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 convergenc... 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 convergenc...