while you can not stop rain, there are watering problems you can try to work with or make note for next year- i.e. raised gardens, heavy mulching with leaves etc
some varieties are prone more to cracking than others

Are you growing heirlooms? If so, it's likely "cats claw" from temps fluctuating too "extreme" for the tomato plants liking. It can happen with organic too etc.

I have 2 tomatoes on different plants that have cracks.. 1 large one and a baby one. "cats claw" isn't a disease and is totally safe to eat.

To prevent? Watch the weather and cover the plants when too cool out at night.

Some varieties are worse than others but I even have episodes of cracking in my high tunnels where plants are esentially covered and the critical factors are more controlable. I find that running circulation fans for an hour each morning to dry plants (and fruits) goes a long way to minimize cracking. However if the weather turns from dry to foggy dampness every morning your only recourse is those crack resistant varieties. I've never seen a Rebelski tomato crack.

My two plants/tomatoes that cracked were under the balcony and didn't get rained on.