Here at the Huntsville, AL Weather Forecast Office (WFO) we’ve pointed out total lightning data’s operational utility a number of times in this blog. After all, the data have been a rather integral part of our severe weather operations for at least 13 years. Anyway…I’m going to do it again. I think it can be beneficial to reiterate the value of certain data sets from time to time, especially to reemphasize their operational utility to new members of the forecasting and research community and perhaps newcomers to the SPoRT blog.
This afternoon and evening was a somewhat typical summertime convective event for the Tennessee Valley. Showers and thunderstorms developed in the early afternoon and gradually increased in coverage and intensity during the mid to late afternoon hours. By the time I arrived on shift at about 3 pm CDT, a few thunderstorms were showing signs of intense updrafts (~50 dBZ at the -10C isotherm level), but were still not to the level of producing severe weather. Nevertheless, multiple outflow boundaries interacting with the hot, humid and unstable airmass caused decent coverage of shower and thunderstorm activity, especially in northeastern portions of Alabama during the mid afternoon into the early evening. A few thunderstorms contained strong updrafts, heavy rainfall, frequent lightning and wind gusts up to about 40 mph. The first of these started showing signs of strengthening in eastern portions of DeKalb County, AL shortly after 3 pm CDT. The first image below (image 1) shows a snapshot of total lightning data (flash extent density) from the North Alabama Lightning Mapping Array (NALMA) at 2014 UTC. Values at this time in the developing storm were just around 10 flashes per 2-minutes. By 2022 UTC however, flashes had increased to nearly 50 flashes per 2-minutes (Image 2).
Importantly, increases in total lightning activity are directly related to updraft strength within storm cells so it was no surprise that reflectivity values increased correspondingly. The next two images show the increases in Multi-radar Multi-sensor (MRMS) isothermal reflectivity (dBZ) at the -20 C level during the same period (Images 3 and 4).
Data such as the MRMS isothermal reflectivity when used in conjunction with other data such as total lightning (or flash extent density) allow for a good evaluation of updraft development within thunderstorms and their evolution through time. Environmental parameters on this day suggested that severe weather was not likely. Nevertheless, the strengthening updrafts were followed by wind gusts around 30 to 40 mph, which were recorded at a few of our surface observation sites. Special Weather Statements were used to address this marginal thunderstorm threat during the afternoon and evening. Interestingly, notice that the total lightning data at 2022 UTC (Image 2) indicated that the updraft in the northern cell in DeKalb County was perhaps the strongest at the time (due to higher values on flash extent density), while MRMS reflectivity values were higher at the same time in the southern cell (image 4). Subsequently, the northern cell strengthened and became the dominant cell over the next 30 minutes. On days such as this when there are often multiple thunderstorms ongoing at any one time, and this happens often here in the TN Valley in the summertime, total lightning data can be an effective situational awareness tool for evaluating storms that are undergoing strengthening and helping to provide proper focus for operational meteorologists.