Don't ride in lightning.
Riding in rain can be fun, especially if it's warm. The rain keep you cool, and there's a certain satisfaction in doing it. If you don't have fenders, it can be a bit of a drag, perhaps, but fenders aren't very expensive. Just be sure to have some good taillights and wear bright colored clothing so motorists see you. remember that when it's raining really hard, motorists have much reduced visibility, so choose your route carefully.
And don't ride in lightning.