Despite the utility of parameters in the forecast setting, what is currently unknown is the variability of atmospheric efficiency. Here, we define efficiency to mean the frequency of the occurrence of severe weather given the presence of certain conditions that favor it. Thus, in this study, we seek to answer the following two questions: (1) What is the frequency of days with actual severe weather compared to days with parameters favorable for severe weather?; and (2) How does this efficiency vary, including spatially, seasonally, and by phase of other known modes of climate variability? To answer these questions, we calculated STP, SCP, and other parameters using 20 years of rawinsonde data from approximately 50 upper-air stations across the U.S. We then developed thresholds for combinations of parameters and calculated the frequency of days with at least one severe report in the NOAA Storm Events data base within 150 km and 3 hr of the upper-air observation. Preliminary results indicate interesting geographic variability of efficiency, and these and other results will be presented at the meeting.