textproduct: Honolulu

SYNOPSIS

A powerful kona storm is expected to affect the state this week and into the weekend, bringing several potential hazards to the islands. The combination of deep tropical moisture, strong winds, and increasing instability could produce periods of heavy rainfall with significant flash flooding, damaging winds, and strong to severe thunderstorms from Tuesday through at least Saturday. Flooding concerns may begin as early as Tuesday and Tuesday night over Kauai and Oahu as deep moisture spreads into the western end of the state, then expands eastward across the island chain later in the week. The potential for strong to severe thunderstorms is expected to increase late in the week and into the weekend.

SHORT TERM UPDATE

Issued at 903 PM HST Mon Mar 9 2026

Current observations show winds have veered to a southeast direction and radar shows some showers with embedded thunderstorms approaching the coastal waters west of Niihau. These thunderstorms are expected to continue their eastward track through the night. We are also watching some light to moderate showers develop over the Koolau mountains. The forecast so far remains on track.

PREV DISCUSSION

The weather regime heading into the evening is expected to be rather typical of a veered east-southeast flow for the eastern half of the state with showers generally favoring the southeast and east facing slopes of the Big Island and Maui. For the western end of the state, showers with embedded thunderstorms will begin to move in from the west.

Forecast confidence continues to increase regarding the wet and unsettled pattern that is expected to develop tomorrow through the rest of the week and weekend. Impacts, including the potential for significant flash flooding, damaging winds, and strong to severe thunderstorms, will initially be felt across the western end of the state and will gradually expand eastward over the next several days. While this is a long duration event, it looks like the overall evolution can be broken into two primary stages.

The initial stage is expected to start tomorrow or tomorrow night and potentially last through Thursday. As a broad low deepens to the northwest of the state, winds will veer southerly for the western half of the state and southeasterly for the eastern half heading into Tuesday. A broad swath of deep atmospheric moisture with precipitable water values (PWats) ranging from 1.5 to 2 inches, which can be seen as an area of showers and thunderstorms roughly 200 miles west-northwest of Kauai this afternoon, will be drawn up in this southerly flow and move over the western end of the state Tuesday into Wednesday. At the same time, a shortwave will round the base of a potent upper level trough to the northwest, providing instability, and upper level jet dynamics will support large scale ascent across the area. Together, these ingredients will support periods of heavy rainfall with embedded thunderstorms and gusty winds. Impacts during this stage are expected to be focused primarily across the western half of the state.

The second stage looks to start on Friday and persist into the weekend. The upper level trough to the northwest will continue to sharpen and take on a negative tilt as a jet streak on its western flank strengthens and another shortwave trough will round the base of this trough. The axis of deep atmospheric moisture will shift eastward and spread out across the island chain, with model guidance showing PWats exceeding 2 inches. By late week and into the weekend, soils may already be saturated, increasing the likelihood that additional rainfall will produce rapid runoff and dangerous flooding conditions, increasing flooding concerns statewide, particularly where heavier rainfall persists over the same areas for multiple days. While thunderstorms embedded within showers are possible midweek as mentioned above, the potential for strong to severe thunderstorms will increase Friday into the weekend as the environment becomes increasingly unstable, moisture increases and the upper level jet dynamics become even more favorable for large scale ascent.

In addition to the rainfall threat, south to southwest winds may strengthen later in the week and into the weekend as the pressure gradient tightens between the developing low to the northwest and high pressure to the east. If this materializes, strong kona winds and localized downslope winds on north through east facing sides of the islands may be capable of downing trees and power lines.

For the Big Island summits, increasing moisture and colder air associated with the upper trough may support periods of heavy snowfall with significant accumulations later in the week and into the weekend, potentially requiring the issuance of a winter storm watch/warning.

While some uncertainty remains regarding the exact timing and placement of the heaviest rainfall and strongest storms, confidence continues to increase that the islands will experience a prolonged period of unsettled and potentially impactful weather this week. Residents and interests across the state should continue monitoring forecasts as details regarding the timing and location of impacts become clearer.

AVIATION

Issued at 903 PM HST Mon Mar 9 2026 Rainfall and thunderstorm chances will quickly ramp up overnight through Tuesday across the western end of the state, particularly over Kauai tonight, as a powerful kona low evolves and draws closer. This activity will lead to MVFR conditions, with LIFR conditions expected in the heavier showers and storms that move through.

Light to moderate southeast winds will gradually veer southerly and increase over the western end of the state through the day Tuesday. Any storms that develop and move through will be capable of producing damaging winds. These impacts will gradually advance eastward down the island chain by midweek.

AIRMET ZULU is in effect for light icing for Kauai and Oahu. AIRMET SIERRA for mountain obscuration will likely be needed overnight through Tuesday, especially for Kauai and Oahu. Additionally, AIRMET TANGO for turbulence may also become necessary.

MARINE

Issued at 903 PM HST Mon Mar 9 2026

A powerful kona storm is expected to bring heavy rain, thunderstorms, and rough seas to the islands and adjacent waters Tuesday into the weekend. Winds will veer southerly and increase to fresh to locally strong speeds Tuesday through late this week as the kona storm approaches. A Small Craft Advisory (SCA) will go into effect for waters north and west of Kauai beginning early Tuesday morning, when winds will begin to increase. Later this week, possibly as early as Thursday night, winds will approach, or possibly reach, gale force for portions of the area. These strong winds will slowly creep eastward with the front heading into the weekend.

A series of small, medium- to long-period west-northwest swells returning tonight will bring small surf to north and west facing shores through the rest of the week. The first of these will peak on Tuesday, resulting in a slight bump in surf along north and west- facing shores. Another small, medium-period north swell is also possible late this week.

Choppy surf along east-facing shores will gradually trend downward, falling below seasonal levels by mid-week as winds veer southerly. Surf along south-facing shores will remain small, though will become rough and choppy as southerly winds increase. A series of small, long-period south swells will also impact south-facing shores through late this week.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch from 6 AM HST Tuesday through Saturday afternoon for Kauai East-Kauai Mountains-Kauai North-Kauai South-Kauai Southwest-Niihau.

Flood Watch from Tuesday evening through Saturday afternoon for Central Oahu-East Honolulu-Ewa Plain-Honolulu Metro-Koolau Leeward-Koolau Windward-Oahu North Shore-Olomana-Waianae Coast- Waianae Mountains.

Flood Watch from Wednesday morning through Saturday afternoon for Haleakala Summit-Kahoolawe-Kipahulu-Lanai Leeward-Lanai Mauka-Lanai South-Lanai Windward-Maui Central Valley North-Maui Central Valley South-Maui Leeward West-Maui Windward West- Molokai Leeward South-Molokai North-Molokai Southeast-Molokai West-Molokai Windward-South Haleakala-South Maui/Upcountry- Windward Haleakala.

Small Craft Advisory from 6 AM Tuesday to 6 AM HST Wednesday for Kauai Leeward Waters-Kauai Northwest Waters-Kauai Windward Waters.