Factors that Influence the Growth of Tornadic Supercells into MCSs after Sunset

Tornadic supercells sometimes transition into mesoscale convective systems after sunset in the Central United States.  A comprehensive understanding of the atmospheric factors that determine whether or not this transition occurs remains an elusive goal, and forecasting this transition is particularly challenging.  Furthermore, the societal threats change quickly from tornadoes to damaging winds and flooding as supercells grow into MCSs, which underscores the importance of adequately understanding and forecasting this transition process.

This research compares the atmospheric environments supercell-to-MCS transition events (hereafter ‘MCS events’) to the analogous environments of supercell events that do not result in an MCS after sunset (hereafter ‘null events’).  Low-level warm air advection (WAA) typically strengthens (relative to the afternoon) below 1.5 km above ground level (AGL) immediately after sunset in MCS events, whereas WAA strengthens at a slower rate, and maximizes at at a higher level AGL later in the evening in null events.  The earlier onset of WAA, combined with its lower level AGL in MCS events results in better overlap between WAA induced lifting and air parcels with high CAPE and low CIN, and thus improves the probability of widespread sustained convection.  The better overlap between CAPE and WAA in MCS events is encapsulated by the ingredients-based propagation index (IPI) parameter, which is a normalized grid-point product of WAA and CAPE.  IPI is statistically higher after sunset for MCS events than null events, which suggest that this parameter may be useful in forecasting the initial growth of MCSs (IPI has previously been used to predict the direction of MCS propagation).