THE INFLUENCE OF THE ROCKY-MOUNTAINS OH THE 13-14 APRIL 1986 SEVERE WEATHER OUTBREAK .2. EVOLUTION OF A PREFRONTAL BORE AND ITS ROLE IN TRIGGERING A SQUALL LINE

In this paper, Part II of a series, the evolution of a prefrontal bore on the leeside of the Rockies and its subsequent propagation and init iation of convection farther downstream over eastern Colorado and west ern Nebraska are investigated. The observational evidence for this seq uence of events was obtained from combined i analyses of high-resoluti on GOES satellite imagery and Program for Regional Observing and Forec asting Services mesonetvork data over the Colorado region for the seve re weather event that occurred during 13-14 April 1986. A 2D nonhydros tatic numerical model is used to further understand the initiation of the bore and its ability to propagate farther downstream and trigger C onvection. Analysis of satellite imagery and mesonet data indicated th at an internal bore (ahead of a cold front), a moderate downslope wind storm, and a quasi-stationary hydraulic jump were generated within a f ew hours along the leeslope as a Pacific cold front and its attendant upper-level jet streak advanced over the Rockies. The bore and the col d front then propagated eastward for several hours and interacted with a lee cyclone, a dryline, and a warm front, initiating severe weather over Nebraska and Kansas. Wave-ducting analysis showed that favorable wave-trapping mechanisms such as a capping inversion above a neutral layer and wind curvature from a low-level jet, which appeared to be th e most dominant ducting mechanism, existed across eastern Colorado and western Nebraska to maintain the bore strength, Numerical simulations of continuously stratified shear flow specified from upstream and dow nstream soundings suggested that the creation of a density current alo ng the lee slopes, a downstream inversion height lower than the upstre am inversion height; and a strong curvature in the wind profile of the low-level jet are all needed to initiate and sustain the integrity of the propagating bore. Based on the synthesis of observational analyse s and 2D nonhydrostatic model simulations, a schematic illustration of the time evolution of the bore ahead of the Pacific cold front, the h ydraulic jump associated with a mountain wave, and the arctic air intr usion from the north to the lee of the Rockies are presented in the co ntext of severe weather occurrence over western Nebraska and Kansas.