Before covering my pool with a solid cover on 10/10/09, my tests were as follows:

FC: 13
PH: 7.8
TA: 120
CH: 220
CYA: 90

I pulled back the cover to test the water today:

FC: 3
PH: 7.8
TA: 80
CH: 160
CYA: 90

I was surprised that I still had FC. Is the drop in CH normal?

The CYA will go down a bit when I add more water when I open the pool.

Since the weather is getting warmer, I may have to add CL prior to opening the pool. Can I do this with the cover still on or do I have to open the pool at that point?

The drop in TA and CH might indicate that some calcium scaling took place over the winter, or it might just be testing error. Test results are not reliable unless the pump has been running to mix all of the water together, especially after an entire winter of sitting around.

If you want to add chlorine you will need to create some circulation to mix it into the rest of the pool water. By far the simplest way to do that is to wait till you have opened the pool, but it could be done with a cover pump or lots of brushing.

I did the same thing and got a CH of 80. That seemed impossible even with the 2+ feet of water that I'd had to pump out because of the rain in Seattle, so I ran the pump for a couple three hours, retested and got 160.