Destructive flash flooding struck Hawaii on Monday and Tuesday, damaging dozens of homes and causing bridges to collapse. Evacuation orders were issued as rain fell at rates near three inches per hour, threatening dams and leaving some roads impassible.
Oahu and Maui were hardest hit, where up to 20 inches of rain fell in just 48 hours between late Sunday and Tuesday. In northern Oahu, hundreds were forced to flee their homes Tuesday in the town of Haleiwa because of surging floodwaters before the evacuation order was lifted early Wednesday.
The heaviest downpours hit Maui on Monday before shifting to Oahu, though torrential downpours continued off and on across the archipelago even into Wednesday.
Gov. David Ige (D) signed an emergency proclamation in the wake of Monday’s and Tuesday’s flooding.
Deluge on Maui
On Monday, heavy rainfall over the Kaupakalua Reservoir in Maui sparked fears of overtopping of the dam, prompting downstream evacuations and the closure of the Hana Highway. West Wailuaiki on northeast Maui picked up 19.21 inches of rain through late Tuesday, while Puu Kukui, a 5,788-foot-high volcano on western Maui, recorded 11.32 inches.
“East Maui is not accessible,” wrote the National Weather Service in Honolulu in a flash flood warning for Maui issued Monday. “Rain was falling at a rate of 2 to 3 inches per hour.”
Advertisement
The downpours that produced the flooding were very slow-moving but highly localized; Hana Airport on Maui only received 3.75 inches during that same time frame.
The Weather Service in Honolulu also noted “severe flooding from Haiku to Makawao,” reporting that six homes were severely damaged or destroyed. The Peahi Bridge on Maui was washed out, while the Kaupakalua Bridge was “displaced and impassible.”
Torrential rains in Oahu
The heavy rain shifted west to Oahu on Tuesday, where rainfall rates flirted with four inches on the windward, or northeastern, side of the island where a flash flood emergency, the most severe flood alert, was declared.
“CATASTROPHIC FLOODING FROM OPAEULA STREAM: EVACUATE NOW from Haleiwa,” tweeted Oahu Emergency Management on Tuesday. Evacuation sites were opened at the Waiahole and Kahuku Elementary schools, Kahaluu Community Park and Kaneohe District Park.
Advertisement
The Kahana rain gauge on northeast Oahu reported a total of 10.78 inches late Tuesday. Upsloping, or the process through which moist air is forced up a mountain and transformed into heavy rain, was a major contributor to the high-end totals.
Video emerged from Haleiwa, home to nearly 4,000 residents, showing vehicles submerged and water up to the front doors of homes. The Kamehameha Highway was closed for a time, with at least two homes reportedly suffering flood damage early Tuesday afternoon. The Weather Service warned of potential landslides in the steep terrain.
Heartbreaking footage of the flooding going on in #Hawaii right now. This is Haleiwa town. It’s nearly 4,000 residents were asked to evacuate.
Vid Cred: Ryan De Seixas pic.twitter.com/oLtKNw7IeL
The Waiahole Stream, located above the Kamehameha Highway, reported flow rates of nearly 9,000 cubic feet per second. The previous record, obtained through 19 years of maintaining data, was set in 2012 and was 432 cubic feet per second. Typical flow is around 90 cubic feet per second — just 1 percent of what was observed Tuesday.
Advertisement
The band of downpours continuously back-built for several hours, meaning it grew upwind at the same rate that clouds and individual updrafts were drifting downwind. Consequently, a number of locations sat beneath the heavy band for hours on end.
This one is not getting the attention it deserves. Catastrophic flooding is occurring in parts of Hawaii from nearly stationary INSANE rain right now. This is not going to end well. pic.twitter.com/byP9fUod5p
— Marc Weinberg (@MarcWeinbergWX) March 10, 2021Once again, amounts were more ordinary outside the band. Daniel K. Inouye International Airport in Honolulu reported a two-day total of 1.23 inches. The airport is only located 12 miles from where the heaviest rain fell, but on the west side of the mountains — meaning the city was in the “rain shadow” and escaped most of the precipitation.
By Wednesday, the axis of heaviest rain had shifted west to Kauai, but downpours had yet to abandon the island chain. Flash flood warnings were up both on Kauai and the Big Island — at opposite ends of the volcanic archipelago — where dangerous flooding continued.
Heartbreaking footage of the flooding going on in #Hawaii right now. This is Haleiwa town. It’s nearly 4,000 residents were asked to evacuate.
Vid Cred: Ryan De Seixas pic.twitter.com/oLtKNw7IeL
The Kuhio Highway was closed at the Hanalei Bridge on Kauai, though Doppler radar indicated some improvement was probably around sunrise.
Advertisement
Significant flash flooding was unfolding on the Big Island, meanwhile, where over half a foot of rain had already fallen by 5:30 a.m. local time. The stagnant band could drop another six inches or more by the time it either moves away or dissipates, and it’s probably that some locations could wind up with totals exceeding a foot.
A flash flood watch remained in effect for all of Hawaii through Wednesday afternoon local time.
March is often a stormy time in Hawaii, a time of year when larger-scale storm systems can bring heavy precipitation and occasional severe weather. This week last year featured twin tornado warnings issued in the state, the first since 2008. And on March 9, 2012, a record 4.25-inch hailstone fell on Oahu from the same rotating thunderstorm that produced an EF0 tornado.
ncG1vNJzZmivp6x7uK3SoaCnn6Sku7G70q1lnKedZMSmrdOhnKtnYmV%2FcnuPbGZqaF%2BdrrityKJkn6SRqLVussuopp2hnpx6prnEq56eppOufA%3D%3D