Analysis of the 1 December 2011 Wasatch Downslope Windstorm

John Lawson Department of Atmospheric Science, University of Utah, Salt Lake City, Utah, and Department of Geological and Atmospheric Science, Iowa State University, Ames, Iowa

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John Horel Department of Atmospheric Science, University of Utah, Salt Lake City, Utah

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Abstract

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. The strongest surface winds began suddenly at 0900 UTC, primarily in the southern portion of the damage zone. Surface winds reached their peak intensity with 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 ~1100 UTC in the midst of strong (>35 m s−1) easterly surface wind intersected a rotor and sampled the strong inversion that surmounted it. The windstorm’s evolution was further examined via Weather Research and Forecasting Model simulations initialized from North American Mesoscale Model analyses ~54 h before the windstorm onset. The Control model simulation captured core features of the 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 zone of strong surface winds, and a downstream rotor located farther west than observed. A second simulation, in which the nearby east–west-oriented Uinta Mountains were reduced in elevation, developed weaker easterly flow across the Wasatch during the early stages of the event. This result suggests that the Uinta Mountains block and steer the initial northeasterly flow across the Wasatch.

Corresponding author address: John Lawson, Dept. of Geological and Atmospheric Science, Iowa State University, 3018 Agronomy Hall, Ames, IA 50011. E-mail: johnroblawson@gmail.com

Abstract

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. The strongest surface winds began suddenly at 0900 UTC, primarily in the southern portion of the damage zone. Surface winds reached their peak intensity with 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 ~1100 UTC in the midst of strong (>35 m s−1) easterly surface wind intersected a rotor and sampled the strong inversion that surmounted it. The windstorm’s evolution was further examined via Weather Research and Forecasting Model simulations initialized from North American Mesoscale Model analyses ~54 h before the windstorm onset. The Control model simulation captured core features of the 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 zone of strong surface winds, and a downstream rotor located farther west than observed. A second simulation, in which the nearby east–west-oriented Uinta Mountains were reduced in elevation, developed weaker easterly flow across the Wasatch during the early stages of the event. This result suggests that the Uinta Mountains block and steer the initial northeasterly flow across the Wasatch.

Corresponding author address: John Lawson, Dept. of Geological and Atmospheric Science, Iowa State University, 3018 Agronomy Hall, Ames, IA 50011. E-mail: johnroblawson@gmail.com
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