Your 99 Chevy 4.3 wont start when damp?

sounds like you have moisture in the distributer. easiest way to remove is to take distributor cap off and dry with a hair dryer,u also need to dry inside of distributer while cap is off. if you have the time to leave cap off in a warm place for a day or so, most moisutre will evaporate out

Ignition coil shorting to surrounding metal or dead. Open top shroud and look and listen for spark to be taking a short cut to ground instead of going to distributor cap. If you see a spark or hear it crackle, that's it.

Another good way to remove and/or prevent moisture problems in your distributor is to spray the inside of the distributor cap with silicone spray. WD40 spray works too.