Because you'll have CYA in the water, the active chlorine level is actually quite low. Even if your FC were around 40% of your CYA level, the active chlorine (hypochlorous acid) level is the same as found in a pool with 0.6 ppm FC and no CYA. So this is not extraordinarily high. Also, at the much colder temperatures all chemical reactions slow way down.

Nevertheless, to minimize any potential issue, close as late as possible and open as early as possible which is what it sounds like you will be doing. 15 ppm FC with, say, 50 ppm CYA will be like 0.4 ppm FC with no CYA. If you had your CYA higher at 80 ppm that is more typical for SWG pools, then the 15 ppm FC with be like 0.2 ppm FC with no CYA so quite reasonable.