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A Kona storm is lashing Hawaii, bringing heavy rains and the risk of thunderstorms and waterspouts. Flood watches blanket the entire archipelago because of the potential for slow-moving downpours to drop up to 8 inches of rain. The heaviest will fall on the southwestern slopes of the islands, which ordinarily are in the “Kona,” or leeward side, of prevailing trade winds.

Kona storms are most common in the wintertime. Hawaii usually receives two or three per season. The strongest can deliver feet of rainfall, along with damaging winds, hail, mountain snows and even tornadoes.

They pass to the northwest of the island chain, drawing rich tropical moisture northward and setting the stage for deluges. Already, the National Weather Service is cautioning that “urban areas may receive more significant flooding and property damage due to rapid runoff.”

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While sporadic showers and thunderstorms rolled through the Aloha State on Tuesday, the bulk of the action was expected to begin early Wednesday and could last into Friday before tapering off.

Although the downpours bring the threat of flooding, the rain is generally welcome as nearly 94 percent of the state is experiencing drought. In August, the drought contributed to the fire disaster in West Maui that destroyed more than 2,000 structures and killed nearly 100 people.

In addition to the predicted heavy rain, the Weather Service has issued a winter weather advisory for the high-elevation summits of the Big Island, where up to 5 inches of snow could fall. Snow is not unusual on the island’s towering peaks of Mauna Kea and Mauna Loa — at elevations above 13,000 feet — during the colder months of the year.

Where the storm is now

On Wednesday morning, the Kona storm was centered about 250 miles north-northwest of Kauai, Hawaii’s northwestern-most island. It resembled a comma on water vapor satellite. That’s because it is a nontropical system. The tail of the comma represents an approaching cold front, ahead of which moisture will stream northward and help generate showers and thunderstorms.

The storm was actually made up of two different low-pressure centers. That upper-air disturbance will move over the surface low on Wednesday, enhancing it and boosting heavy rainfall.

Expected rain totals

Waves of showers and thunderstorms will pinwheel north through early Friday, with the heaviest coming Wednesday into the first half of Thursday. There will be two areas of greatest rainfall — one near the Big Island close to the axis of greatest moisture, and the other near the far northwestern islands and closer to the surface low-pressure center. Between, heavy downpours will still be common but will be more widely scattered.

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Rainfall totals will be highly variable. Weather models vary wildly in their simulations, but as a general rule, most areas will get 2 to 4 inches of rain. In the higher elevations that are too warm for snow, rainfall totaling 4 to 8 inches is possible, and locally up to 10 inches.

Other impacts

Because of lower temperatures at high altitudes beneath the core of the upper-level disturbance, there will be some “instability,” or energy for thunderstorms. Instability will be greatest in the northwest islands, closer to the upper low where high-altitude temperatures are the lowest.

It’s challenging to pinpoint exactly where thunderstorms will form, but any thunderstorms that do form could contain winds gusting 30 to 40 mph. The greater concern exists for the potential for isolated waterspouts. It’s unlikely any would move ashore, but if they did, wind damage could occur. A number of marine warnings were issued Tuesday night for the waters west of Kauai.

Historic Kona storms

Kona storms are staples of the wintertime in Hawaii. They are nontropical in nature but not necessarily entirely nontropical. On rare occasions, a Kona storm can acquire subtropical or tropical characteristics.

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Even if they remain fully nontropical, they tend to be waterlogged. On Dec. 6, 2021, a Kona storm dumped 10 to 15 inches of rain on the Kona District, which lies on the south side of the Big Island. Honolulu registered nearly 8 inches of rain, its wettest December day on record, and then-Hawaii Gov. David Ige (D) declared a state of emergency.

While it is up for debate whether a Kona storm is fully to blame, tropical moisture interacting with a stalled front left by a Kona-like disturbance prompted the issuance of two tornado warnings on March 9, 2012. A tornadic waterspout came ashore near Kailua on the island of Oahu causing minor damage, and a record hailstone 4.5 inches in diameter fell. Heavy rains also tallied 45.97 inches in Hanalei on Kauai.

Jason Samenow contributed to this report.

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