Dynamics of an unforecast clear air turbulence outbreak over the upper midwest United States

On 21-22 October 1996, widespread clear air turbulence (CAT) was reported over the upper Midwest United States, ahead of a very slowly approaching low and cold front. This outbreak lasted approximately 18 hours and consisted of dozens of reports of moderate-to-severe CAT. However, conventional diagnostics for CAT forecasting, such as the Richardson number, failed to suggest the possibility of a long-lived and widespread CAT outbreak.

In this talk we examine the dynamics of this CAT outbreak using a suite of dynamical diagnostics appropriate for unbalanced, gravity wave-generating flows. By comparing these diagnostics to hourly observations of CAT, it is determined that the inertial-advective wind provided the best guidance for CAT location and intensity during this event. In addition, by studying the ageostrophic wind field it is found that nonlinear interactions between the approaching low and a cut-off low over the Northeast played a role in this outbreak.