Throughout the 21st century extreme temperatures are projected to increase faster than the mean over parts of the U.S. We use the North American Regional Climate Change Assessment Program (NARCCAP) suite of high resolution regional climate models to investigate temperature changes and their causes over North America. Our results show that the eastern half of the continental U.S. is likely to see extreme temperatures diverge from the mean more than in the west, and we also find that large-scale synoptic setups - as characterized by their geopotential height anomalies - cannot explain the amplified warming. This result suggests that other dynamics, especially soil moisture feedbacks, may be driving the faster warming of extreme temperatures as compared to the mean.