Rapid Evolution of Cool Season, Low-CAPE Severe Thunderstorm Environments

Abstract

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.

A Convection Parameterization for Low-CAPE Environments

“…The July-August 2016 period is considered here because the introduction of the low-CAPE convection scheme has little systematic impact on winter guidance. This is consistent with the fact that although the bulk of low-CAPE convection in the southeastern United States occurs in the cool season (King et al 2017), such events are more frequent at night and during the warm season across the remainder of the continent (Evans 2010;Main et al 2018). Forecasts are initialized from operational analyses at 36-h intervals in order to reduce computational load while promoting serial independence.…”

“…The importance of strong synoptic and mesoscale forcing for ascent in the preconvective environment is emphasized by van Den Broeke et al (2005) in relation to the quasi-linear mesoscale convective systems that can develop parallel to cold fronts under low-CAPE conditions. Along the leading edge of these fronts, the poleward transport of moisture in the warm conveyor belt (Carlson 1980;Eckhardt et al 2004) contributes to satisfying the moisture requirement of the ingredients-based perspective (King et al 2017). These associations are reinforced by recent studies of high-shear, low-CAPE (HSLC) convective storms (Sherburn and Parker 2014) and the environments in which they form (Sherburn et al 2016).…”

On the Moisture Origins of Tornadic Thunderstorms

“…The climatological characteristics of DJF and MAM tornadoes have differences that are largely related to the seasonal cycle. DJF is characterized by the southward protrusion of the meridionally oriented polar jet across the CONUS, which leads to a reduction in low-level moisture return due to cold air advection upstream of ETCs (Lewis et al 1989;Sherburn and Parker 2014;Allen et al 2015;King et al 2017). As a result, tornado frequency is lower in DJF as compared to other seasons (Molina et al 2018), primarily favored across the southeastern CONUS and generally independent of the diurnal cycle (Sherburn et al 2016;Childs et al 2018;Krocak and Brooks 2018).…”

Structure and Environment of Tornado-Spawning Extratropical Cyclones around Japan

“…Note that the horizontal resolution of the dataset may affect the mesoscale features, such as the veering of the wind with height and CAPE that can evolve rapidly in small time and space scales (e.g., Apsley et al 2016;King et al 2017;Markowski et al 1998). Thus, Fig.…”