The battery voltage being low can cause this, as well. It causes all sorts of wierd things to be possible (wipers coe on all by theirselves, jumpy speedo/tach... flashing airbag lights, etc)
Purely as a guess, I would imagine that your battery is low, and posible that the rain is a coincedence.
The possibility that water is getting on to the PCM or associated wiring is THERE, but not likely. (its on the passenger side of the engine bay, near the firewall.. check for water up top of the engine...)
The other possibility is the failing sensor, as previously stated.
Ray
