The situation is it is raining: All three forwards are in an offside position as it is played forward by a teammate of the forwards, but right at a defender. The defender kicks the ball one-touch but it squibs off their foot and goes 20 yards down the field out of bounds for the team that was offside to take a throw in.

I signaled for offside. Is that gaining an advantage or a misplay by the defense and rain is rain?

USSF answer (September 20, 2011):
The correct decision would be for the referee to call the offside. See the USSF publication “Advice to Referees on the Laws of the Game”:

The possibility of penalizing a player for being in an offside position must be reevaluated whenever:
1. The ball is again touched or played by a teammate,
2. The ball is played (possessed and controlled, not simply deflected, miskicked or misdirected) by an opponent, including the opposing goalkeeper, or
3. The ball goes out of play.

The result of such a reevaluation, of course, may be that the player remains in an offside position based on still being beyond the second-to-last defender, the ball, and the midfield line. Referees must remember that a player cannot simply run to an onside position and become involved in play. The player’s position with relation to the ball and the opponents must change in accordance with the Law.

In the case of the ball leaving the field in favor of the team whose player was in an offside position and actively involved in play (e.g., a corner kick or throw-in for the attackers), it is traditional to call the original offside offense. If the restart would be in favor of the opposing team (e.g., a goal kick or throw-in for the defenders), it is usually preferable to ignore the offside infringement, as the defending team’s restart gives them the possession under circumstances not much different than the indirect free kick for offside-and often with less controversy.