Predictability and error growth in medium-range forecasts of the 1 December 2011 Wasatch Windstorm

The downslope windstorm of 1 December 2011 led to considerable damage along a narrow 50-km swath at the western base of the Wasatch Mountains in northern Utah. Operational forecasts issued by the Salt Lake City National Weather Service Forecast Office provided accurate guidance for the event at lead times of 1-2 days, based in part on their locally-generated high-resolution numerical forecasts, but also suggested the windstorm's occurrence at medium range (up to 5 days prior). To address the apparent enhanced predictability of this windstorm, three 11-member three-domain ensemble forecasts were initialized from 0000 UTC on 25, 27, and 29 November 2011, using the Weather Research and Forecasting (WRF) model with initial and boundary conditions supplied by reforecasts from the Global Ensemble Forecast System (GEFS/R2). In the 29 November forecast (60 h), eight of the eleven members generated a strong, localized windstorm with the outliers arising from reduced cross-barrier synoptic-scale flow. Analysis of kinetic energy error growth shows that this was unlikely to result fortuitously from underdispersion within the GEFS/R2 boundary conditions, and instead is perhaps related to a local reduction in error growth downstream of the synoptic-scale mid-tropospheric ridge. The ensemble initialized two days earlier (27 November) contains fewer members that generate a windstorm. No members generate a windstorm in the ensemble initialized two days earlier still (25 November), due to more rapid error growth in the synoptic regime during the first 60 h of this ensemble. This suggests the large-scale regime at the initial stage of a forecast period may be important in forecasting windstorms along the Wasatch.