Efficiency of severe thunderstorm environments in the U.S

Significant progress has been made to identify environmental conditions that are favorable for severe weather in the U.S. The classic storm setup that would be evident on both the synoptic and mesoscales is well-known: intense, well-defined jet streams and strongly baroclinic weather systems. These types of environments, which are often characterized by a broad mid- and upper-tropospheric trough, abundant surface moisture, instability (high convective available potential energy (CAPE) values combined with low convective inhibition (CIN) values), large values of storm-relative helicity (SRH) and well-defined surface boundaries, are now able to be quantified as parameters, including the Significant Tornado Parameter (STP) and Supercell Composite Parameter (SCP), both of which have shown to be useful in discriminating severe weather environments from those that are not severe.

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.