Parameter Assessment for a Marginal, High-Shear, Low-CAPE (HSLC) Cold Season Tornado Event in Indiana

On 31 December 2018, a quasi-linear convective system (QLCS) moved through the lower Ohio Valley. Driven by a strong low-level jet and plentiful near-surface moisture, the system produced two EF1 tornadoes in southern Indiana, including one in northern Spencer County. Nearby soundings taken from the Rapid Refresh model (RAP) indicated lower values of CAPE (near 200 J kg-1) and relatively high 0-3 km bulk shear (near 33 m s-1) than are normally associated with higher-end tornado threat. From these parameters, the event meets the definition of a HSLC tornado event. We evaluated two relevant HSLC parameters, SHERBS3 and SHERBE, which have been shown to be good indicators of tornado potential in HSLC scenarios. The SHERBS3 parameter calculated from RAP soundings yielded a “hit” for tornado potential, but a “missed detection” for SHERBE. Area forecast discussions from impacted National Weather Service County Warning Areas, as well as Storm Prediction Center Mesoscale Discussions, also show mixed signals for forecasters’ concern about tornado potential. Slight concern for brief tornadoes was mentioned, but damaging wind gusts were the main focus of the event. The general consensus was that QLCS tornado potential was limited by perceived minimal instability. It can be shown that SHERBE’s failure to convey tornado potential came from the effective shear calculation. Strong low-level jets, seen in this HSLC case, can make effective shear calculation less meaningful for conveying tornado potential. These results highlight the inherent difficulty in forecasting such marginal HSLC events.