The Generation and Propagation of a Nocturnal Squall Line. Part I: Observations and Implications for Mesoscale Predictability

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Abstract
The initiation and forcing of a squall line on 26–27 May 1985 near Wichita, Kansas is examined. The squall line is secondary convection in the sense that it is preceded in the diurnal cycle by other mesoscale convective systems. The squall fine develops in synoptic conditions frequently associated with convective weather in the central United States. This includes moderate shear, moderate-to-high convective available potential energy, a quasi-stationary dryline, a low-level southerly jet, and an 85 kPa moist tongue. The initiation of free convection is specifically attributed to a gust front that propagates into a low-level jet with attendant horizontal vorticity, convergence and moisture. The gust front initially propagates as a gravity current and subsequently as an internal undular bore. Observations of gravity currents, gravity waves and the “collision” of radar echo “boundaries” provide valuable clues for short period forecasting. They establish the presence of potential triggering mechanisms that can propagate from convectively less-favorable to more-favorable environments. Such observations in themselves, however, are insufficient to establish the dynamical causes of convection. The impact of shallow boundary layer disturbances as generators of convection is briefly discussed with respect to mesoscale predictability. To forecast squall line precipitation in metropolitan areas beyond one to four hours is a major challenge given the ubiquity of weakly coupled, shallow disturbances in the nocturnal PBL. While it is necessary to model and accurately forecast larger scale conditions, this is likely to be insufficient for advances in locationally specific 6–12 h forecasts of convective precipitation. Finally, the squall line described herein form in a region where there exists, climatologically, a nocturnal maximum in thunderstorm occurrence. If this event is typical, then it suggests that the nocturnal maximum is due, in part, to an interaction between earlier disturbances that have formed over the Rockies and unstable conditions in the vicinity of the Great Plains southerly jet. This may be viewed as a form of discrete propagation for mesoscale convective systems.

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