Tornadogenesis and Operational Considerations of the 11 August 1999 Salt Lake City Tornado as Seen from Two Different Doppler Radars

Lawrence B. Dunn National Weather Service Forecast Office, Salt Lake City, Utah

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Steven V. Vasiloff National Severe Storms Laboratory, Norman, Oklahoma

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Abstract

On 11 August 1999 a climatologically rare F2 tornado developed just west of downtown Salt Lake City, Utah (SLC), and moved directly through the city during the noon hour. Tornadogenesis was observed from a mountaintop WSR-88D Doppler radar 76 km (41 n mi) to the northwest of and 0.7 km (2300 ft) above SLC and also from a Terminal Doppler Weather Radar (TDWR) located only 22.2 km (12 n mi) north of and at the same elevation as SLC. Data from the TDWR offer an unambiguous view of the development of a nondescending tornado as an intensifying updraft became juxtaposed over enhanced cyclonic shear along a surface-based convergence zone. The convergence zone intensified and developed upward with a circulation center directly beneath the updraft eventually contracting to the scale of a tornado vortex. After tornadogenesis, the previously disorganized thunderstorm displayed characteristics commonly associated with supercells, such as a hook echo, bounded weak-echo region, a WSR-88D algorithm detection of a mesocyclone, and a visible wall cloud.

The mountaintop WSR-88D was able to identify the tornado in the base velocity data and via the latest operational version of the Tornado Detection Algorithm. However, interpretation of velocity products produced by the radar system for real-time operations was problematic due to degradation with range of the displayed data. Without access to the full-resolution velocity data in real time, it would be impossible for a forecaster to corroborate the algorithm tornado detection.

Corresponding author address: Dr. Lawrence B. Dunn, National Weather Service Forecast Office, 2242 W. North Temple, Salt Lake City, UT 84116. Email: larry.dunn@noaa.gov

Abstract

On 11 August 1999 a climatologically rare F2 tornado developed just west of downtown Salt Lake City, Utah (SLC), and moved directly through the city during the noon hour. Tornadogenesis was observed from a mountaintop WSR-88D Doppler radar 76 km (41 n mi) to the northwest of and 0.7 km (2300 ft) above SLC and also from a Terminal Doppler Weather Radar (TDWR) located only 22.2 km (12 n mi) north of and at the same elevation as SLC. Data from the TDWR offer an unambiguous view of the development of a nondescending tornado as an intensifying updraft became juxtaposed over enhanced cyclonic shear along a surface-based convergence zone. The convergence zone intensified and developed upward with a circulation center directly beneath the updraft eventually contracting to the scale of a tornado vortex. After tornadogenesis, the previously disorganized thunderstorm displayed characteristics commonly associated with supercells, such as a hook echo, bounded weak-echo region, a WSR-88D algorithm detection of a mesocyclone, and a visible wall cloud.

The mountaintop WSR-88D was able to identify the tornado in the base velocity data and via the latest operational version of the Tornado Detection Algorithm. However, interpretation of velocity products produced by the radar system for real-time operations was problematic due to degradation with range of the displayed data. Without access to the full-resolution velocity data in real time, it would be impossible for a forecaster to corroborate the algorithm tornado detection.

Corresponding author address: Dr. Lawrence B. Dunn, National Weather Service Forecast Office, 2242 W. North Temple, Salt Lake City, UT 84116. Email: larry.dunn@noaa.gov

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