Will crank but will not start when it's raining

How long since a tune-up? Are there any applicable trouble codes? Have you changed the plug wires, lately? In the dark, raise the hood, start the vehicle, does it look like the great northern lites under there, fire running everywhere? When the problem is ongoing, if it cranks good but won't start, have a helper crank it, while you visually check for spark at the spark plugs.

At night, start the engine, open the hood, and then mist water on the plug wires, distributor cap. See if anything is sparking where it shouldn't be.