Southern Wasatch Front
The date of August 1st, 2006 will be remembered by many residents in Salt Lake and Utah counties as violent storms impacted the area. During the morning hours the large scale weather pattern became quite supportive for severe thunderstorm development across northern Utah. A cold front and associated upper level trough (Fig 1) moved across the Great Salt Lake and Wasatch Front during the morning hours of August 1st, 2006. The air mass was very moist and unstable ahead of the front despite the event occurring during the typically more stable time of the day. Other ingredients necessary to develop and sustain severe thunderstorms were present across northern Utah, including an upper level jet. The jet was in a position which placed northern Utah under strong upper level divergence (Fig 2) which can act like a vacuum near jet stream level. This vacuum effect can cause the low level air to lift enhancing the development of precipitation. The presence of the jet also placed northern Utah in a deeply sheared environment (shear is essentially a condition in which the wind speed and/or direction change substantially with height), which can contribute to the organization of convective storms (i.e., thunderstorms). When storms develop in environments with strong shear and sufficient instability they are much more likely to be long-lived and severe.
II. Storm Evolution
A broken band of showers and thunderstorms moved across the northwest Deserts before sunset on the evening of July 31st, 2006. The southwest to northeast oriented line of precipitation began to intensify on the south side of the front when it moved across the Salt Flats along Interstate 80 by 900 AM MDT on August 1st. As this line of storms approached the community of Tooele they began to evolve into what is called a bow echo (Fig 3). Bow echoes are organized lines of thunderstorms which typically range from 20 to 200 km in length. Bow Echoes can be long-lived and produce damaging winds in excess of 100 mph. A strong bow echo will produce more widespread and intense damage than the majority of tornadoes. Figure 3 depicts how the leading edge of the line began to accelerate forward as noted by the bowed shape. This bowing is indicative of strong winds at the surface and is a signature utilized by National Weather Service meteorologists to anticipate damaging winds associated with a line of storms.
By 944 AM MDT the line was moving through western Salt Lake County. On the backside of the line a structure known as a rear-inflow notch was evident (Fig 4). A rear-inflow notch is an indication that cooler and drier air from the mid levels of the atmosphere is being brought into the backside of the line of storms. Ultimately, this results in the generation of strong and damaging downburst winds. As the line of storms moved into eastern Salt Lake County, and produced the most significant damage in the Sugarhouse and Mill Creek areas, the bowed shape of the storms and the rear-inflow notch were quite evident (Fig 5). Typically, the most damaging winds are near the apex of the bow echo and this was the case in Salt Lake County. This severe thunderstorm complex persisted as it crossed the Wasatch Range and reached the western Uinta Mountains by 1100 am MDT before weakening.
At the same time time, the second line of thunderstorms was strengthening over the Dugway area. By 1100 AM MDT, this storm complex also developed into a well-defined bow echo segment west of Utah Lake (Fig 6). By 1125 AM, the line of storms further intensified and developed a well defined rear-inflow notch (Fig 7). At 1130 AM the backside of the bow echo had two rear-inflow notches (Fig 8). The rear-inflow notches indicate two regions where we would expect to see enhanced winds gusts and the most damage to occur.
On August 1st, 2006 two bow echoes impacted the Wasatch Front click here for a loop of the entire event (yellow boxes in the loop are Severe Thunderstorm Warnings issued by the National Weather Service). Both of these bow echoes produced damaging straight-line winds near the apex of the bow segment which is a characteristic of this type of severe thunderstorm. Peak wind gusts were measured at 92 mph at the Provo Airport and estimated between 60 and 70 mph in East Millcreek of Salt Lake County. Rainfall was very intense with these thunderstorms as many areas in the southern Wasatch Front observed one half to one inch in less than 30 minutes. Hail stones as large as ¾ of an inch in diameter were observed with the thunderstorms in Provo. The hail accumulated on the Provo Airport runways and was still present in piles during the evening hours.
Tree and building damage was extensive across Salt Lake and Utah Counties making it one of the most costly severe thunderstorms in the past 15 years. Preliminary damage estimates in Utah County were around $13 million. Many trees were uprooted and power line poles were snapped. Over 20,000 customers lost power in Salt Lake County and a few thousand lost power in Utah County. There was also considerable damage to small airplanes which were toppled over by the wind and to buildings which had roofs and siding partially or totally torn away at the Provo Airport. The intense rainfall during a short time period caused considerably flooding with raging torrents of water on some streets and ponding of water in low lying areas.
Definitions from the Glossary of the American Meteorological Society:
Bow Echo-A bow-shaped line of convective cells that is often associated with swaths of damaging straight-line winds and small tornadoes. Key structural features include an intense rear-inflow jet impinging on the core of the bow, with book-end or line-end vortices on both sides of the rear-inflow jet, behind the ends of the bowed convective segment. Bow echoes have been observed with scales between 20 and 200 km, and often have lifetimes between 3 and 6 h. At early stages in their evolution, both cyclonic and anticyclonic book-end vortices tend to be of similar strength, but later in the evolution, the northern cyclonic vortex often dominates, giving the convective system a comma-shaped appearance.