This month’s record-breaking Kona-low storm that saw torrential rain, strong winds and floodwaters rushing through the streets of Honolulu was a sneak preview of Hawaii’s future during a warming planet.
Some of the year’s highest tides, on top of higher sea levels, plus some sizable wind-driven waves, conspired to make an already powerful storm that much worse, especially when it came to the flooding in coastal areas such as Waikiki and Kakaako.
“It was a compound event, a classic natural hazard with a climate change twist of lemon,” said Chip Fletcher, University of Hawaii professor of geology and coastal researcher.
The flooding not only damaged hundreds of properties across the island, it triggered at least nine Oahu sewage discharges into the ocean and swamped a Hawaiian Electric substation, knocking out power to thousands of residents and scores of buildings in downtown Honolulu.
As bad as it sounds, it could have been much worse.
John Bravender, warning coordination meteorologist with the National Weather Service in Honolulu, said fewer large bands of heavy rain and thunderstorms traveled across Oahu than forecasters originally feared. What’s more, a particularly large band of rain, estimated to have been dumping up to 3 inches an hour, moved offshore in the channel between Oahu and Kauai.
“If there had been a slight shift in location (of that band), Oahu could have gotten a lot more rain,” Bravender said. “Actually, we were lucky.”
Adam Weintraub, spokesman for the Hawaii Emergency Management Agency, said the Kona storm could have lingered over the islands for as long as two more days.
“The worst-case scenario was a lot worse than what we got,” Weintraub said.
As it was, heavy rain and strong winds swept over the state for three days, Dec. 5 to 7, dropping anywhere from 20 inches on the south-facing slope of Haleakala on Maui to nearly 6 inches at Hilo International Airport.
OAHU CONSISTENTLY appeared to get much of the worst of it, with three-day totals across the island ranging from 8-plus inches to nearly 13 inches.
Most of that precipitation fell Dec. 6, when Daniel K. Inouye International Airport recorded nearly 8 inches of rain, breaking the daily rainfall record and the single- day record for December. The 7.92 inches also came in second for the all-time single-day rainfall record, falling short of the previous record in 1958.
Kona storms typically occur during the hurricane offseason and are linked to an area of low pressure that sets up west or northwest of the islands.
Hawaii is ordinarily dominated by the tradewinds that blow in from the northeast. But the counterclockwise flow around a Kona low turns things around, bringing southwesterly winds sweeping over the islands from the Kona, or leeward, side.
These storms, which get their energy from the jet stream, draw moisture from the warm tropical waters to the south and unleash their fury in the form of heavy rain, hailstorms, flash floods, landslides, high winds, large surf, waterspouts, thunderstorms and blizzards atop Hawaii’s tallest mountains.
“This was a classic Kona low,” said Steven Businger, University of Hawaii professor of atmospheric sciences. “It really focused its atmospheric river on Maui and Oahu.”
Flash floods filled the parched gulches west of Kaupo on Maui and closed Piilani Highway and roads in the Kula area, while floodwaters swept over the Maui Meadows subdivision in South Maui, damaging homes and washing away vehicles.
ON OAHU a section of the H-1 freeway westbound through Honolulu was closed with waist-deep water near the Vineyard Boulevard offramp. On the Windward side of Oahu, flooding shut down Kamehameha Highway from Waiahole to Waikane.
Fletcher, the climate scientist and associate dean of UH’s School of Ocean and Earth Science and Technology, said the storm’s flooding was exacerbated by different factors, including the fact that the Honolulu tidal gauge showed ocean water levels 6 to 8 inches above the normal tide, which was already supposed to be at high tide. Strong Kona winds and waves of 6 to 10 feet also pushed water onshore and raised the water level on the coast.
“Then it was, ‘Let’s rain on top of that mess,’” he said. “Mother Nature was conspiring to bring multiple forces at once.”
Fletcher and his UH research group have identified coastal flooding as a major problem as sea level rise continues under the force of climate change and lifts the water table ever higher.
A 2017 study by colleague Shellie Habel and Fletcher found that groundwater inundation in urban Honolulu and Waikiki eventually will threaten some $5 billion in taxable real estate, nearly 30 miles of roadway and lots of other facilities and assets.
And it will happen regardless of whether any seawalls are built, a fact that will require special engineering, costly public works and other measures to deal with the standing water that will become commonplace in the decades ahead, the study said.
“Hopefully, we can learn something from this,” Fletcher said.
