We just got back from a 3000 mile R&D/vacation trip. On our way down through Illinois last week we ran into some really intense rain. I guess reports where 3-4inches per hour.
Normally rain is not a problem as I am geared up and can run in rain without barley changing speed however in this stuff what I noticed was something new. Hydroplaning.

Most of the cars were pulling off the road and all of a sudden the front started wondering slightly and it was time to slow things down. Under 45mph things were still fine but we found the first exit and sat this one out.
I guess things could have been worse if the wind was also pushing but none the less it was a good lesson learned and adjustments have been made in the form of narrower tires up front.

My buddy fared worse as he doesn't have a full face helmet and ended up riding totally blind until we hit the exit. Proper gear for the weather is still the key.