Pool closing · Washington
When to Close Your Pool in Seattle, WA: Deadline, Window & Checklist
Last updated: July 15, 2026 · Model window year: 2026 · dates · checklist · FAQ
In Seattle, the closing window runs from September 21 to October 1. Let the water cool out of the algae-growth range before covering — close too warm and you lift the cover onto a green surprise in spring — but finish ahead of the first freeze, which normals place around November 23. The live estimate below shows where Seattle's water sits today.
Seattle closing dates at a glance
| Closing window | September 21 – October 1 |
|---|---|
| Close by (deadline) | October 1 |
| First freeze, 50% probability | November 23 |
| Open by (recommended) | May 31 |
| Opening window | May 24 – June 14 |
| 61°F crossing (7-day mean) | June 14 |
| Swim-season length (80°F+ days) | 0 days |
| NOAA normals station | Seattle Sand Pt WSFO · 5.6 mi · 60 ft |
Seattle banks only about 0 days of 80°F-plus afternoons — early opening weeks are the cheapest season extension available.
Put dates aside and follow the water: the Seattle curve says roughly 50°F by mid-April, 60°F by mid-June, 68°F in mid-August, then back down through 56°F in mid-October. The summer ceiling sits near 68°F, and every window above is just a line drawn on this curve.
The 12-step Seattle winterizing checklist
A closing is a plumbing project with a chemistry warm-up. Start a few days ahead of your target date, keep every dose per its product label, and don't skip the photographs — spring-you reassembles from them.
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Balance the water a few days ahead
Three or four days before closing, adjust alkalinity and pH into label ranges. Balanced water is gentler on the liner, plaster, and equipment through the long covered months ahead.
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Deep-clean the pool
Make the last cleaning the best one of the year: full skim, full brush, careful vacuum. Debris left behind steeps all winter and greets you as April's water problem.
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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
Run the winter kit through moving water: dose each product per its label with the pump on, give it a few hours to distribute, then start the shutdown. Chemistry added to still water stays where it lands.
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Lower the water level
Check the cover manufacturer's spec before touching the hose: solid covers typically want water below the skimmer mouth, mesh often barely lower than normal. Full draining is off the table entirely.
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Blow out the lines and plug returns
Work line by line: push air until the return spits dry mist, plug it against the flowing air, move on. Skimmer, returns, cleaner line, in whatever order your plumbing prefers — dry pipes are the entire point of closing.
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Protect the skimmer
Seat a skimmer guard or bottle in the throat — ice that forms there needs a sacrifice, and a two-dollar bottle beats a plumbing repair under the deck.
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Add pool antifreeze where blowouts fall short
Any line you can't prove is dry gets pool-grade antifreeze at the label's rate per foot. Automotive antifreeze is toxic in this context — pool-rated only, always.
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Drain the equipment
Every vessel on the pad — pump, filter, heater, chlorinator — gets its drain plugs pulled. Drop all the plugs in the pump basket; that's the one place everyone looks first in spring.
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Set the air pillow and cover
Pillow first, cover second, tension last. A properly centered pillow turns the winter ice sheet from a wall-pressing ram into a harmless dome.
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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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Winterize the water features
Waterfalls, slides, and spillover spas hold water in places gravity won't clear — blow those lines separately and plug them, or they'll be the one crack you find in spring.
What to buy before the rush
A small stack of supplies turns the checklist above into a single-weekend job. Buy before Seattle's September rush and the whole list ships at leisure instead of sprinting.
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Winter cover
The one purchase every other closing step depends on.
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Cover pump
Keeps rain and melt off a solid cover all season.
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Expansion plugs + skimmer guard
Cheap rubber that stands between ice and your fittings.
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Pool antifreeze
Pool-rated glycol for the lines air couldn't clear.
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Winter closing kit
Season-length winter chemicals in one label-dosed box.
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Air pillow
Centers under the cover so ice pushes inward, not outward.
How Seattle compares locally
Statewide context: across the 21 Washington cities we model, Seattle's October 1 deadline sits in the later half. Nearby, Kirkland (8 mi) closes around October 1 and Bellevue (8 mi) around October 1 — treat gaps under a week as microclimate noise, not strategy. Spring planning restarts at the opening guide; the Seattle pool season page keeps the whole year in one view.
The measuring stick here is Seattle Sand Pt WSFO — 5.6 miles to the northeast, elevation about 60 feet. Its 1991–2020 record is what the model reads for Seattle; your backyard in King 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 Seattle owners
Gas heaters get the manual, not a guess
Every heater brand sequences its winterizing differently — drain plugs, blower considerations, gas supply, control settings. The generic advice (drain fully, close the valve) is right but incomplete; ten minutes with your model's manual protects the most expensive component you'll winterize.
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.
Cold water is the whole point
A pool closed at 55°F barely changes all winter: algae are dormant, chemicals hold, and spring opens with a light dusting instead of a bloom. A pool closed at 72°F runs its own quiet ecosystem under the cover for a month. The date matters less than the water temperature it represents.
Hard-winter homework
Where winter is long — Seattle banks only about 0 warm-swim days — the closing carries months of load. Bury the effort where it counts: verified-dry lines, fully drained equipment, a skimmer guard, and a cover secured for real wind. A short season forgives a late opening; it never forgives a cracked pump.
Seattle pool closing FAQ
What temperature should water be to close a pool?
Below roughly 65°F, and trending down. Water closed warm keeps feeding algae under the cover for weeks; water closed in the 50s goes dormant almost immediately. Seattle's cool-down lands near September 21 in the normals, which is why the window opens there.
Can you close a pool too early?
You can, and warm-water closings are why "we opened to a swamp" stories exist. The fix is patience: let Seattle's water cool through the mid-60s°F — around September 21 by our model — then close inside the window that ends October 1.
Do I need antifreeze in pool lines?
It depends entirely on your confidence in the blowout. Lines that blew fully dry need nothing; anything uncertain — low runs, water features, a stubborn cleaner line — gets pool-grade antifreeze at the label's rate per foot. With Seattle's freeze clock starting near November 23, uncertainty is the thing to eliminate.
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. Seattle 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 Seattle?
Our model's practical deadline is October 1 — set by the cool-down plus ten days (the freeze normal, November 23, 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 21–October 1 window, none of that drama applies.
Email me when Seattle hits the closing window
Data: NOAA 1991–2020 climate normals via Seattle Sand Pt WSFO (5.6 mi); live outlook by Open-Meteo. Full attribution on the sources page. Model assumptions and error bars on methodology.