Unusual late season Kona low mostly fizzles on neighbor islands, drenches O‘ahu

An unusual late-season Kona low had Hawai‘i on edge last week as it formed to the north of the islands. Gov. Josh Green even declared a state of emergency to make sure resources were ready and available to help if and when needed after the storm struck.

The neighbor islands, including the Big Island and Kaua‘i, were fortunately spared from most of the storm’s impacts, including heavy rain from bands of precipitation that lingered for days in some places.

O‘ahu was right in the crosshairs.

“Big Island saw little significant rainfall, as did Kaua‘i,” said National Weather Service meteorologist Thomas Birchard, who works at the agency’s Honolulu forecast office, adding the office also was not aware of any significant impacts on Kaua‘i from the kona low.

Heavy precipitation associated with the storm’s main rainband moved west onto O‘ahu the early morning of May 16 and strengthened before drenching the island the last two days of the week.

Preliminary data from automated rain gauges collected several notable preliminary 2-day rainfall totals.

The highest was at Maunawili, with 10.84 inches. The lowest of the most notable totals was 5.50 inches recorded by the gauge at Kahuku Training Area. Ten others collected from 6.35 to 9.68 inches of rain.

The rain caused minor flooding of streets in multiple areas of O‘ahu. A rockslide also happened May 16 near the Pali Highway tunnels. Streams where rain gauges were collecting precipitation indicated elevated flow levels, but none overflowed.

A report from Hawai‘i News Now said there were downed trees in several communities and the severe weather caused a high school graduation to be relocated.

The rainband that drenched O‘ahu continued moving west the afternoon of May 17, toward Kaua‘i. After making some locations around the Garden Isle soggy, the storm started to weaken early Sunday morning and shift north, moving away from the state that night.

Forecasters already were keeping close eyes on the skies over Hawai‘i by the beginning of last week as the kona low started to take shape.

A flood watch was issued for the entire state by late afternoon May 13 because of the potential for rainbands to produce prolonged heavy precipitation and possibly thunderstorms from midweek through the weekend.

There also was concern the rain falling over the same areas for extended periods of time would cause major flooding and also flash flooding.

The threat of flash flooding on the Big Island diminished by the afternoon of May 14 and the island was removed from the statewide flood watch. While heavy rain and flooding were no longer an issue, windward areas, particularly in Hilo and lower Puna, did experience some locally strong southerly winds for the next day or so.

Hilo saw just 0.16 of an inch of rain from May 14-17. Kailua-Kona recorded a trace of precipitation during the same time period.

After making some locations around the Garden Isle soggy, the late season kona low finally started to weaken early Sunday morning and shift north, away from the state that night.

Kaua‘i and Ni’ihau were the last bastions of the statewide flood watch issued May 15 to be removed after flooding was no longer expected to pose a threat.

The highest 24-hour rainfall totals recorded by automated rain gauges on Kaua‘i as of 11:45 a.m. May 18 were in Mount Waiʻaleʻale with 1.39 inches and Mana with 1.37 inches. Pu’u Lua received 1.20 inches, Waiakoali got 1.08 and Waiʻalae saw 1.05 inches.

All of the other gauges around the island by that time had received less than an inch of rainfall.

By late Sunday morning, the highest 24-hour rainfall total recorded on the island was 0.58 of an inch by the Waimea Tank gauge. Rain had already began to cease by that time, with 3-hour totals at 11:45 a.m. at just three of the automated gauges recording a nominal 0.01 of an inch and all others dry.

Līhuʻe saw a total of 1.02 inches of rain from May 17-19.

Last week’s storm appears to be the latest kona low to directly affect the main Hawaiian Islands in at least the past 20 years.

A kona low is a subtropical cyclone that typically forms northwest of the state, outside the band of warm waters that fuel tropical storms. The counter-clockwise flow around a kona low results in southwesterly winds.

Kona means leeward, or dry side of an island, in ‘Ōlelo Hawai‘i (Hawaiian language). It also is a name for a southwest or south wind.

The storms most often form in late fall, winter and spring because they are associated with a southwest plunge of cold air over the Central Pacific Ocean. Hawai’i’s dry season also normally starts at this time of year.

“Kona lows mainly affect the state from November through April,” said National Weather Service meteorologist Maureen Ballard, who also works at the Honolulu forecast office. “They are mainly cool season systems, so having one in May is unusual.”

These storms, unlike their hurricane and tropical storm cousins, form around a cold center, originating and pinching off from a deep kink in the jet stream after sinking south. That forms a low-pressure system that feeds off abundant moisture from warm waters around the islands, lingering for days.

The last late season kona low to impact the state was in May 2002. It was stronger and closer to the islands, moved through from May 4-7. The storm caused flash flooding along the southeast slopes of the Big Island and significant flash flooding on O‘ahu.

It’s also interesting that last week’s unusual subtropical cyclone happened just weeks before the Central Pacific hurricane season begins, Ballard is not aware of any correlations between late season kona lows and the hurricane season.

The Central Pacific hurricane season runs from May 1 through Nov. 30 each year. The National Oceanic and Atmospheric Administration will host a news conference today to announce their outlook for anticipated hurricane activity this year.

A proclamation by the governor marking May 19-25 this year as Hurricane Preparedness Week in Hawai‘i will also be presented, as well as a review of the 2023-24 wet season and outlook for the 2024 dry season from the National Oceanic and Atmospheric Administration.

Watch Big Island Now and Kaua‘i Now for information from the news conference later today and tomorrow morning.