It has nothing to do with weather, like txhatch says, brakes have operating temps, if a pad is not in its operating temp, and it generally will not perform as well until you get some heat in the pads. Stock Pads usually have a pretty low operating temp so they stop well when you first hit the brakes. The AEM's that I had seemed to always perform better after I had used the brakes 2 or 3 times when autocrossing, they still stopped when they were "cold" but the defiantly stopped better when "warm" mean they had better bite with some heat. Axxis Ultimate’s stop well through out their operating temperatures with good bit, however I was just doing some searching and Carbotech is Offering the Bobcat pad which is better than the ultimate’s for like $69, which is a great buy, especially if you are going to be auto-crossing.

However for just street driving I would get either OEM or even some of the autozone offerings over the AEM because of the "cold" performance.