| Description |
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. The strongest surface winds began suddenly around 0900 UTC, primarily in the southern portion of the damage zone. Surface winds reached their peak intensity (gusts to 45 m s-1) at ~ 1600 UTC, while the strongest winds shifted later to the northern end of the damage swath. The northward shift in strong surface winds relates to the rotation of synoptic-scale flow from northeasterly to easterly at crest level, controlled by an evolving anticyclonic Rossby wave-breaking event. A rawinsonde released at ~1100UTC in the midst of strong (>35 m s-1) easterly surface winds initially travelled horizontally before ascending rapidly within a downstream rotor.The sonde subsequently intersected a strong inversion at the top of the rotor due to dry air descending sharply from above crest level. Weather Research and Forecasting (WRF) model simulations 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 1-2 days in advance. A simulation was initialized from North American Mesoscale analyses at 0600 UTC 29 November 2013 and forced on the outermost boundary over the next 72 h by subsequent analyses at 6-h intervals. 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. |
