I have the same question

Heavy rain is an expected weather pattern following a nuclear explosion, as the heat and debris from the explosion in the air will act as nuclei for condensation. This happened in Japan following the nuclear detonations, and considering the amount of missiles fired in the Final Solution event it's not surprising that there's a period of seemingly endless rain.

Following the rain should be a period of intense cold caused by the massive change to the atmosphere above Knox Country by the explosions releasing so much heat, which I believe was added by slayer in the most recent update.

Correct.