A shift in wind direction over Hawaii this week will set the island chain up for heavy rain, thunderstorms and high-elevation snow.
This will be a one-two punch of Kona Lows. The normally fair-weather pattern of prevailing northeasterly trade winds shifts to a southwesterly wind direction as low pressure develops west of Kauai – twice in less than a week.
Low-pressure systems have a counterclockwise flow around them. That will tap into warm, tropical Pacific moisture and douse areas that don’t normally see rain and snow. “Kona” is the Hawaiian word for the leeward side of an island (sheltered from the trade winds) that becomes the windward side when the Kona Low develops according to NASA. This is a cold-core low and not a tropical cyclone, which is a warm-core low.
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“A developing Kona Low will then bring the potential for heavy rain and thunderstorms to the islands next week beginning with Kauai and Oahu on Tuesday and then spread statewide from late Tuesday into next weekend,” National Weather Service forecasters in Honolulu wrote in a forecast discussion on Sunday. “Significant snowfall remains in the forecast for the highest summits of the Big Island.”
The first Kona Low developed Sunday and shifts east Monday and Tuesday. The NWS calls for heavy, flooding rain and thunderstorms with the first round for Oahu and Kauai.
“Another Kona Low drops in from the northwest on Tuesday and sets up just west of Kauai by Wednesday,” NWS Honolulu said. “This second low is stronger than the first one with deeper southerly winds that will draw up deep tropical moisture into the eastern half of the state. This unstable tropical moisture will spread periods of showers, heavy at times, and thunderstorms across all islands from Wednesday through Saturday.”
The NWS issued a Hydrologic Outlook for all islands from Tuesday through Saturday, warning of brief periods of heavy rain and flash flooding. As computer forecast model guidance becomes clearer on where the downpours are expected, the NWS said it could issue a Flash Flood Watch in the next 24 to 48 hours.
“Since the leeward side of the islands don’t get as much rain, they tend to be drier with less vegetation and shallower soil,” NOAA stated. “These conditions make these areas more vulnerable to floods, along with landslides and mudslides, especially in mountainous areas with steep, sloping inclines. The state’s Big Island has endured some of the worst impacts after experiencing ‘catastrophic flooding’’ where two feet of rain fell in some areas and knocked out power throughout the islands.”
The Kona Low that dropped 2 feet of rain in December 2021 closed highways on Maui and washed cars away. On Oahu, one highway was under waist-deep water, according to the NWS. Tropical-storm-force winds (39-plus mph) blew a shed into planes at the Honolulu Airport, damaging the aircraft. Winds gusted over 100 mph at the summit of Mauna Kea, which created near-blizzard conditions, blowing around the 10 inches of snow that fell.
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“Kona Lows originate from a deep kink in the jet stream (a band of strong winds high in the atmosphere that steer weather systems) that sinks south and pinches off, leaving behind a low-pressure circulation that is cut off from the main core of the jet,” NASA stated. “These cut-off lows can linger for several days.”
A Kona Low dropped a record-setting 49.69 inches of rain in 24 hours over Waipa Gardens on Kauai on April 14-15, 2018.
Easing the drought is the silver lining to the flood threat for Hawaii. Over 93% of the state is in moderate drought, and 75% is in severe drought, according to the latest U.S. Drought Monitor.
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Officials say drought conditions helped spark the deadly Lahaina firestorm over the summer.