Lower-Tropospheric Precursors to Nocturnal MCS Development over the Central United States

Abstract

Composite analyses are examined to identify signals in the late afternoon surface and lower-tropospheric environments that indicate the expected location and degree of nocturnal mesoscale convective system (MCS) development over the central United States. The authors concentrate on two features: 1) the forcing for the low-level jet (LLJ), and 2) the frontogenetic character of lower-tropospheric fronts, or other types of airmass boundaries, with which MCSs are associated. Results show that very large, long-lived, nocturnal MCSs are likely to mature downwind of a late afternoon surface geostrophic wind maximum if that region is frontogenetic at 850 mb. The significance of the afternoon surface geostrophic wind maximum is that it identifies the region where the core of the elevated nocturnal LLJ will develop atop the surface-based nocturnal inversion. Where the forecast LLJ will encounter the frontogenetic boundary defines an area of potentially enhanced nocturnal low-level ascent through convergence and warm advection, which would predispose that region to significant mesoscale convective development and heavy rain. Composites and case studies show that smaller, less significant MCSs also mature north of maxima in the late afternoon surface geostrophic wind but that those regions appear to lack a strong frontogenetic signal at 850 mb. Case studies illustrate how well these indicators applied to four different situations during the summer of 1992. Finally, a tentative design for an operational product that incorporates key features of these findings for forecasting the location of heavy rain is proposed.