High-shear, low-CAPE (HSLC) environments, here characterized by surface-based CAPE ≤ 500 J kg−1, most unstable parcel CAPE ≤ 1000 J kg−1, and 0–6-km shear vector magnitude ≥ 18 m s−1, occur at all times of day, across all seasons, and throughout the entire United States. HSLC environments represent a unique challenge for forecasters, as they occur frequently but produce severe weather a relatively low percentage of the time. Recent studies have primarily focused on improving nowcasting and warnings for events through the identification of radar signatures commonly associated with HSLC tornadoes. Few studies have investigated the forecasting of HSLC severe weather, despite the acknowledged poor performance of traditional tools and techniques. A general climatology of HSLC significant severe weather is presented, focusing on regional, diurnal, and annual trends. Through this climatology, it becomes apparent that multiple types of HSLC environments are possible, including surface-based cases with low lifted condensation levels and high-based convection cases. A statistical analysis of HSLC events and nulls from the southeastern and mid-Atlantic states is utilized to assess the performance of conventional composite parameters in HSLC environments. Additionally, a new composite parameter is introduced that utilizes the product of the statistically most skillful parameters in HSLC environments: the 0–3-km lapse rate, the 700–500-hPa lapse rate, and multiple wind and shear metrics. The strengths and weaknesses of these ingredients-based techniques are then reviewed, with an eye toward improving future HSLC severe weather forecasts.

Severe convection occurring in environments characterized by large amounts of vertical wind shear and limited instability (high-shear, low-CAPE, or “HSLC,” environments) represents a considerable forecasting and nowcasting challenge. Of particular concern, NWS products associated with HSLC convection have low probability of detection and high false alarm rates. Past studies of HSLC convection have examined features associated with single cases; the present work, through composites of numerous cases, illustrates the attributes of “typical” HSLC severe and nonsevere events and identifies features that discriminate between the two. HSLC severe events across the eastern United States typically occur in moist boundary layers within the warm sector or along the cold front of a strong surface cyclone, while those in the western United States have drier boundary layers and more typically occur in the vicinity of a surface triple point or in an upslope regime. The mean HSLC severe event is shown to exhibit stronger forcing for ascent at all levels than its nonsevere counterpart. The majority of EF1 or greater HSLC tornadoes are shown to occur in the southeastern United States, so this region is subjected to the most detailed statistical analysis. Beyond the documented forecasting skill of environmental lapse rates and low-level shear vector magnitude, it is shown that a proxy for the release of potential instability further enhances skill when attempting to identify potentially severe HSLC events. This enhancement is likely associated with the local, in situ CAPE generation provided by this mechanism. Modified forecast parameters including this proxy show considerably improved spatial focusing of the forecast severe threat when compared to existing metrics.

Low-CAPE (i.e., CAPE ≤ 1000 J kg−1) severe thunderstorms are common in the greater southeastern United States (including the Tennessee and Ohio valleys). These events are often poorly forecasted, and the environments in which they occur may rapidly evolve. Real-data simulations of 11 low-CAPE severe events and 6 low-CAPE nonsevere events were performed at convection-allowing resolution. Some amount of surface-based destabilization occurred during all simulated events over the 3-h period prior to convection. Most simulated severe events experienced comparatively large destabilization relative to the nonsevere events as a result of surface warming, cooling aloft, and surface moistening. The release of potential instability by large-scale forcing for ascent likely influenced the cooling aloft in some cases. Surface warming was attributable primarily to warm advection and appeared to be an important discriminator between severe and nonsevere simulated events. Severe events were also found to have larger low-level wind shear than nonsevere events, particularly during nocturnal cases. Because of the rapid destabilization that occurred within 3 h in the simulated events, it is evident that 3–6-hourly model output may not be adequate for forecasting severe events in high-shear, low-CAPE environments. Monitoring of high-resolution model forecasts and surface observations may be necessary to identify a rapidly changing severe environment.

Environments characterized by large values of vertical wind shear and modest convective available potential energy (CAPE) are colloquially referred to as high-shear, low-CAPE (HSLC) environments. Convection within these environments represents a considerable operational forecasting challenge. Generally, it has been determined that large low-level wind shear and steep low-level lapse rates—along with synoptic-scale forcing for ascent—are common ingredients supporting severe HSLC convection. This work studies the specific processes that lead to the development of strong surface vortices in HSLC convection, particularly associated with supercells embedded within a quasi-linear convective system (QLCS), and how these processes are affected by varying low-level shear vector magnitudes and lapse rates. Analysis of a control simulation, conducted with a base state similar to a typical HSLC severe environment, reveals that the key factors in the development of a strong surface vortex in HSLC embedded supercells are (i) a strong low- to midlevel mesocyclone, and (ii) a subsequent strong low-level updraft that results from the intense, upward-pointing dynamic perturbation pressure gradient acceleration. Through a matrix of high-resolution, idealized simulations, it is determined that sufficient low-level shear vector magnitudes are necessary for the development of low- to midlevel vertical vorticity [factor (i)], while steeper low-level lapse rates provide stronger initial low-level updrafts [factor (ii)]. This work shows why increased low-level lapse rates and low-level shear vector magnitudes are important to HSLC convection on the storm scale, while also revealing similarities between surface vortexgenesis in HSLC embedded supercells and higher-CAPE supercells.

Recent research has improved our knowledge and forecasting of high-shear, low-CAPE (HSLC) severe convection, which produces a large fraction of overnight and cool season tornadoes. However, limited near-storm observations have hindered progress in our understanding of HSLC environments and detection of severe potential within them. This article provides an overview of a research project in central North Carolina aimed toward increasing the number of observations in the vicinity of severe and nonsevere HSLC convection. Particularly unique aspects of this project are a) leadership by student volunteers from a university sounding club and b) real-time communication of observations to local National Weather Service Forecast Offices. In addition to an overview of sounding operations and goals, two case examples are provided that support the potential utility of supplemental sounding observations for operational, educational, and research purposes.