2 trillion gallons of water trigger historic flooding in Hawaiʻi

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More than 2 trillion gallons of water-enough to fill 3 million Olympic-sized swimming pools-inundated Hawaiʻi in March. The accumulated rainfall over 14 days reached as high as 3,000% of normal historical levels for this time of year, culminating in a destructive "rain bomb" over Oʻahu. Through the University of Hawaiʻi's Hawaiʻi Mesonet and the Hawaiʻi Climate Data Portal, researchers captured the scale of these back-to-back Kona low systems, mapping localized threats and providing crucial data on the state's severe flooding.

Between March 1 and March 23, statewide rainfall averaged 18.25 inches-more than 2.6 times the standard March average of 6.85 inches.

While the first storm (March 10-16) brought hurricane-force wind gusts of 135.4 mph to Hawaiʻi Island and up to 62 inches of rain to Maui, the second Kona storm between March 19 and 23 triggered a new wave of devastation. The second storm dumped up to 61 inches of rain in localized areas, producing destructive floods across eastern and central Molokaʻi, West Maui and Oʻahu.

Communities such as Waialua and Haleiwa on Oʻahu's north shore experienced devastating inundation. Hawaiʻi Mesonet stations highlighted the widespread intensity of the storm: the Kaʻala station recorded the island's highest two-day rainfall of roughly 22 inches, including 19.67 inches in a 24-hour period beginning the evening of March 19. Almost simultaneously, the nearby Kalaheʻe Ridge station above Waimea Valley recorded 9.75 inches in 24 hours.

Mānoa, Palolo flash floods

As the two-week rainy period neared its end, an intense, localized atmospheric event struck Mānoa and Palolo valleys on March 23. Honolulu Mayor Rick Blangiardi described it as a "classic rain bomb," heavy rain caused by a stationary storm cell. This "rain bomb," over Mānoa and Palolo valleys, dropped 2 to 4 inches of rain per hour.

Six Hawaiʻi Mesonet stations in the Nuʻuanu-Mānoa area recorded between 3.5 and 6.5 inches of rain, the majority of which fell within a three-hour window. Falling on already saturated ground, this turned streams into raging torrents and triggered significant flash floods.

The Hawaiʻi Mesonet, a statewide network of state-of-the-art weather stations, is proving to be a critical source of weather information, especially valuable during extreme events.

"We are building the mesonet to serve multiple purposes, including research, resource management, support for farmers and ranchers, and others," said Thomas Giambelluca, Hawaiʻi Mesonet project lead, and former director of the UH Water Resources Research Center. "But, providing data when and where it is most needed before and during extreme events like floods and wildfires, might be its most important purpose. Mesonet data will make us better prepared for future events by improving weather forecasts and enabling emergency managers to plan for and respond to extremes."