Transmission slips after heavy rain

Friend has '04 LS and he says that after a heavy rain, his LS will only go about 25 mph. Water is getting into something but where? This lasts for about 2 weeks and then all is OK. Water is finally evaporating, I assume, and problem goes away.

What makes you/them think that the transmission is slipping? I'd be more inclined to think the issue is due to water getting in to spark plug wells and causing the misfire. This seems to be a documented issue with the V8 and there are other posts on this forum that describe it.

-Rod

Engine isn't misfiring.

So then is the engine speed running really high and smooth, yet the car will not make it over 25mph, and that's why the transmission is suspect?

Yes