The post-frontal pressure gradient is locally tighter than nearby pressure gradients and generates a low level ribbon of strong easterly winds oriented nearly perpendicular to the mountain range. Deep convection develops where the post-frontal easterly ribbon is lifted by the higher terrain. Individual cells drift northward in the weak southerly steering currents as the slow southward progress of the surface high and its associated post frontal low-level easterly flow focus the convection over the same general area for several hours. The approaching shortwave and upper level divergence maintain favorable deep tropospheric forcing throughout the storm's duration. Radar cross sections show sloping updrafts and a low-echo centroid.
Warm cloud processes dominate precipitation production.
Moist mid to upper levels, deep warm cloud processes, and cloud bases near the ground severely limit cooling by melting andevaporation thereby minimizing the formation of cool moist downdrafts and propagation of the system away from the mountains. These ingredients often do not result in the delicate equilibrium necessary for a terrain induced quasi-stationary convective system. Nevertheless, they should be heeded as strong warning signs that a life threatening flash flood is possible. Although this paper focused on a single scenario in which to create this equilibrium, there are other ways it can be achieved. Of course, as shown by Maddox et al.