Analysis and Predictability of the Wasatch Windstorm of 1 December 2011

A downslope windstorm on 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. Surface winds, gusting to 45 m/s, gradually moved northward, controlled by an evolving anticyclonic Rossby wave-breaking event. A rawinsonde released in the midst of strong (over 35 m/s) easterly surface winds initially travelled horizontally before ascending rapidly within a downstream rotor.

Weather Research and Forecasting (WRF) model simulations, initialized with North American Mesoscale analyses, were completed to assess: (1) the fidelity of high-resolution models (1.3-km horizontal grid spacing) to resolve the dynamics of this downslope windstorm, and (2) why there was apparent enhanced predictability in high-resolution model guidance, compared to other windstorm locations. The model simulation captured core features of the downslope wind event, including the spatial extent and timing of the strongest surface winds. However, the model developed stronger mountain-wave breaking in the lee of the Wasatch, a broader hydraulic jump, and a downstream rotor located farther west than observed. To investigate the predictability of this windstorm, an 11-member ensemble of 72-h WRF high-resolution forecasts was initialized from 0000 UTC 29 November 2011 reforecasts from the Global Ensemble Forecast System. Eight of the eleven members generated a strong, localized windstorm with the outliers arising from reduced cross-barrier synoptic-scale flow.