A pool hit by a heavy North Texas rain needs a chlorine dose the same evening, after the baskets are emptied and free chlorine is tested. The next day, lower the water to mid-skimmer and correct alkalinity before pH.
The timing matters because almost all of the damage happens in the forty-eight hours after the storm rather than during it. We get four inches in an afternoon, the pool comes over the coping, the water goes flat and gray, and by Thursday it is green.
Those storms come in two seasons. The National Weather Service’s Dallas/Fort Worth climate narrative describes our rainfall as “typically favoring a bimodal distribution of wet spring/fall and dry summer/winter,” with thunderstorms and severe weather peaking in the spring. Those two wet windows are where most of the green pools we get called about begin, and the North Texas pool year maps out the rest of the calendar around them.
What a heavy rain does to your water
A heavy rain does four separate things to your water at once, which is why the aftermath is worse than any one of them would suggest.
| Effect | Mechanism | What you see |
|---|---|---|
| Dilution | Rain contains no chlorine, no cyanuric acid, and almost no minerals | Every reading drops, chlorine and alkalinity most visibly |
| Organic load | Wind and runoff bring in leaves, pollen, soil, grass clippings, and fertilizer | Cloudy or brown water, debris on the floor, chlorine consumed fast |
| Phosphate and nitrate delivery | Lawn fertilizer and soil wash off the yard into the pool | Nothing immediately, then algae with a food supply |
| Alkalinity crash | Rain is slightly acidic and carries no buffering capacity | pH that will not hold anywhere after the storm |
| Overflow | The pool passes the skimmer throat and keeps rising | Skimming stops entirely, surface film stays put, deck and pad get wet |
The interaction is the problem. Dilution cuts your chlorine roughly in proportion to the water added, which after four inches on a normal pool is not catastrophic on its own. But the same storm delivers an organic load that consumes what remains, and it strips the cyanuric acid (CYA), sold as stabilizer, that was protecting your chlorine from the next morning’s sun. That is how a pool that read 3 ppm on Saturday reads zero on Sunday afternoon.
Runoff into the pool is worse than rain into the pool
Rain landing in the water is mostly a dilution event. Water sheeting off a sloped yard, a flower bed, or a fertilized lawn into the pool is a nutrient delivery event, and it is the one that produces algae a week later. If your yard drains toward the pool, that grading problem is worth solving once rather than treating every spring. Mulch and soil in the skimmer after every storm is the tell.
The same evening, if you can get outside
Fifteen minutes of work the same evening, while the deck is still wet, saves a recovery. In priority order:
- Empty the baskets. Skimmer and pump basket both. A packed basket starves the pump, and a starved pump is not circulating the chlorine you are about to add.
- Test free chlorine. This is the number that decides whether the next few days are routine or expensive. If it is at or near zero, dose tonight.
- Chlorinate, using liquid. Liquid chlorine adds no cyanuric acid and works immediately, which is exactly what a diluted pool needs. Tablets are the wrong tool in this moment, for the reasons in liquid chlorine versus tabs.
- Run the pump. Overnight, not on the usual schedule. Circulation is what turns a dose into a treated pool.
- Get the big debris out. Net what floats. Leaves left on the bottom overnight start staining and start consuming chlorine.
- Look at the pad. Standing water around the equipment, a tripped breaker, or a pump that did not restart is a separate and more urgent problem.
What does not need doing tonight: pH, alkalinity, calcium, and anything involving a filter. Those matter, and they matter tomorrow.
Getting the water level back down
A pool sitting above the skimmer throat is not skimming. The surface film, pollen, and floating debris that the skimmer would normally pull off sits there, and that film is where a good deal of the organic load lives. Getting the level back to the middle of the skimmer opening is a real chemistry action, not housekeeping.
| Method | Good for | Watch out for |
|---|---|---|
| Submersible pump on a hose | Any pool, any amount, most control | Where the water goes. Not into the yard next to the pool. |
| Filter valve to waste, sand or DE (diatomaceous earth) | A quick few inches if you have a multiport | Never with a cartridge filter, which has no waste setting |
| Letting it evaporate | An inch or two in a hot dry week | Useless in a wet week, which is when this happens |
| Doing nothing | A pool that only came up an inch | Above the tile is where grout and coping start to suffer |
Send the discharge away from the house and away from the pool. The clay under a Collin County backyard swells when saturated, and the clay soil guide explains why not to add water to it. If your city is running any kind of discharge restriction, treated pool water going to a storm drain is generally not allowed, which is covered in more detail in draining a pool here.
The next day, in order
Alkalinity before pH
Rain is soft, unbuffered water, and a big storm can pull total alkalinity down noticeably. When alkalinity is low, pH will not stay where you put it, so correcting pH first is work you will repeat. Bring alkalinity back into range, let it circulate, then read pH and adjust. The water chemistry guide argues for the same order in general.
Then cyanuric acid
Stabilizer washes out with everything else, and a pool that had 40 ppm before a storm that added a fifth of its volume has something closer to 33. Unprotected chlorine in August sun disappears in hours. If the reading has fallen well below 30, that is worth correcting deliberately rather than waiting for tablets to do it slowly. The reverse problem, stabilizer that has climbed too high over a season, is the subject of the cyanuric acid guide.
Then brush, and only then filter
Brush the walls and floor before you vacuum. Storm debris packs into corners and behind steps, and a vacuum run over settled silt clouds the whole pool. Once it is brushed and circulated, vacuum to waste if the load is heavy and you have the valve for it, otherwise vacuum normally and expect to clean the filter afterward.
Wait to clean the filter
The instinct after a storm is to backwash or pull the cartridge immediately. Resist it. Your filter is about to do the heaviest work it does all year, and a filter performs better slightly dirty than perfectly clean because the accumulated layer catches finer material. Clean it when pressure climbs 8 to 10 psi above baseline, or when the water is clear and the job is done, whichever comes first. The full logic is in pool filter cleaning.
Why the pool goes green on day five
A pool goes green around day five after a storm because algae needs a few days of warm water and sunlight to multiply from invisible to visible. The classic pattern is a storm on Saturday, a pool that looks fine on Monday, and a green tinge on Wednesday or Thursday. That delay confuses people into thinking the storm was not the cause. It was.
Free chlorine hits zero on Sunday, algae starts on Monday, and by Wednesday there are enough cells to tint the water. By the time you can see it, the population is already large, which is why green pool recovery is a job rather than a dose.
The prevention is unglamorous: hold free chlorine above the floor for your stabilizer level every day of that week. A pool that stays above that floor rarely goes green, whatever the storm delivered.
Hail, wind, and the equipment pad
Hail and damaging winds occur nearly every year in the Metroplex, according to the same NWS climate narrative, and they damage equipment rather than water. Before you worry about chemistry, walk the pad.
- Did the pump restart? A power blip can leave a variable speed pump in a fault state or reset its schedule to something wrong. Confirm it is running its program, not just humming.
- Is there standing water at the pad? Motors and control boards do not tolerate submersion. If the pad flooded, shut the breaker before touching anything and get it looked at dry.
- Check the automation and timers. Clocks reset. A pool running two hours a day instead of eight after an outage is a slow-motion green pool. See how long to run your pump.
- Hail on a heater or a filter lid. Cracks in plastic housings and dented heater cabinets are worth photographing while the storm date is fresh, because they are a homeowner’s insurance conversation.
- Fence and gate. Wind moves gates and breaks latches, and a self-closing gate that no longer closes is the most serious thing in this list. Barrier requirements exist for a reason and a storm is a common way they quietly stop being met.
Austin’s tip
Keep a jug or two of liquid chlorine in the garage through spring and fall. The storms arrive in the evening, the pool store is closed, and the single highest value action available to you is a dose that night rather than one on Monday. Store it out of the sun and off the concrete, and follow the handling rules in chemical storage.
What a service visit does after a storm
A service visit after a heavy rain should be longer than a normal stop: a chlorine correction sized to the dilution rather than a standard dose, an alkalinity check, a full brush, and basket work at both ends. On a route, the storm also reorders the week. Pools that took the most water, pools with a yard that drains toward them, and pools we already know run thin on stabilizer move to the front, because a stop on Monday is worth two on Thursday. Pools under cover, in shade, or on a spa-sized volume that a storm barely moved slide back. That triage is the part of weekly service that doesn’t appear on a price sheet.
It is also the argument for weekly rather than every other week through the wet months. A biweekly pool that gets hit by a storm on day two of its cycle can spend twelve days at zero chlorine with fertilizer runoff in it, and the arithmetic on that is in weekly versus biweekly service.
None of this is complicated, and all of it is available to you on the evening the rain stops, which is earlier than any company can get there. Two days at zero free chlorine is what turns a pool green, and a storm is the most reliable way to get there. The dose you add that evening keeps a four-inch afternoon an inconvenience rather than the start of a recovery.
