PM Thunderstorms, crowd level and FL+

I'm keeping an eye on the forecast for this weekend and as of now they're calling for scattered PM showers on Saturday, and scattered showers on Sunday.

I had planned on getting FL+ for myself and my friend on Saturday, but obviously don't want to drop $298 if the weather makes for lower crowd levels.

I know this is all hypothetical--but does anybody have any tips for crappy weather and if there would then be a need to drop extra $ on FL? Even with being a Saturday in July?

If we do not get the FL+ to begin with but the crowd level seems to rise, about how long into the day does FL+ availability last? I think I once read that they rarely 'sell out', but I could be wrong.

Saturday's in July are going to be crowded 99% of the time. Storm or not. Plus, it's way too early to accurately predict weather for Saturday/Sunday. Also, fastlane should be available until probably around 1.