A bit more detail on my reply. First check pads are not worn down too far.
The biggest issue with disc brakes is normally contamination. This is when oil or similar gets into the pads, calliper and rotor. This means the brakes are being lubricated and the ability to brake is greatly compromised causing the screeching noise. You will get this with perfectly clean brakes in the wet but after a couple of pulls on the brakes it will go, except in heavier rain.
The solution to this if contaminated is to remove the old pads, these generally absorb oil like a sponge and cannot be cleaned. The with disk brake cleaner, thoroughly clean out the calliper and the rotors all over. If you use white kitchen roll once you stop getting black marks on the kitchen roll you know it is clean. Then fit new brake pads and get up to speed and brake hard a few times. This will fix it unless you have fluid leaking from the brakes.
Sources of the contamination are an over oiled dirty drive chain ( chain, cassette etc.), debris off the road, leaning the bike against something oily, touching the disks with oily hands, not being careful when oiling the chain etc / putting too much oil on the chain. Generally disc brakes do not get contaminated unless you are doing something to cause it or there is a build up over a long time, but you can be unlucky.