I can't figure this out, usually I keep the 91 in the garage but this past rain storm she had to sit outside. There have been a couple of other times that it sat out when it rained and each time the next day after the rain stops the car won't start. This time I don't know what to do. It cranks but won't turn over what could it be?

Wow.. odd... My best guess would be something with the dist&cap. Though I would expect a 92-96 to have more problems with that than yours. Hopefully someone else with similar experiences will ring in.

Glock94 is right you shoud check the dis for any cracks in it and look at the rotor and make sure its not worn out also check the in side center of the cap and look at the center carbon pile that should be a dull black not shiny.

Could be a crack in the distributor cap, or -
(How hard a rain are we talking about?) a clogged gas drain hole.
When I first got my "88 coupe, I bought one of those nifty "gas-bibs" that goes around the fill throat from one of the catalogs. DO NOT USE THIS ITEM!!! Each time it rained heavy the car ran like hell. It took Hurricane Floyd to help me figure out why. During that storm, the entire gas fill recess was filled to the brim like a mini in-ground swimming pool, completely over the top of the fill cap. The damn gas bib was covering the drain hole and allowing water into the gas tank. In previous storms it had probally been some water. This time it was alot of water, and was a nightmare to fix. I immediately threw out the gas bib thingy and added two extra drain holes in either corner.
Even if you're not using one of theses bibs, check to see that your drain hole isn't clogged with something. A friend of mine had a cherry seed clogging his and experienced a similar problem.

I have on several occasasions that the same problem with my 91.It will not start after sitting out in a heavy rain.
One thing that I did notice was water that had collected in the hood latch(driver side)was dripping on the 12v junction box.I also happen to have my MSD unit 12v wire hooked in there.
I have since installed my own drain tube in the bottom of the bracket.MidAmerica also sells a kit.
Phil