Large drops in FC with a CC of zero. Is this possible?

[FailedChemistry]

Hey all. I've been a bit perplexed by my pool this season.
Since opening, I've been having to add double the Chlorine daily that I've had to in previous years.
I try to keep an FC of 5. In previous years I'd see an average of 4 most mornings.
This season I keep waking up to 2s.

I can't figure out why the FC is dropping so hard. The water is clear, the CC is consistently 0 or .5
But it's like the FC is just going poof!
My feeble-minded understanding was that CC would read high if the FC was killing algae. I thought if CC tested 0 that meant the pool was ok.

Now — reading through the forum — I'm thinking i need to SLAM.
So i'll probably do that and the OCLT tests. But in the meantime, what are y'alls thoughts?
Are large drops in FC levels with no rise in CC abnormal?

This morning's test:
FC 2
CC 0
pH 7.8
TA 90
CH 500
CYA 35

 

[CraigChing]

You could try an OCLT to rule out organics, but it sounds to me like it’s probably normal UV degradation. I’m using more than normal right now as well, 3 quarts per day which is high for me, but it’s also been unprecedentedly hot here the past week.
Overnight Chlorine Loss Test

 

[mknauss]

Are large drops in FC levels with no rise in CC abnormal?

No. A pool full of algae can have 0 CC. CC is not a definitive test for organics in your water. It is an indicator, but only when supported by other methods.