Remember, rain is acidic. Hence, pool overflowing from rain causes the Alkalinity and pH levels to lower. Very low pH causes pool water to turn very clear and acidic, which destroys your pool surface and equipment.
To prevent potentially dangerous electrical issues, it's imperative that you turn off the power to your pool equipment — such as pumps, motors, filters, heaters, chlorinators, and lighting fixtures. Even if you turn off the power to your pool equipment, it can still be damaged by wind, rain, and debris.
The pool will only overflow by the amount of rain in excess of the amount of rain needed to fill the pool to the top. So if your pool is the normal 3” below the top, the rain would need to exceed 3” before any additional rain would be available to overflow onto the deck.
So shock your pool and keep the pump running.
And if you're wondering if you can shock a pool IN the rain, the answer is yes. Just remember that rainwater adds more contaminants, so it won't be as effective as shocking during dry weather.
Should I Shock My Pool? Shocking your pool isn't necessary, although, it's not a bad idea either. If you get an extremely heavy rain fall, you could shock your pool for good measure. This will help fight off any contaminants that the rain may have brought to your pool.
Swimming pool owners can add disinfecting and stabilizing chemicals as they please when it rains. However, continuous rain will adversely affect the water's chemical balances. This is because rain picks up atmospheric contaminants, such as organic materials and even algae spores, when it falls.
But large quantities of precipitation combined with an overflowing pool and poor drainage can cause problems such as flooding, structural damage to the surrounding buildings as well as out of balance swimming pool water chemistry. No fun.
If the pool overflows, now only will the pool chemicals be diluted, but they may contaminate the pool deck and surrounding landscape. Removing excess water quickly is important to prevent this.
You don't need to drain your pool, as there is no risk to your pool by it being full. The only thing you lose with a pool filled to the rim is your skimmer's surface cleaning action. Overall, it still draws water and the equipment is just fine.
Go to the back of the filter and unscrew the drain plug. Turn the pump back on and let the water run until it reaches the point where you want to drain the water out. Turn the pump back off and put the plug back into the drain plug.