The rain doesn't cause the algae bloom--the reason that a lot of people have algae after rains is because they're not as diligent about testing and keeping their chlorine levels where they need to be when it's raining as well as they do when it's sunny outside

Until you have stabilizer in the water to the point where you're able to maintain a chlorine level all day, then I would add that half dose of dichlor before a group--even a family of 5--swims...