Short, intense storms gain intensity across Hawaiʻi

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While long-term data shows that Hawaiʻi is getting drier overall, short, intense downpours that cause sudden, dangerous flash floods are getting stronger in the islands, according to a new study by researchers at the University of Hawaiʻi at Mānoa.

Published in theInternational Journal of Climatology, the research from the College of Tropical Agriculture and Human Resilience (CTAHR) is the first comprehensive study to analyze extreme hourly rainfall patterns across the state over 50 years (1966-2020). The team examined decades of hourly data from 117 rain gauges across Kauaʻi, Oʻahu, Molokaʻi, Maui and Hawaiʻi Island, uncovering climate trends that traditional 24-hour daily averages often miss.

Bursts of intense rain

In steep, mountain-filled tropical watersheds like Hawaiʻi's, bursts of intense rain lasting one to six hours can overwhelm stream channels and storm drains before emergency alerts are even issued.

"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 Professor Yinphan Tsang of CTAHR's Department of Natural Resources and Environmental Management and the principal investigator.

The study's major findings include:

• Common heavy rains, happening once every two years, are generally becoming less intense across the state.
• Short, sudden bursts of rain lasting one to three hours - the severe storms that usually occur only once every 20 to 100 years - are getting significantly stronger.
• Storms lasting 6 hours or longer are weakening more noticeably than shorter one- to three-hour storms.

Researchers noted that rainfall trends vary widely even between neighboring areas. Because of Hawaiʻi's unique mountains and island terrain, rainfall intensity can increase in one neighborhood while decreasing just a few miles away.

Researchers also identified two distinct peaks when Hawaiʻi is most vulnerable to extreme hourly rain events: October through December and February through March. These heavy rains are overall driven by cold fronts, Kona storms and other winter weather systems moving through the islands. The extreme events occurring from February through March, near the end of the wet season, were unexpected and could take people by surprise. Other systems, such as tropical storms, could also bring extreme rainfall during the summer season.

Vital data for climate adaptation

The findings offer updates for emergency responders, county planners and infrastructure design.

The study was a collaborative effort at UH Mānoa bringing together experts from CTAHR, the School of Ocean and Earth Science and Technology, the College of Social Sciences and the Water Resources Research Center. The research was primarily compiled by Maxime Gayte with co-authors including faculty members Professor Yinphan Tsang, Professor Pao-Shin Chu (Hawaiʻi State Climatologist), Professor Thomas W. Giambelluca, Jr., Researcher Yu-Fen Huang, and Researcher Hanpei Zhang.