Kona Low Storms and Flooding on Oahu: The Record From December 2021

What is a kona low, and why does it flood Oahu?

A kona low is a storm that brings a steady, moist flow of air and rain to the islands, often for long periods. Its rain can fall on ground that is already wet, so the water has nowhere to go. The National Weather Service Honolulu office tracks these storms and reports their totals and impacts.

The storms are dangerous because of their duration as much as their intensity. A slow-moving system keeps adding rain for hours or days, and the drains, the streams and the yards fill up. Water then travels toward the lowest buildings, which is where the damage is usually found.

The office's records show that the storms do not all behave the same way. Some bring a single heavy day of rain, others bring long, moderate rain, and some produce flash flooding on ground that is already saturated. Each pattern reaches buildings in a different way.

What did the December 2021 storm do?

The December 2021 kona low was the most damaging storm in this record. The office reports five to ten inches of rain across the island, with peaks of 10 to 13 inches, and state and city emergency proclamations followed. Flooding hit the south side especially from Pearl City to Kahala.

The storm also flooded an Iwilei power substation, which cut power to downtown businesses and residents. A flood that takes out power as well as water means a building may need its electrical system checked before it is switched back on, even if the water has been removed.

The lesson for a homeowner is that the damage from one storm can last well after the rain has stopped. Buildings that were wet in 2021 and not fully dried can still hold moisture in their walls and floors, which is why the readings matter.

What happened in the storms between 2021 and 2026?

The later events were smaller in scale but still caused damage. The office lists a February 2023 event with two kona lows and limited impacts on Oahu, a late November 2023 storm with minor flooding, and a May 2024 kona low that caused mostly street flooding. Each one left some buildings wet.

The February 2023 storms had limited impacts on Oahu, but one reading at Poamoho recorded 5.20 inches in 24 hours. Rain of that amount on a single station shows how localized a kona low's heaviest rain can be, even when the island as a whole is less affected.

The May 2024 kona low was unusual because it came late in the season. It caused mostly street flooding, and a rockslide near the Pali Highway tunnels. Street flooding reaches ground-floor rooms first, and the entry points are where the damage usually begins.

Why was the March 2026 storm so damaging?

The March 19 to 24, 2026 kona storm was large and very slow-moving, and it brought flash flooding mainly around Oahu on ground that was already saturated. Saturated ground cannot absorb more rain, so the water runs off into streets, yards and buildings.

The slow movement mattered as much as the rain. A storm that stays over the island keeps adding water to ground that cannot take it, which is why flash flooding can happen even when the rain is not extreme on any one day.

For a building, a storm like this means the water can enter through the yard, the drains and the lowest openings at the same time. The drying and the readings must cover all the routes, not only the one that is visible.

What did the October 2026 flood advisory mean?

A flood advisory for Oahu was issued on October 7, 2026, tied to a low north of the islands drawing tropical moisture over them. An advisory is a warning that flooding is possible or occurring, and it is the signal to check drains, entries and the readings in buildings that are still drying.

A building that dried after an earlier storm can take water again from the next one. The advisory is a reason to look at the entry points, to clear drains where they can be reached safely and to call the line if water appears.

The record shows a pattern: the island's storms come in clusters, and each one can reach buildings that were not fully recovered from the last. The homeowner should keep the drying record and the readings, so the next visit starts from the true state of the building.

What does the record mean for a building in Honolulu?

A building in Honolulu should be read after every storm, not only the first one. The record shows that flood water, street water and flash flooding each reach buildings in a different way, so the entry points, the lowest rooms and the yard must all be checked.

The island's storms are a reason to keep a written record of each event. The date, the water line, the readings and the work done after each storm give the next crew the starting point it needs and show an insurer the history of the building.

Calls for storm damage come in quickly after a storm, and crews are dispatched 24/7 as fast as one is available. The phone line is (833) 490-5225. Keep the photographs and the readings, and ask the crew to show you the record before the equipment comes out.

Where can the storm records be checked?

The National Weather Service Honolulu office publishes the storm summaries and the event pages. A homeowner who wants to check the record for a particular storm can read the office's pages, which give the rainfall totals and the impacts reported for each event.

The office's pages are the source for the figures in this guide. Where a figure is not on those pages, this guide does not state it. A reader should use the office's own records for any figure that matters to a claim or a repair.

The records are updated as storms are reviewed, so the pages are the best place to check a figure before relying on it. The crew can also explain the water route for a building once it has been read.

How should a homeowner use the storm record?

The storm record is most useful as a dated list. A homeowner can match the date of a water event to the storm that caused it, and use the list to show what was known at each point. That helps when the building is read, when the insurer asks for the history and when the drying is planned.

Write down the date when water was first seen, and match it to the nearest event in the record. If the water appeared during a storm, the record gives the rainfall and the impacts reported for it. If it appeared days later, the record shows whether a later event was the cause or whether the water came from another source.

Keep the record with the readings and the photographs. Each visit adds a line to the history, and the history is what a crew, an insurer or a future buyer will need to see. A building with a clear dated record is easier to assess than one where the water history is unknown.

The record is also a reminder that the island's storms come in clusters. A building that was dried after one event should be read again after the next, and the dated list makes it easy to see when that next read is due.