Rough Start after rain

RIOs around your model year have problems with the low voltage plugs that connect to the coil packs (ignition coils. They become brittle and fall apart.
Spark plug wires as well as the coils break down over time and moisture on either can cause misfires from the high voltage jumping from them to ground..
Another "problem" is the IAC (idle air control) valve on the throttle body..It gets gummed up with oil and soot. If it can't constantly readjust the idle throttle the engine either sticks at high RPM or stalls... If you look inside the throat of the throttle body (take off big rubber air hose) and see black soot on insides and on butterfly valve (round metal disk) clean it with choke and carb cleaner and a solft toothbrush and cotton cloth. The IAC is external and makes a buzzing sound with the ignition on and engine NOT started. It may also need cleaning.
Take some time to read the past threads in the RIO section as there is a wealth of information there...
Dave

Thanks for the info. Like I said, this only happened after the rain. The closest thread I found was the weak battery after raining thread. So I thought I could post the question. Anyways thanks for the info.

Another problem is condensation from under the engine cover and getting the ignition wires causing misfiring. Get the engine up to full temp so as to dry the wires fully then turn engine off and spray wires with silicone spray. This will seal them, curing the problem if is fact this is the problem.