Case Study of a Downslope Windstorm April 23, 1999 Wasatch Range, UT Claire Hruska ABSTRACT An easterly downslope windstorm hit the western side of the Wasatch Range on April 23, 1999. The storm was very damaging to three surrounding counties and winds of up to 113 mph were recorded at the Bringham City Airport. Two factors made this such a damaging storm. Vertically propagating mountain waves hit one of three possible critical levels and returned to the surface with intense winds. The other factor was the strong pressure gradient from Riverton, WY to Salt Lake City, UT that enabled gap flow to be forced through breaks in the range. 1. Introduction Downslope windstorms can be hazardous to the inhabitants on the lee side of a mountain range. These wind storms have winds in excess of 55 knots, or 34 m/s, with the potential to topple trees, blow tops off buildings, and even cause injury or death. They are also very dangerous to aircraft, with the intense wind creating clear air turbulence at flying altitudes. The downslope windstorm that occurred between 6 and 12z on the western side of the Wasatch Range in Utah on April 23, 1999 brought gusts of up to 113 mph winds, knocking over trees, power lines, and closing down a highway. The 113 mph winds were recorded at Bringham City Airport, and set a new record for low altitude wind speed in Utah. Throughout three counties, residential property damage cost an estimated $3 to $5 million dollars, with the most damage occurring along the Northern Wasatch Front. Bringham City Airport reported toppled planes and damaged hangars. The wind blew semis over on I-15 between Bringham City and Centerville, closing the highway, and Weber State University reported blown out windows and toppled trees. An estimated 10,000 people lost power due to snapped power poles. Unlike many of the windstorms that are well documented and studied, this particular windstorm is different because the cross-barrier flow comes from the east, not the west. Figure 1 is a picture from an easterly downslope windstorm in the Sierra Nevadas. Figure 1 Picture of a downslope windstormThis wind flow pattern is typical for this area and downslope windstorms on the western side of the Wasatch Mountain Range occur from a variety conditions, ranging from purely gap flow winds, to mountain waves, to a combination of both. The strongest winds tend to happen with mountain waves, but there is almost always some contribution from gap flows, as was the case in the April 23 downslope windstorm. This windstorm was a bora wind scenario, as cold air from the northeast was advected over the mountain barrier, displacing a warmer air mass on the western side of the range. The downslope windstorm on the western side of the Wasatch Mountain Range was a product of strong mountain waves enhanced by a combination of critical levels, including a mountain top inversion, but the reason extreme winds occurred was due to the aid of a strong pressure gradient over the range. This pressure gradient allowed intense gap flows to occur, which in combination with the mountain waves, produced intense winds of up to 113 mph. 2. Data Data for this case study was obtained from a downslope windstorm case study of the Wasatch Range by Larry Dunn, as well as the damage reports from NOAA. Data for the Colorado Park Range windstorm came from a publication put out by NOAA’s National Weather Service Weather Forecast Office. Papers and publications on downslope windstorms and gap flows were obtained from MetEd at UCAR and Gabersek and Durran of the University of Washington, respectively. 3. Synoptic Overview On April 23, a cutoff low pressure system was situated in the southwestern U.S, just below Utah, as seen in the model data of Figure 2. Figure 2 Sea level pressure at 12z 23 Apr 99 A very strong pressure gradient existed in southwestern Wyoming, western Colorado, and into Utah. The cyclonic rotation of the winds from where the low pressure system was located, along with the strong pressure gradient is what caused the easterly cross-barrier winds in northern Utah. Figure 3 shows surface streamlines from 12z on the 23rd. Figure 3 Surface streamlines and temperatures in degree F at 12z 23 Apr 99Cold air from the northeast, namely Montana, was being advected into northern Utah, replacing a warmer air mass on the western side of the Wasatch Range. All of this was fueled by cyclogenesis in a trough centered over the Great Basin area on the morning of April 22. An upper level jet positioned in the base of this trough helped to turn the trough into a strong, closed low pressure system. As the low pressure system became cut-off, intensified by cyclogensis, the easterly cross-barrier flow at 700 mb strengthened and the cross-barrier sea level pressure difference between Riverton, WY and Salt Lake City, UT was over 19 mb. The importance of the strong pressure gradient will be discussed in the mesoscale analysis. 4. Mesoscale Analysis As mentioned before, in a downslope windstorm, there is almost always some contribution from gap flow. The contribution from gap flow in this case was enhanced by both the synoptic and mesoscale pressure gradients that were present. The synoptic closed low pressure system with a strong pressure gradient over southwestern Wyoming, as well as the pressure gradient created from blocking of the wind by the topography at low levels created an ideal situation for strong gap flow. Gap flow is sometimes referred to as pressure driven channeling, so as the easterly winds found gaps or channels through the Wasatch Mountain Range, the high pressure on the east side of the range forced the wind through the gaps at high speeds. The major mesoscale events in this case study were the lee side mountain waves caused by the upslope flow. The atmosphere surrounding the Wasatch Mountain Range was very stable on April 23.