Highs will warm to the mid to upper 70s by Wednesday under mostly sunny skies. It was a nice afternoon with highs in the low 70s, which is right around the seasonal average for Seattle. It was a nice afternoon with highs in the low 70s, which is right around seasonal average for Seattle. High pressure will build Tuesday into Wednesday, warming highs into the mid to upper 70s with mostly sunny skies. High pressure will build Tuesday into Wednesday, warming highs into the mid to upper 70s with mostly sunny skies.