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.
Give your pool a good shock treatment 1 to 2 days before the storm hits. You can bring the chlorine level up pretty high to prolong the pool being depleted of chlorine.
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.
When preparing your pool for a storm, leave it uncovered. Installing any kind of cover across the pool will not do much to protect against dust and contaminants because storms often bring strong winds and heavy rain that can cause the cover to lift off your pool.
At the first sign of an incoming storm, you should turn off and unplug your pump. If this is left running during a storm, the motor may suffer electrical damage and short out.
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.
Pool Losing Water After Heavy Rain
Due to a heavy rainstorm, water loss is more common with vinyl liner pools that can be damaged, come loose, or float to the top. With other types of inground pools, it is vital to get the water level down. If not drained, the water balance will change, causing cloudy water and more.
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.
Answer: It should not matter much. It could help to filter debris and contaminants the rain washes into the pool. However, lightning can be a concern with the filter running.
Protect the pump motor from water damage
In addition to removing circuit breakers, consider moving the motor indoors or covering it with a waterproof material to prevent water from getting inside.
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.