December and January have been very busy for tornadoes across the NWS Huntsville County Warning Area. Eight tornadoes, including two EF-2 tornadoes, impacted the area on December 16. One additional tornado occurred on December 29. Forecasts and outlooks leading up to January 11 suggested that it, too, would be another busy day for the region.
Flash Extent Density data from GLM were crucial while issuing warnings during the December 16 event. So one of the first things I did on January 11th was load a procedure that combined the mesoscale sector legacy IR band (10.3 µm), one-minute ENTLN cloud to ground and cloud flash lightning data, and the one-minute GLM flash extent density. This was done as a quick way to identify strengthening updrafts, as it is easy to see cloud tops cool and lightning activity increase using one-minute data. This proved to be especially useful on December 16th as lightning jumps were noted before most tornadoes occurred, and therefore became a key component of increasing lead times with warnings. On January 11th as the QLCS was starting to surge into west central Alabama, we noticed a strong lightning jump that preceded the deadly Pickens County tornado. So, from that point we knew GLM was going to be useful once again in warning operations.
Tornado warnings had been issued based on the Three Ingredients Method earlier as the line crossed Cullman County, as can be observed in the image loop below. However, it became increasingly challenging to determine whether to extend tornado warnings downstream due to data limitations:
- few if any damage reports
- limited radar data since KHTX was down at the time
- KGWX and KBMX only capturing the mid-to-upper level portions of the storms
- ARMOR data coming in sporadically
KGWX and KBMX indicated rotation aloft over Cullman and Marshall counties, but we did not have a good feel for what was happening closer to the surface. A modest increase in lightning over Cullman County provided the confidence to issue a downstream tornado warning at 1907 UTC. At 1910 UTC, we noticed a relatively sharp uptick in lightning activity on GLM, the sharpest we had seen in our area all day, which further increased our confidence. The strongest uptick was noted at 1919 UTC, which preceded the damage at Union Grove by approximately 5 minutes. This, combined with the line segment becoming oriented more perpendicular to the 0-3km bulk shear vectors (upper left box), created enough confidence to go ahead and issue another warning downstream into portions of Jackson and Madison counties.
The first warning including Union Grove was issued 17 minutes before an EF-2 tornado hit Brindlee Mountain Primary School (indicated by Home marker in image below), and the second was issued as the tornado was impacting the school. We also noticed that the modest increase in lightning over Cullman County occurred close in time to both the Holly Pond and Joppa, AL tornadoes.
GLM was critical to our warning decision-making process at a time when radar data were limited. Furthermore, it helped us communicate the urgency of the situation to Marshall County first responders as the Union Grove event unfolded.
— Ashley and Brian (WFO Huntsville)