The National Hockey League team in Tampa Bay is called the Lightning for good reason. Central Florida has long been the lightning capital of the United States, and 2018 was no different (sort of). Vaisala recently issued its Annual Lightning Report, and not surprisingly, Florida had the highest mean cloud-to-ground (CG) lightning flashes per square mile, followed by Louisiana, Arkansas, Mississippi, and Oklahoma. Keep reading however, and you'll see that a couple of other states can rightfully claim "lightning superiority" depending on the perspective of the question. Irrespective of who claims leadership, there was something surprising in the lightning report by Vaisala.
Lightning strikes in the U.S. decreased by 11% in 2018. Why?
The National Lightning Detection Network (NLDN) is an important asset used by meteorologists, and this week at the American Meteorological Society (AMS) Annual Meeting, Vaisala released its lightning report. They operate the NLDN and collect a wealth of information about lightning each year. According to a press release issued by the company,
there were nearly two million fewer lightning strikes in the U.S. in 2018 compared to the 10-year average...2018 had 17,804,321 negative cloud-to-ground flashes, the most common cloud-to-ground lightning. This marks the third-fewest flashes in the last 10 years and a 1.9 million flash decrease from the 2009 - 2018 average of 19,728,634 lightning flashes across the U.S. annually.
"Okay," you might be saying, "this is interesting, but what does some of that terminology mean?" Most lightning is intracloud or cloud-to-cloud (CC)lightning. Cloud-to-ground (CG) lightning flashes have a huge "so what" factor because that is what can strike you, me, a house, or a tree. CG flashes transport negative charge to the ground. There are positive CGs, but they are less common. However, they can also be dangerous because they are often associated with "blue sky" lightning as I have previously discussed in Forbes. Kevin Ambrose wrote in the Capital Weather Gang of the Washington Post that "positive lightning carries a much greater charge and a longer flash duration than negative lightning, reaching up to 1 billion volts and 300,000 amps, compared with 300 million volts and 30,000 amps with negative lightning." Plenty of people are struck by positive and negative CG flashes.
Okay, why did lightning activity decrease in the United States last year? The answer probably lies in basic meteorology. According to Ryan Said, a lightning expert at Vaisala,
[T]he likely reason for the reduction is simply that there were fewer big storms... Specifically, there were fewer days with very strong air mass contrasts across the Central Plains and Upper Midwest during the spring and summer of 2018, which contributes to the severe weather season.
This explanation is also consistent with reports that tornadic activity at the highest end of the Enhanced Fujita scale was also down in 2018.
The Vaisala report confirmed that the Tampa Bay Lightning probably doesn't have to consider a name change anytime soon. As noted early, Florida had the highest mean flash counts per square mile and was home to 14 of the 15 counties that receive the most lightning (first graphic above). Florida is so lightning-active because of its unique geography. I actually studied Florida storm activity for my doctoral dissertation many moons ago at Florida State University. Because Florida is a peninsula, there are prevalent interactions among sea breeze fronts from both coasts, outflow boundaries from previous storms, and lake breezes from Lake Okeechobee. Such interacting boundaries create a source of lift, a needed ingredient for storms, in the very moist Florida environment.
Though Florida is still the leader in terms of flash counts per square mile, the Vaisala report noted that Texas had the highest total flash count at 2,483,805 negative cloud-to-ground flashes. Florida ranked second with roughly 1,385,710. Geographically, Texas is physically larger so this makes sense. Harrison County, Mississippi, had the highest 10-year average of flashes but not for 2018.
I guess the answer to who leads depends a little bit on how you ask the question.
