Temps of 150* is really cooking powder. I can't imagine storing ammo where it might get that warm for any length of time...but that's probably because I live in Maine!! Moisture is more our problem, especially for those who live on the coast, or those who try to keep their powder in a moist basement
Section 7.3 of the UN Surveillance Manual provides temperatures for ammunition left out in the sun, it can get pretty hot.
Surplus ammunition can be stored in bunkers which help keep it cool, but a lot of ammunition is stored in ammo cans under tarps or ISO shelters. If you ever get into an ISO shelter that is in direct sunlight, it is plenty hot.
As for 150 F, I have found references to weapon systems that because of location, the ammunition gets to that temperature and higher. It is a concern because primers will dud (there are high temp primer compositions) and the ammunition is unsafe after a short exposure.
The bottom line is that heat is bad, the more heat, the more bad.
Moisture is bad because water is a polar molecule and that breaks down gunpowder. I have found references where too much water damages the stabilizer. Gunpowder is a very complicated mix.