On the evening of Monday, October 6th, several severe thunderstorms producing large hail moved across the Tennessee Valley region. I, along with another colleague, was on the radar desk for severe weather operations at the Huntsville, AL Weather Forecast Office. Some thunderstorms in the region had already produced large hail up to the size of golf balls to our north. As a vigorous short wave moved through the region, leading to increased lapse rates and deep layer shear, the threat for large hail was expected to continue into the early evening hours. As usual here at the Huntsville WFO, total lightning data from the North Alabama Lightning Mapping Array (NALMA) were incorporated into our warning decision process.
At 548 pm CDT, my colleague issued an initial severe thunderstorm warning for a storm cell located over northwestern portions of Jackson County, AL, primarily for the expectations of large hail. Reflectivity from the KHTX radar and the initial polygon can be seen below in image 1.
Image 1. KHTX 0.5 degree reflectivity (dBZ) with warning polygon issued at 2248 UTC (548 pm CDT) 6 Oct 2014. The black circle near the top center of the image is the KHTX “cone of silence”.
I had taken over warning responsibility for this severe thunderstorm by 6 pm and was having to decide whether or not to continue the warning when it expired at 615 pm CDT. This storm was tracking very close to the KHTX radar (noted by the black circle) and it was difficult to make out some of its higher level features and characteristics (although the Advanced Radar for Meteorological and Operational Research (ARMOR) was also being utilized at this point). The storm had wavered in intensity since the warning issuance and was only expected to be at the low-end of severe criteria. Another factor complicating the warning decision was that this storm was tracking over an area with very low population density. So, severe weather reports providing ground-truth were difficult to come by, and in fact, we had not received any yet allowing for verification of the warning.
Nevertheless, this is where the NALMA data came into play. Just shortly after the severe thunderstorm issuance, source densities within the storm surged, with values reaching well over 400 sources (image 2) between 550 and 552 pm CDT.
Image 2. Source density values from the North Alabama Lightning Mapping Array, 2-min period ending 552 pm CDT (2252 UTC) 6 October 2014.
Over the next series of updates from the NALMA, source densities maintained relatively high values. As late as 2302 UTC, when I was beginning to consider the continuance of the warning, source values were still around 400 or higher. Afterward, values did gradually decrease. However, with the understanding that hail production will take some time following the strengthening updraft and that severe weather may not manifest up to about 30 minutes (or longer in some cases) after sustained surges in total lightning, I decided to continue the warning (Image 3). As the storm continued eastward, we finally received our first reports of one inch diameter hail in the town of Stevenson. Interestingly, the hail accumulated to the depth of a few inches according to one report.
So, yet again, this was another case in which total lightning provided value-added data and significant help for an operational warning decision.
Image 3. KHTX 0.5 degree reflectivity with warning polygon (yellow), valid 617 pm CDT 6 October 2014. The town of Stevenson, AL is highlighted where one inch diameter hail was reported covering portions of Hwy 72.