For carp in particular I've found that when a cold front approaches there's zero activity, but as soon as the front breaks and it starts raining the fish go bananas. My 3 most memorable carp fishing sessions all happened exactly that way - the first two being luck of being at the right place at the right time and the third time the penny had dropped and I went fishing in the rain.
In the salt it seems to be a little less predictable, I guess because there are more variables involved. I caught my PB grunter of 6kg with a thunder storm fast approaching - I was wading a mud flat and at one point during the fight panicked a little because of the lightning and actually considered breaking off to run for cover.
On the other hand I've blanked in seemingly perfect conditions as well as atrocious conditions and at other times got some of my biggest fish on clear, calm days when I thought it was hopeless.
Ultimately I don't get to fish often enough to be picky about conditions anyway, if it's not life threatening I'll be out there. As my dad always says, there are only two good fishing times: when it rains and when it doesn't.