Short, Intense Storms Are Getting Stronger Across Hawaiʻi, Study Finds

Hawaiʻi's long-term rainfall is declining overall, but the short, intense downpours that trigger sudden flash floods — flooding when heavy rain overwhelms streams and drains faster than people can react — are getting stronger, a new study by the University of Hawaiʻi at Mānoa has found, published in the International Journal of Climatology.

The research, from the University of Hawaiʻi (UH) Mānoa's College of Tropical Agriculture and Human Resilience (CTAHR), is the first comprehensive analysis of extreme hourly rainfall across the state, covering 50 years of data (1966-2020) from 117 rain gauges on Kauaʻi, Oʻahu, Molokaʻi, Maui and Hawaiʻi Island — trends that 24-hour daily averages often miss. "If we want to understand extreme weather, like storms and floods, we really need to look at short-duration, high-temporal-resolution data rather than daily or monthly averages," said Yinphan Tsang, a CTAHR professor and the study's principal investigator.

Common heavy rains, occurring about once every two years, are generally becoming less intense statewide. But short, sudden bursts lasting one to three hours — rare, severe storms that typically strike once every 20 to 100 years — are getting significantly stronger, while storms lasting six hours or longer are weakening more. Because of Hawaiʻi's mountains and island terrain, rainfall intensity can rise in one area while dropping a few miles away.

Researchers identified two periods when Hawaiʻi is most vulnerable to extreme hourly rain — October through December, and February through March — generally driven by cold fronts, Kona storms and other winter systems. The February-March events, near the end of the wet season, were unexpected and could catch people by surprise; tropical storms can also bring extreme rainfall in summer.

The findings are meant to inform emergency responders, county planners and infrastructure designers. The study was a collaborative effort across UH Mānoa's CTAHR, School of Ocean and Earth Science and Technology, College of Social Sciences and Water Resources Research Center, compiled primarily by Maxime Gayte with co-authors including Tsang and Pao-Shin Chu, the Hawaiʻi State Climatologist.