A surprising outcome

[chem geek]

If you had enough rain to overflow and therefore dilute the pool water then it could lower the TA and pH (assuming acidic rain, which is typical, though it would take a lot to move the pH). From historical weather data, it looks like you had about 0.16" of rain every day so over 3 weeks that would be 3.4" so with a 4.5' avg. pool depth that would only be a dilution of 6.3% so I wouldn't expect much drop in TA or pH. It's possible the water didn't mix well (since your pump wasn't working) so unless you were taking a sample from depths, you might be measuring closer to rainwater measurements.

[AnnaK]

I take the test sample from approximately 12" down.

I had expected the decrease in FC, of course. That CYA, TA, and pH also dropped was a bit of a surprise, but so was that I had any free chlorine left at all. I had fully anticipated coming back to a slightly green pool.

The new pump is up and running and doing a fine job.