Disc brakes squealing after rain do not automatically mean a fault: water or dirt on the rotor can cause temporary noise. If the sound persists, possible causes include contaminated pads or an uneven layer of pad material left on the rotor by incorrect bedding-in.
Start with cleanliness. A rotor can be cleaned with isopropyl alcohol and a clean cloth. Keep bare fingers off its braking surface and avoid picking a cleaner at random: additives can affect braking. Do not cool a hot rotor with water, as it can warp.
Check pad and rotor wear too, following the instructions for the particular system and the minimum thickness marked on the rotor. New pads and rotors need bedding-in. If noise persists or braking power has dropped, have a workshop inspect the brakes; do not continue riding with poorly functioning brakes.