2026 · dates · checklist · FAQ
Circle September 18 on the Missoula calendar. Closing earlier traps warm, algae-friendly water under the cover; closing later gambles the plumbing against the first freeze, which the 1991–2020 normals place near October 1. The window opens September 8 — the live widget below shows how this year is actually tracking against it.
Missoula closing dates at a glance
| Closing window | September 8 – September 18 |
|---|---|
| Close by (deadline) | September 18 |
| First freeze, 50% probability | October 1 |
| Open by (recommended) | June 7 |
| Opening window | May 31 – June 21 |
| 61°F crossing (7-day mean) | June 21 |
| Swim-season length (80°F+ days) | 57 days |
| NOAA normals station | Missoula 2 NE · 3.0 mi · 3420 ft |
Missoula banks only about 57 days of 80°F-plus afternoons — early opening weeks are the cheapest season extension available.
The same model in water terms: Missoula's estimated pool temperature runs about 44°F in mid-April, 59°F in mid-June, 69°F in mid-August, and 48°F in mid-October, peaking near 70°F. Those four checkpoints — not any calendar holiday — are what the windows above are protecting.
The 12-step Missoula winterizing checklist
Sequenced against Missoula's September 8–September 18 window: chemistry while the pump still runs, blowouts before the equipment drains, cover last. Labels dictate every dose.
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Balance the water a few days ahead
Start midweek for a weekend close: bring alkalinity and pH into their label ranges and let the water settle. What you seal under the cover is what the pool soaks in until spring.
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Deep-clean the pool
Leave nothing organic behind: skim the surface, brush every wall and step, vacuum the floor slowly. What goes under the cover dirty comes out worse — winter only ever compounds what it's given.
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Service the filter one last time
Backwash sand or DE, or pull and rinse cartridges, per the manual. A filter stored dirty cakes over winter and starts spring half-clogged.
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Apply winter chemicals per label
Add a winterizing kit or your usual closing chemicals exactly as their labels direct for your volume, with the pump still circulating so everything distributes before shutdown.
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Lower the water level
Your cover's manual sets the number — commonly a few inches under the skimmer for solid covers, barely below normal for mesh. Stop there. The remaining water isn't laziness; it's ballast holding the shell in the ground.
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Blow out the lines and plug returns
Push air through skimmer, return, and cleaner lines until they run dry mist, then seat expansion plugs at each return while air still flows. This is the single most important freeze-protection step.
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Protect the skimmer
Install a skimmer guard bottle (or a plug rated for your skimmer) so ice that forms there crushes the sacrificial piece instead of cracking the skimmer body.
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Add pool antifreeze where blowouts fall short
If any line can't be verified dry, add pool-grade antifreeze per its label. Use only pool antifreeze — automotive products don't belong in pool plumbing.
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Drain the equipment
Open the drains on everything that holds water and let the pad empty completely. Cartridges and small equipment overwinter far better on a garage shelf than outside.
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Set the air pillow and cover
Center an inflated air pillow, then fit the cover and secure it with water bags, cable, or straps as designed. The pillow gives ice a place to push besides your walls.
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Remove and store ladders and rails
Pull ladders, rails, and the diving-board hardware; rinse, dry, and store them out of the weather. Anchor sockets get a dab of protectant so spring bolts turn freely.
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Stage the cover pump
Solid covers need drainage all winter: set a cover pump or siphon before the first storm, not after. Standing water strains seams and invites a mid-winter emergency.
What to buy before the rush
Every item below sells out somewhere in Montana every September. Stocking the short list before the rush costs nothing extra and saves the mid-project store run — the chemicals guide explains what each category actually does.
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Winter closing kit
The under-the-cover chemistry, measured for your gallons.
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Air pillow
Gives ice somewhere to push besides your pool walls.
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Winter cover
Sized to overlap; the cheapest insurance the pool wears all winter.
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Cover pump
Standing water is a cover killer; this is the counter.
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Expansion plugs + skimmer guard
The blowout's finishing move — one per return, one for the skimmer.
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Pool antifreeze
Pool-rated glycol for the lines air couldn't clear.
How Missoula compares locally
Two nearby data points to triangulate with: Coeur d'Alene, 142 miles from Missoula, models its close at September 26 (about a week later); Spokane Valley, 160 miles out, at September 27. Missoula's own window ends September 18. For the other half of the calendar see when to open in Missoula, or scan the full year on the season page.
The measuring stick here is Missoula 2 NE — 3.0 miles to the northeast, elevation about 3420 feet. Its 1991–2020 record is what the model reads for Missoula; your backyard in Missoula County will run a touch warmer or cooler with shade, wind, and pavement, which is exactly the slack the two-week lead absorbs.
Field notes for Missoula owners
The mesh-cover spring surprise, prevented in fall
Mesh-covered pools green up early because late-winter sun plus nutrient-carrying meltwater reaches the water. The fall counter-moves: close late and cold, dose the winter kit exactly per label, and plan an early-spring peek under the cover rather than a Memorial Day reveal.
Blowout first, antifreeze second
Air is the only guaranteed freeze protection: a line that's verifiably dry cannot burst. Antifreeze (pool-grade, per label) is the fallback for lines you can't confirm — long runs, low spots, water-feature plumbing. Doing antifreeze instead of a blowout, rather than in addition, is how most cracked returns happen.
Salt cells overwinter indoors
Remove the cell at closing, inspect and clean per the manufacturer, and store it inside with the unions capped. A dummy cell or spacer keeps the plumbing sealed. Cells left in outdoor plumbing through freezes are a common — and completely avoidable — spring casualty.
Closing for a real winter
A Missoula closing has to hold for months of freeze-thaw, not a few frosty mornings. Spend the effort where winters bite: prove every line dry, drain every vessel on the pad, guard the skimmer, and tension the cover for wind that will actually come. The reward is a spring opening that's a rinse, not a rebuild.
Missoula pool closing FAQ
What temperature should water be to close a pool?
Cold enough that biology has clocked out — below 65°F and falling, ideally low 60s. The widget above tracks Missoula's actual water; the normals say the durable cool-down arrives near September 8, and anything inside the window to September 18 closes clean.
Can you close a pool too early?
Early closing is the mistake the whole model is built to prevent from the other direction. A cover installed over 70°F water is a terrarium: sanitizer decays, algae compound, nobody looks for months. Missoula's water isn't reliably out of that zone until about September 8 — the calendar's first cold weekend doesn't change that.
Do I need antifreeze in pool lines?
Treat antifreeze as a backup, not a substitute: the real protection is air in dry lines. Where a full blowout isn't possible, pool-grade antifreeze per label is cheap insurance against a cracked pipe — worth it anywhere freezes are routine, and Missoula sees them from about October 1.
How far should I drain my pool for winter?
Follow the cover's instructions first: solid covers usually want water a few inches below the skimmer; some mesh setups run higher with the skimmer sealed. The hard rule is never empty — hydrostatic pressure can lift or crack an empty pool, a far worse outcome than any freeze.
What happens if you don't winterize a pool?
Two failure modes. Where freezes reach the plumbing, expansion cracks pumps, filters, and fittings from the inside. Where they don't, an unwatched pool simply drifts green and unbalanced by spring. Missoula has no published freeze normal to pin the date, so the winterizing above plus forecast-watching covers both risks.
When is the last safe date to close in Missoula?
Our model's practical deadline is September 18 — set by the cool-down plus ten days (the freeze normal, October 1, leaves room to spare). Push much past it and you're winterizing in freeze-warning weather, rushing the blowout, and hoping the cover goes on before the first hard night. Inside the September 8–September 18 window, none of that drama applies.
Email me when Missoula hits the closing window
Data: NOAA 1991–2020 climate normals via Missoula 2 NE (3.0 mi); live outlook by Open-Meteo. Full attribution on the sources page. Model assumptions and error bars on methodology.