This paper analyzes recent trends related to delays, airborne holding and diversions in the National Airspace System (NAS) during the summer convective weather season. A weather variable is introduced to help analyze these performance metrics in a way that factors out differences in the location and intensity of thunderstorms. Regression analysis indicates a nearly 50% increase in flights delayed more than an hour from summer 2003 to 2005. The increase in delay is associated with a growing concentration of flights at busy hub airports over the past five years.