Theoretically the US System takes weather into account in setting a course rating but the system does not work all that well on exposed courses that experience highly variable weather. There are days on my course where I know it is extremely unlikely I will shoot my handicap and others where shooting my handicap is a mediocre score. Supposedly for that reason, some clubs have declined to have their courses rated and one may not post scores for handicap purposes at such courses.
However, beyond weather, there are all sorts of variables that impact how difficult a course plays. Scores in tournament play, particularly stroke play tournaments, tend to be higher because the greens are firmer, the pin positions are trickier and everyone is putting out on every hole. In my area, the grass is much thicker in June due to something about that stage in its growing cycle. In the fall, the un-mowed areas on my course are basically out of bounds because bluestem plants grow to shoulder height by that time of the year. In July - the course is often soft due to soaking rains that make the course play much longer but make it much easier to hold a green on approach shots.
I have always favored eliminating the current method for rating courses which is based on length adjusted for a number of difficulty factors as applied to a hypothetical scratch player who hits the ball 250 yards off the tee (I cannot think of many scratch players that come close to meeting that description) in favor of a rating based on scoring statistics. Even that approach, however, will not account for day to day variation and I am unsure how you could make such an adjustment on a statistically valid basis or whether such an effort would be worthwhile.
Overall I think it all comes out in the wash.