What do I do? I try to harvest as much Calcium-free water as I can!

Unless you get massive amounts of debris or surface runoff in the pool, rain really won't mess things up too bad. A big pool party probably does worse. The pH can get weird, since a lot of times the rain picks up CO2 on the way down, which drives pH down, but a hard pounding also aerates, so pH will go up. If you overflow, you'll lose some CYA. But really, if you have a test kit, once you can get out there afterwards, you can get things fixed in an hour.
 
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