21st Conf. on Severe Local Storms and 19th Conf. on Weather Analysis and Forecasting/15th Conf. on Numerical Weather Prediction

Thursday, 15 August 2002: 10:45 AM
The Structure of a tornadic bow echo in Idaho
James G. LaDue, NOAA/NWS, Norman, OK
Poster PDF (1.2 MB)
A storm structure analysis of a climatologically rare tornadic squall line in Northern Utah and Southern Idaho on February 14, 2000 is the subject of this study. This event occurred during an Intensive Observation Period (IOP4) of the Intermountain Precipitation Experiment (IPEX). The substantial increase in observations associated with IPEX in addition to the fortuitous proximity of the Pocatello and the Salt Lake City WSR-88Ds to the most severe part of the squall line has allowed a higher than average observation density of the near-storm environment and internal storm structure for this area. In addition, five reported tornadoes developed at close range to the Pocatello, Idaho WSR-88D. The National Weather Service in Pocatello, ID also conducted a damage survey of the six reported tornadoes providing relatively accurate damage path information.

Despite the shallow nature of the convective layer and a relatively cold environment (surface temperature=10 o C), this squall line exhibited characteristics typical of many squall lines in warmer environments. These included a line of strong reflectivity cores followed by a trailing stratiform precipitation region and rear inflow jet. The leading gustfront remained within 3 km of the high reflectivity cores, a feature typical of intense squall lines observed and modeled in warmer environments. However, this squall line also exhibited a slight forward tilt of the core and a more extensive forward anvil overhang with respect to its motion. In an environment consisting of greater than 50 kts of shear in the convective layer, and a CAPE of around 500 j/kg, this study will attempt to answer how this squall line retained its upright character.

For the 30 minutes prior to the first observed tornado, a quasi-two-dimensional bounded weak echo region (BWER) was observed by radar underneath a forward-tilted elevated core. This linear BWER exhibited 3-D variations in intensity, as inferred by the changes in minimum reflectivity at a constant altitude, along its length. This BWER structure suggests that a strong linear updraft was in progress from near cloud base up to 3 km above radar level with local along-line variations in strength. This background updraft with the along-line variations in intensity above a gustfront with intense convergence may have allowed locally enhanced tilting of horizontal vorticity in the lowest two kilometers above ground level to produce small 1-2 km diameter tornadic circulations.

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