I have a Ford F150 Triton Super cab that once and a while it will not start after a heavy rain. When I turn the key it sounds like I am not getting a spark or gas. Besides that everything comes on normal.
What year?
Does it have a distributor and plug wires?
If so crank it up one dark night, use a spray bottle to mist the ignition parts and look for a light show.
If it’s coil-on-plug I’d suspect the crank or cam position sensors. Check their connectors.
Oops, sorry. It is a 2000 5.4L heme. I will start with those sensors.
The problem might be with the boots for the coil-on-plugs. http://www.rockauto.com/catalog/moreinfo.php?pk=1217573&cc=1362495
Just like old ignition systems where spark plug wires are used, if moisture gets inside the spark plug boots it causes the spark to short to ground instead of to the spark plugs when it’s damp out.
Tester
Use some dialectric grease on the boots and see if that corrects the problem.