A moderate shower should not cause you any worries or be alarmed as to how you would get about caring for your swimming pool to ensure it remains as perfect as it should be. On the contrary a thunderstorm could upset the equilibrium in your pool water because rain water is generally acidic and could put the chemistry in your pool water in a right royal spin. The Total Alkalinity, PH and chlorine levels could turn topsy-turvy after a heavy thunderstorm with ample water from the sky falling into your swimming pool.
The extra water falling as rain would dilute your pools water and hence a concentrated effort would need to be put in place to ensure the water in your pool is brought back to acceptable limits. A heavy thunderstorm would not only bring water but would blow in all types of dirt and debris into your pool. If you have been prudent enough and envisaged the impending thunderstorm and have a cover for your pool and had put it in place, your task would become that much easier. If not with all the debris floating on the water and some at the bottom of the pool your task of cleaning up would be quite daunting.
The primary task after you have removed all the debris and dead leaves in the pool water check out the water level in your pool. If it has risen due to the rains then it would need to be brought back to the original level. To do so you could run the pool pump and execute a backwash which would also clean the pool filter. It is imperative that the water level is brought back to acceptable levels before you would go to the next step. Check the water for the Total Alkalinity levels, PH value and the chlorine mix.
These three would also need to be at acceptable limits and if they are not ensure you add the right amounts of required chemicals and bring it back to where it was before the thunderstorm. Run the pool pump and filter for about 8 to 10 hours to ensure there is a good mix up of the added chemicals.
Try to clean the pool and vacuum it when the pool pump and filter are operating which would ensure all loose and released grime, dirt and debris are filtered out. After the cleaning and vacuuming it would be prudent to recheck the chemical composition again which is not a very arduous task but one which would be helpful to ensure your pool is back to normal and good for use. You need your swimming pool to be clean and healthy at all times and hence it would be necessary that you bring a sense of discipline into your care of the pool.
The cleaning up and chemical adjustments need to be carried out before anyone would use the pool after a heavy thunderstorm. It may not be advisable to use it without going these chores because you would not know what the thunderstorm has done for your swimming pool.