18th Conference on Weather and Forecasting, 14th Conference on Numerical Weather Prediction, and Ninth Conference on Mesoscale Processes

Thursday, 2 August 2001
The role of mesoscale dry intrusion in the Washington, D.C. snowstorm of March 9, 1999
Richard P. James, Penn State University, University Park, PA; and J. H. E. Clark and R. H. Grumm
A series of MM5 simulations at 18 km horizontal resolution of the 9 March 1999 DC snowstorm is described. The purpose is to determine the processes responsible for the strong mesoscale structure of the snowfall and to ascertain the MM5 sensitivity to initial time and inital data quality.

We find that strong cold air damming played a crucial role in the event. A ducted internal gravity wave that moved eastward through Virginia initiated the heaviest snow fall. It was confined to the strong inversion capping the weak surface northeasterlies. This wave was linked to a disturbance in the moist and unusually warm southwesterlies above the stable layer. This disturbance preceeded a mid-troposphere dry slot. A deep layer of upward motion was triggered by a juxtaposition of the two disturbances.

We find that conditional symmetric instability played a role, albeit not dominant, in triggering the event. We conjecture as to the reasons for this.

A series of MM5 runs is described to assess model sensitivity of snowfall forecasts for the event. We discuss the reasons for the lack of accuracy in the actual operational forecasts for the event.

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