A critical level is the level in the atmosphere that prevents the gravity wave energy from continuing upward. A mean-state critical level can also be a stable layer topped by an unstable layer. A self-induced critical level can be visualized as the turbulence generated by a breaking wave. Consider a breaking ocean wave, where, as the wave builds and then finally breaks, a region of turbulence is created where the flow may either reverse or go to zero. This region acts to prevent energy from propagating vertically through it.
Reverse shear is cross-barrier flow that decreases with height.
Moderately cold air over Montana was forecast to move south into Wyoming at the same time and easterly upslope and dynamic ascent were also forecast to produce 700 mb cooling over Wyoming. The forecast cyclogenesis took place. Below are analyses, observations and some imagery from around 1200 UTC April 23, 1999. The 500 mb analysis shows a deep closed low over the southwest United States. The spreading of the 20 knot SLC raob wind by the ETA analysis is the likely reason the 700 mb analysis is so much different than what was forecast.