The past two weeks have been an active period of severe weather for the Tennessee Valley, which is covered by the Huntsville, Alabama and Nashville, Tennessee National Weather Service (NWS) Offices. The region has seen destructive winds, baseball sized hail, and several powerful tornadoes. Throughout all of these events, the North Alabama Lightning Mapping Array (NALMA) has been providing supporting observations to the hard working NWS forecasters. The NALMA provides an observation of “source densities”, which can be thought of as individual stepped leaders (or pieces) of a lightning strike and can observe intra-cloud lightning. Essentially, higher densities indicate greater lightning activity and a stronger thunderstorm updraft.
SPoRT trains forecasters to look for lightning jumps, which are rapid spikes of source densities. These jumps typically precede severe weather events. Figure 1 shows the lightning jump that occurred as a thunderstorm intensified in northwest Lawrence County on 2 April 2009. The jump shows the source densities rapidly increasing from 4 (at 2100 UTC) to 145 (at 2110 UTC). This was followed by a rapid drop off in lightning activity, indicating the potential for severe weather. While the NALMA was not the primary reason, the NALMA supported radar observations that the storm was intensifying and a severe thunderstorm warning was issued at 2118 UTC as the storm entered Limestone county. Reports of dime sized hail and strong winds were reported starting at 2124 UTC in Athens in Limestone County. Figure 2 shows the real-time progression of NALMA source densities as seen in AWIPS.
Watching for lightning jumps is very useful to detect the onset of severe weather, but sometimes the NALMA observations serve a less direct role. In the case of the widespread severe weather on 10 April 2009, forecasters were already aware of the potential of severe weather. The CAPE values were exceptionally high across the Tennessee Valley at nearly 3000 J kg-1. Additionally, helicity values and other environmental factors clearly indicated that conditions were favorable for severe weather. When severe weather began to occur, it was widespread and moving rapidly through the region. The NWS forecasters relied on their training and skill interpreting radar data as the primary warning tool. This was particularly evident in the strong radar velocity signatures associated with the EF-3 tornado near Guntersville Lake in Alabama and EF-4 tornado in Murfreesboro, Tennessee. (Figure 3) Additionally, there were many storm reports coming into the NWS offices. In this strong, synoptic environment, NALMA data were not the primary forecast tool. However, NALMA observations provided vital data to give additional confidence to forecasters in their warning decisions.
Overall, for the past two weeks, NALMA data have provided a vital, supporting data set to NWS forecasters. In these cases, when environmental conditions favor strong thunderstorms, radar has been the primary warning tool. However, the NALMA data have provided additional information to give forecasters confidence in their warning decision making process.