Thursday, 7 October 2004
On 18 August 2002, several strong storms occurred in the Tennessee Valley region of the United States. Subtropical moisture resulted in showers during the early morning hours which, in turn, set up a weak boundary in the region as indicated by radar data for storms to form along later in the day. Sounding data indicated light winds at the surface and weak vertical shear aloft. A strong multicell storm formed over the Huntsville, Alabama area between 2030 and 2230 UTC. A severe thunderstorm warning was eventually issued for this storm as it produced small hail, damaging wind, and localized flooding. Some areas around the National Space and Science Technology Center (NSSTC) recorded rainfall amounts of 4 inches during this time frame. At 2117 UTC, an anemometer on the roof of the NSSTC recorded a gust of 34 m s-1, indicating a microburst event.
One of the more impressive aspects of this storm was its duration and slow movement, along with the lightning flash rates produced during its lifecycle. Over the two-hour time period, radar data showed the storm remaining fairly stationary over the Huntsville area while other storms in the region moved quickly off to the ESE. Data from the NASA Lightning Mapping Array (LMA) and NLDN indicated strong intracloud (IC) and cloud-to-ground (CG) lightning flash rates within this storm complex. In the region of highest reflectivity and inferred updraft, the storm produced an overall flash rate of approximately 4 flashes/second. Though other storms in the region exhibited impressive lightning flash rates, none came close to producing the same rates as those contained within this microburst-producing storm.
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