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Abstract
High-shear low-CAPE (HSLC) severe weather events, defined as those in which surface-based CAPE is below 500 J/kg and most unstable CAPE is less than 1000 J/kg and have a 0-6 km bulk wind difference of at least 18 m/s, are uniquely challenging to forecasters due to their rapid evolution times and relatively infrequent production of severe weather. Additionally, these environments tend to occur during the cool season and overnight hours, which present their own challenges in terms of current conceptual understanding, forecaster verification, and societal impacts. To better understand how rapid environmental evolution contributes to severe weather production in HSLC environments, 224 tornado reports from HSLC events in the southeastern U.S. between 2014-2018 were categorized based on their radar signature near the time of tornadogenesis. More than half of the reports were from supercells, with the next largest contributor being bowing segments. Next, Rapid Refresh (RAP) model analysis soundings are used to characterize the near-storm environment and its spatiotemporal evolution, focusing on the time of tornadogenesis, as well as the hour before. For comparison, two other storm-relative locations (offset to the north and south, forming a grid) also had their environmental changes quantified. Perhaps unsurprisingly, as is also the case for radar signals of HSLC tornadogenesis, many of the observed environmental differences were very subtle. The most useful parameters for mesoanalysis as events unfold appear to be related to environmental moisture, as instability uniquely decreases preceding tornadogenesis for many cases within the dataset, and kinematic variables show very marginal changes. The key for increased potential for tornadogenesis appears to be most directly related to the spatial variations in surface and mid-level moisture, both of which are most favorably enhanced at the tornadic inflow environment.