Unless it's windy, I doubt the rain will mess up operations. If things do get delayed/canceled Delta will let you change to connect through Detroit. Plus worse case scenario you could always rent a car and drive to ATL and pick up your ATL-SEA flight.
Side note: These kinds of threads make me happy! It's a welcome change from all the COVID talk. It reminds me that people are still traveling and gives me a slight sense of normalcy. Thank you!