The quick answer
Rain is only part of the issue. Storms add water, pollen, soil, leaves and runoff while sometimes interrupting circulation.
Dilution can shift readings
Heavy rain may lower salt, stabilizer or other dissolved levels, especially when the pool overflows or water is removed.
Debris increases demand
Leaves, soil and organic material consume sanitizer and load the filter. Remove debris before relying on chemicals.
Check water level and skimming
Excess water can reduce surface skimming, while wind creates blockages. Restore the normal operating range appropriately.
Test after circulation
Allow water to mix, then check sanitizer, pH and relevant values with a reliable method before correcting.
Inspect equipment
Look for tripped breakers, lost prime, blocked baskets or leaks. Do not operate damaged electrical equipment in standing water.