The simple answer is yes, most often it is advantageous to use before a rain storm. However, care should be taken to avoid having actual fertilizer granules from entering flowing water runoff. Direct entry of the fertilizer into water runoff is the ONLY WAY that EW4H fertilizer could have any detrimental effect on waterways.
Growing an active landscape, particularly a lawn, is the best way to create a natural filter and improve the vitality of all of nature’s waterways. So please grow a robust lawn and landscape at your house, but don’t throw fertilizer into running water!