A record-breaking rainstorm over Kaua’i, Hawaii in April 2018 caused severe flash flooding and estimated nearly $180 million in damage. The deluge damaged or destroyed 532 homes and landslides left people along Kaua’i’s north shore without access to their homes. In a recently published study, atmospheric scientists from the University of Hawai’i at Mānoa revealed that severe supercell thunderstorms were to blame.
The rainstorm inundated some areas with nearly 50 inches of rainfall in a 24-hour period, shattering the previous 42-inch US 24-hour rainfall record set in Texas in 1979. An interesting finding is that the rainstorm described in this article was associated with a Kona Depression and not a tropical cyclone as featured in previous US rainfall records.
Terrence Corrigan, a graduate student, and Professor Steven Businger, both in the Department of Atmospheric Sciences at the UH Mānoa School of Ocean and Earth Science and Technology, sift through extensive NOAA National Weather Service weather radar data to uncover the contributing factors to the historic event. Their analysis revealed large changes in the direction and speed of winds in the lower atmosphere. As these shifting winds collided with the sheer cliffs of Kaua’i, thunderstorms with rotating updrafts were unleashed. The rotation scale seen in radar data and the amount of precipitation seen as bounces are consistent with supercell thunderstorms.
“This finding was a surprise that has interesting implications for other mountain regions around the world,” said Businger.
“Rotating thermals are more intense and long-lasting and have been observed to produce large hail and tornadoes in Hawaii,” Businger said. “In this case, the updrafts were forced through the sheer mountain cliffs of Kaua’i, with the result that the thunderstorms were more violent and anchored to the terrain, setting a new 24-hour rainfall record in the United States!”
Although supercell thunderstorms are the least common type of thunderstorm in Hawaii, they are the most likely to produce severe weather, including large hail, tornadoes, and strong straight-line winds.
“Understanding the dynamic interaction of our tropical atmosphere and steep mountains will help weather forecasters better anticipate severe weather events and flash floods in our state and elsewhere.”
Businger and Corrigan’s next steps are to use computer models to simulate the interaction between different wind currents and terrain configurations to further shed light on the interaction of mountains and severe thunderstorms.