A messy area of thunderstorms in the Pacific Ocean, seen here Tuesday morning, is likely to strengthen into a tropical storm as it nears Hawaii. It could drop anywhere from less than a half foot to more than 3 feet of rain on the Big Island and Maui.
NOAA/NESDIS Center for Satellite Applications and Research
Service members with the Hawaii National Guard walk through a field of debris after Hurricane Lala in Naalehu, Hawaii, on Aug. 17. Another big rain system could hit Hawaii this week.
Stephen Lam/S.F. Chronicle
A potential tropical storm could drop anywhere from a few inches to more than 3 feet of rain on Hawaii this week. The forecast is very uncertain, but the Big Island and Maui are most at risk from the system, which poses the biggest threat Thursday through Saturday.
The wide range in potential rainfall stems from a messy area of thunderstorms southeast of the state. The National Weather Service gives it a 90% chance of developing into a tropical storm, but where it goes after that is unclear.
“The general consensus is that the storm will remain south of the islands,” the weather service said. “However, there is a large spread concerning how far south … from just barely to several hundred miles.”
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The accuracy of the forecast is “not likely to improve until the system actually forms and becomes something that the models can ‘sink their teeth into,’” the weather service said.
Air near Hawaii is forecast to hold record amounts of moisture later this week, regardless of tropical storm formation. This abundant water vapor means rainfall could be unusually heavy, especially near the islands’ volcanoes.
Record levels of moisture are forecast in the atmosphere around Hawaii on Thursday. This particularly humid air will make flooding more likely if the tropical storm makes a close pass to the state.
Baron/Lynx
The risk of flooding and landslides will be much higher if the storm moves close to Hawaii.
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“This makes all the difference as modeled rainfall totals run the gamut from being measured in inches over a day or so to measured in feet over several days,” the weather service said.
The National Weather Service forecasts 15 to 20 inches of rain on the southeast side of the Big Island, the same place that was hammered by flooding from Hurricane Lala. Forecast totals may increase or decrease depending on the exact track of the tropical rainstorm.
National Weather Service
Moderate rain could begin as early as Wednesday on the Big Island and possibly Maui, but downpours were expected to increase Thursday into Friday.
Of particular concern is the slow movement of the potential tropical storm.
Unlike Hurricane Lowell, which accelerated as it passed Kauai earlier this month, this week’s system is crawling across the Pacific. An extended window of heavy rain through Saturday would expose the Big Island and Maui to a heightened risk of flash floods and landslides. Rainfall in excess of 3 feet is possible on the Big Island if the storm tracks close, which could even surpass totals from Hurricane Lala.
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“There is a significant potential for a prolonged heavy rain event over windward and southeast Big Island due to the deep tropical moisture and weak steering flow,” the weather service said.
Downpours are likely even with a tropical storm that stays farther away from the islands. Rainfall totals in this case would probably be less than a foot, but damage could be just as bad if it comes within a few hours.
The weather service said it planned to issue and flood watch and wind advisory for at least some portion of the state.
“How strong the system gets, combined with how far away it is, will determine wind speeds,” the weather service said. “At this time, we are forecasting gusts over 70 mph on the summits of Big Island (over 60 on Haleakala) late this week, but that is subject to significant change.”
Ocean temperatures around Hawaii are record high, in part due to long-term global warming, and short-term effects of El Niño. These warm waters fuel hurricanes, and make downpours more extreme.
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In the past two months, four tropical storms have passed within a few hundred miles of Hawaii. Accumulated cyclone energy, a unit that accounts for the duration and strength of tropical storms, is about three times higher than a typical year in the central Pacific, including the waters around Hawaii.
Flood impacts from winter kona storms to summer hurricanes are compounding across the state.
“There is concern that the heavy rainfall in the case of a closer brush with this system will occur over the same areas impacted by Hurricane Lala, though this remains far from certain,” the weather service said.