Downslope Windstorms In The Green Mountains: Operational Modeling Requirements To Forecast High Impact Wind Events

Damaging easterly downslope windstorms occasionally develop along and immediately west of the spine of the Green Mountains in Vermont and are known colloquially as shirkshires. These events are typically driven on the synoptic scale when an unusually deep surface low tracks just south of New England with large easterly wind anomalies across central and northern New England.

The mechanisms speculated to contribute to these events are mountain wave breaking, gap winds, and turbulent mixing. Of particular importance is the magnitude of the wind near the ridge top and the extent to which a critical layer is formed above the ridgeline. If a critical layer traps these breaking waves, the potential for hydraulically forced wave breaks and damaging easterly winds reaching the surface are greatly enhanced, especially along the steeply sloping terrain of the western Green Mountains.

We use the Weather Research and Forecast model to analyze a particularly strong high wind event from April 16, 2007 to determine if it is possible to produce prognostic output that would alert operational forecasters to a high impact wind event instead of a moderate wind event. To test this we use varied grid resolutions, initial and boundary conditions, and model physic packages to diagnose what combination would be needed for the operationally forecaster.

The goal of this research is to identify key model resolution and physics requirements needed to operationally produce a high impact wind event forecast of damaging easterly winds in the lee of the Green Mountains. It is argued that the predictability of the magnitude of the hazardous events will increase as operational models decrease in resolution.