The National Weather Service posted a flash flood watch this morning for the state of Hawaii, including Hawaii Island. As of now, the watch is effective through Friday evening.
According to forecasters, a kona low pressure system developing north of the state will bring the potential for heavy rain and thunderstorms over portions of the state through Friday and possibly continuing through the weekend.
“It’s a low pressure front with southerly winds, so it taps into the moisture from the tropics,” said Gavin Shigesato, a National Weather Service forecaster in Honolulu. “These definitely bring potential for heavy rainfall and flooding, due to the fact that there is a lot of moisture associated with it. These things end up lingering for days, so they are rainmakers.”
According to the watch document itself, the highest chances for excessive rainfall appear to be over Oahu and Maui counties later today and tonight, then slowly shift westward, with the focus mainly on Kauai by later Friday.
“The Big Island’s kind of tricky to forecast, actually, because a lot of that moisture, it’s going to be pocketed near Oahu and Maui. So the Big Island remains a little bit more of a question than a slam dunk like some of the smaller islands,” Shigesato said. “A lot of the moisture for (today) and Thursday is focused over the smaller islands, from Kauai through Maui County. There is that potential, though, with the southerly swell, that we would see rainfall over the interior sections, from Hilo, up north, through Laupahoehoe and Honokaa. But right now, it remains uncertain with the Big Island right now.”
According to the text of the watch, significant flooding could occur due to the overflow of streams and drainage ditches. Roads in several areas may be closed, along with property damage in urban or low lying spots due to runoff. In addition, landslides may also occur in areas with steep terrain.
NWS said it doesn’t have to be raining where you are for flash flooding to occur. If you notice signs of heavy rain or thunderstorms upslope, you’re advised to move quickly away from flood prone areas.
AccuWeather, a commercial weather forecasting firm, issued its own advisory concerning the kona low.
“While the heaviest rainfall is forecast to occur from Maui to Niihau, the Big Island can still see periods of heavy rain, especially in the higher terrain,” said Alex DaSilva, AccuWeather long-range meteorologist. “Localized areas in the higher terrain could see rainfall amounts in excess of 10 inches, which could cause very dangerous flooding and mudslides. Winds can be very strong in the higher elevations as well.”
Kagimoto advised the public to continue to monitor weather forecasts and to be prepared to take action should a flash flood warning — which means that flash flooding is imminent or already occurring — should be posted.
Email John Burnett at jburnett@hawaiitribune-herald.com