Sunday is likely to be the reverse of Saturday. Rain in the north early on in the day drifts south. This could produce some spells of heavier rain for a time before weakening. Into the afternoon, south Wales receives the rain (what's left of it), further north there should be a good clearance of cloud the north coast should see sunshine to end the day. Everywhere will be milder with highs of 9C.
As the name suggests a cold front will introduce colder air, so while it will be sunnier again, the temperatures will take a tumble.