The NWS office in Huntsville, AL (HUN) has had a long history with the use of total lightning data in operations, which stretches back to the office’s inception (after NWS modernization) in 2003. Back then, and until its removal to South America for GOES validation testing, the HUN office largely used data from the North Alabama Lightning Mapping Array (NALMA). Lightning data sources from NLDN and ENTLN have also been used to varying degrees, but the advent of the GLM aboard GOES-16 brought a new era of lightning observations. Because of the office’s participation in early operational testing of the GLM, its use and familiarity have gradually increased over the past year. This was probably made easier due to our familiarity with total lightning data from the NALMA network. Generally, GLM data have been used in much the same way as those from the NALMA network, especially with regards to situational awareness purposes (i.e., airport weather warnings, real-time weather watches for EM partners, initial cell electrification, etc.). The use of the data to aid in severe weather warning decisions has been a bit slower to evolve, as might have been expected. After all, there are differences in the way the NALMA observes lightning as compared to the GLM. Values from the GLM have typically been “muted” compared to those from NALMA, so forecasters have had to make internal adjustments and recalibrate, if you will, what is considered significant. However, the physical mechanisms that generate increases in total lightning, that is, increases in mixed-phase updraft volume, are essentially observed either way. Thus, GLM data can still be useful to relate important information about storm/cell evolution, and can help to “tip the scales” in the balance of evidence about whether or not a warning may be needed.
This particular application of the GLM data occurred this morning with operational meteorologists at the HUN office. The short image loop below shows thunderstorms moving across northwestern portions of Alabama between 1226 and 1300 UTC. The top panel of the image contains data from the KGWX radar (0.5 degree reflectivity), while the bottom panel contains GLM 1-minute Flash Extent Density (FED) data. Notice that lightning activity is relatively limited initially as the storm moves across western Franklin County, AL (near center of image), with 1-min FED values ranging between 2 and 6 flashes per minute. Then, at the 12:37 UTC time mark, flashes begin an increase that manifests in a statistical lightning “jump” (GLM sigma > 2). The warning meteorologist at the time was watching this cell for potential severe weather, and observed the sudden increase in FED values. This, together with other radar and satellite observations (not shown here), suggested that a severe weather warning was necessary as wind signatures aloft gradually increased. A warning was subsequently issued at 1245 UTC. Incidentally, this thunderstorm did end up producing some wind damage, with trees reported down in south-central portions of Franklin County. Notice also that a number of strong cell signals were detected by radar as indicated by the higher dBZ values across the domain. Another use of the GLM is allowing meteorologists to focus on the cells with the strongest updrafts, making the overall radar interrogation and warning process more efficient. This case can help to demonstrate that the GLM can be used as an important indicator of storm evolution and as a useful operational tool for the evaluation of severe weather potential.