High Shear/Low CAPE (HSLC) Convective Operations at WFO GSP

Environmental Assessment: GSP forecasters loosely define low CAPE environments as situations in which 0 < surface-based CAPE < 500 J kg-1. Forecaster experience and climatological studies have shown that CAPE values in excess of this number tend to result in convection with more “classic” or “textbook” structure. Strongly sheared environments are loosely defined as those in which the magnitude of the 0-6 km shear vector exceeds ~25 m s-1. During these situations, GSP forecasters are highly aware of the potential for weak and occasionally strong tornadoes developing from shallow quasi-linear convective systems (QLCS). These tornadoes are often referred to locally as “broken-S” tornadoes, due to the radar reflectivity pattern that often accompanies their development. Research and operational experience has led to the conclusion that these tornadoes do not develop via supercellular processes. I.e., they occur in the absence of a WSR-88D user-defined or algorithm-defined mesocyclone. In fact, the WSR-88D often fails to detect even weak cyclonic rotation in association with these events. It is therefore assumed that the mechanism responsible for tornadogenesis is likely akin to the development of intense mesovortices that sometimes develop within QLCS in strongly-sheared environments (i.e. Weisman and Trapp 2003 and Atkins 2008). Modeling research by Weisman and Trapp (2003) indicates intense mesovortices tend to develop in QLCS when the magnitude of the 0-2.5 km shear vector exceeds 20 m s-1. Forecasters at GSP are encouraged to evaluate this parameter when assessing the non-supercell tornado potential. Forecasters are not necessarily encouraged to assess environmental Storm Relative Helicity (SRH), since this is a parameter that assesses the potential for mesocyclones. (However, this is still an important step in assessing the potential for mini-supercells during HSLC situations.) Additionally, forecasters are encouraged to assess CAPE in the lowest 3 km, since previous research (i.e., Davies 2006) has shown that 0-3 km CAPE may be a reliable predictor of tornadic environments during situations in which the overall CAPE is small.

Radar Interrogation: Due to the absence of radar velocity signatures, issuance of effective warnings for these events is always challenging, and often impossible. Reflectivity data is typically more useful, due to the “broken-S” signature. However, this pattern often does not appear until the tornado has already developed, and is therefore not always reliable as a precursor signal that tornadogenesis is imminent. This signature appears to develop as the result of a descending rear-to-front (in a storm relative sense) air current (i.e., a rear inflow jet [RIJ]) that first distorts the radar reflectivity pattern into a “bulge,” or slight bowing appearance.

Later volume scans often reveal a stretching of the “bulging” portion of the pattern along the direction of the RIJ. This stretching subsequently evolves into the “broken-S” pattern as the reflectivity segment on the south (i.e.”anticyclonic shear” side) of the RIJ is accelerated ahead of the reflectivity segment north (i.e. the “cyclonic shear” side) of the RIJ. The tornado occurs in association with the reflectivity segment that is “left behind” on the cyclonic shear side of the RIJ. This is consistent with the findings of Atkins (2008) and Weisman and Trapp (2003), who found that development of intense mesovortices in QLCS is tied closely to the descent of the rear inflow jet. In order to issue warnings with adequate lead time, forecasters are encouraged to anticipate development of the “broken-S” signature, given a favorable environment. This requires recognition of a descending RIJ via the usual radar interrogation techniques (i.e., the appearance of a rear inflow notch).

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