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Lane’s extreme rain in Hawaii triggers major flooding

By Alex Sosnowski, AccuWeather senior meteorologist
By Kristina Pydynowski, AccuWeather senior meteorologist
August 26, 2018, 9:38:59 AM EDT

Lives and property will remain at risk across Hawaii as Tropical Storm Lane will continue to unload feet of rain, gusty winds and dangerous surf into Saturday night.

Lane has the potential to be the single-costliest hurricane in recorded history of Hawaii and may end up causing the most expensive hurricane damage in all of the United States for the 2018 hurricane season, according to AccuWeather President and Founder Dr. Joel N. Myers.

Hurricane Iniki caused just over $3 billion in damage to Hawaii in September 1992, which would be about $5 billion in today's dollars, according to Myers.

"Lane could potentially cause in excess of $10 billion in damage due to its forecast impacts as well as the increase in population, property, property value and infrastructure on the Hawaiian Islands since then," Myers said. The damage from Lane will be mainly from flooding, high tides and beach erosion rather than wind.

Hurricane Lane Aug. 25 NOAA/GOES Satellite

This animation shows Tropical Storm Lane spinning west of the Big Island of Hawaii on Saturday, Aug. 25, 2018. (NOAA / GOES Satellite)

Lane has already unloaded more than 40 inches of rain across parts of the Big Island, leading to catastrophic flooding and debris flows. A hurricane-force wind gust of 74 mph was reported at Oahu Forest National Wildlife Refuge on Thursday night.

Lane weakened from a hurricane to a tropical storm on Friday afternoon, but that has not lessened the risk to lives and property. As anticipated by AccuWeather, the worst impacts of the storm have and will continue to be extreme rainfall.

AccuWeather is predicting a Local StormMax™ of 60 inches, which would approach the United States tropical rain record of 60.58 inches that was set during Hurricane Harvey near Nederland, Texas, one year ago.

Track Lane 8.25 AM

Lane is expected to increase its forward speed slightly later on Saturday and track to the west during the weekend. Further weakening is expected through this time.

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Flooding rainfall and mudslides are greatest concern

"Residents and visitors to the Hawaiian Islands need to be very cautious and wary this weekend and stay alert, watching out for downed trees and power lines, potential flash localized flooding hazards, mudslides, debris and unusual phenomenon that can occur after a storm," Myers said. "There may be casualties and fatalities as a result of flooding or the effects of the agitated state of the ocean, including rip currents and rough seas. If people heed warnings and forecasts, the death toll will most likely be less than 10. If people don’t heed the warnings, it could be a multiple of that. To remain safe, residents and visitors can track the storm on our free AccuWeather app."

In general, Harvey-like rainfall of 1-3 feet is expected by AccuWeather meteorologists in the higher-elevated areas because of the slow movement of Lane. The highest rain amounts are anticipated on Maui County and the Big Island.

An AccuWeather Local StormMax™ of 60 inches is forecast where rain persists along the steep mountainsides of the Big Island.

That would be the most rain from a tropical cyclone in Hawaii's recorded history. The current record is being held by Hurricane Hiki and the 52 inches that fell in August 1950.

This amount of rainfall will lead to extensive flash flooding, mudslides and debris flows as the torrential rain cascades down the higher mountainous areas into the lower elevations.

Expect more bridges and roads to be washed out or severely damaged.

Honolulu is expected to escape the heaviest rainfall. However, downpours can still trigger localized urban flooding and travel hazards.

Downpours can reduce visibility and heighten the risk of vehicles hydroplaning when traveling at highway speeds.

People should not attempt to drive through flooded areas or venture near the edge of streams, even after the heavy rain has eased. The road may have been compromised and the banks of the stream may give way without warning.

Risk Aug 25 pm

Expect mudslides and landslides to occur without notice and to block roads.

"People living in flood- and mudslide-prone areas should be prepared to evacuate," AccuWeather Hurricane Expert Dan Kottlowski said.

Even after Lane moves away on Sunday and into Tuesday, a continued stream of tropical moisture will enhance showers across the islands. Flooding problems may only grow worse, and any necessary clean-up operations could be hindered.

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Tropical-storm-force winds may trigger power outages

Wind gusts of 25-40 mph will continue to buffet parts of the islands from Maui County to Kauai into Saturday evening. Winds will gradually lessen from east to west as Lane slowly departs.

The winds can push weak trees over and cause additional sporadic power outages.

Prior to Lane's heavy rain arriving, the gusty winds fanned a brush fire in the Kauaula Valley in Lahaina.

Seas remain too dangerous to enter

Dangerous seas will continue to pound the islands to start the weekend, but will gradually lessen from east to west as the weekend progresses.

People should not stand or walk along the immediate coast and not venture onto breakwaters and jetties.

"Expect extensive overwash while relentless, pounding waves are likely to cause damage to structures, walkways and roadways near the coast," Kottlowski said.

Lane impacts Sat Aug 25 pm

The chaotic nature of the pounding waves and frequent and strong rip currents will be too dangerous for anyone to be in the water, including experienced surfers.

"All of the impacts from flooding, wind and heavy seas can be life threatening," Kottlowski said.

When compared to Iniki, it is possible that wind conditions may be less severe, but flooding rainfall may be more severe due to Lane's slow-moving nature.

No immediate additional threats in Lane's wake, but pattern has potential for more hurricane threats

While there are no additional threats from the tropics in the short term, additional threats from tropical storms and hurricanes are likely into the autumn, due to a developing El Niño.

Because El Niño is a plume of warmer-than-average waters over the tropical Pacific Ocean, the warm water can sustain more hurricanes than average over the eastern and central Pacific, cause them to be stronger in nature and allow them to retain strength for a longer period of time as they approach Hawaii.

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