This year marks a rare convergence as two significant cicada broods—the 13-year Brood XIX and the 17-year Brood XIII—emerge simultaneously, an event that occurs only once every 221 years. These broods are examples of periodical cicadas, which synchronize their development and emerge in unison every 13 or 17 years, contrasting with annual cicadas that emerge yearly. And given the scope of this year’s periodical broods (Brood XIX is the largest by geographic reach, and Brood XIII is the largest in size), 2024 will bring a once-in-a-lifetime level of cicada activity to the U.S. Though the co-emergence of Brood XIII and XIX will increase the overall activity of cicadas in the U.S. this year, few locations will be impacted by both broods simultaneously. Researchers calculated the percentage of counties in which either Brood XIII or Brood XIX is expected to emerge, then ranked states accordingly. Researchers also calculated percentages and totals for the individual broods and their overlap, where applicable.