textproduct: Honolulu

SYNOPSIS

An upper level trough over the islands will set the stage for an unstable environment lasting through Monday. A low level trough riding in from the north will trigger another round of moderate to heavy showers later tonight through Monday afternoon. Cool moderate to breezy northeasterly trade winds will blow across the region with decreasing shower trends as high pressure builds in from Monday night onward.

SHORT TERM UPDATE

The satellite imagery this evening shows the most unstable region with this upper level trough will likely remain east of the Hawaiian Islands. However, increasing northeasterly trade winds will combine forces with a low level trough, remnants of an old frontal boundary, moving in from the north later tonight. These stronger trade winds will lift this unstable moisture over island mountain ranges as the trough moves from north to south down the island chain. Periods of moderate to heavy showers remain in the forecast from Kauai to Maui into Monday afternoon. Flood threats continue as smaller amounts of additional rainfall may cause rapid runoff over the ground still saturated by recent heavy rainfall.

This synoptic setup justified an extension in time for our Flood Watch now in effect through Monday afternoon for most of the state. The latest forecast guidance shows the low level trough weakening as the enhanced shower band drifts into the Big Island around 9 AM HST Monday morning. We may need to expand the Flood Watch to include the Big Island with the morning forecast package to cover potential flooding threats associated with this trough.

PREV DISCUSSION

Issued 301 PM HST Sun Feb 22 2026

Cold northwest flow aloft supports moderate instability noted on this afternoon's soundings out of Lihue and Hilo. CAPE on the order of 500-1000 j/kg is noted over island interiors this afternoon as 500mb temperatures have fallen to around -15C and 700-500mb lapse rates are on the order of 7C/km. The environment through tonight will be weakly or even neutrally forced aloft as the bulk of PV advection already occurred yesterday. Instead, the islands reside in a favorable thermodynamic environment that is particularly sensitive to developing low-level convergence, namely among emerging sea breeze boundaries this afternoon. Deep, slow- moving convection/tstorms are therefore be possible over island for the next couple of hours. Thunderstorms may yet develop over the higher terrain of the Big Island this afternoon where they are considerably less likely to result in a flash flooding threat.

Trades then return tonight as the environment remains weakly forced overall. The mechanism for low-level convergence will shift to terrain and will therefore refocus the flash flooding threat over windward and mauka zones for tonight. Trades will gradually strengthen, becoming breezy by early morning Wednesday which will lead to decreasing convergence and increasingly progressive shower motion with time. However, at least the first half of the night will see rather slowly building trade winds and correspondingly slow- moving convection embedded within. The Flood Watch therefore remains in effect for for Kauai, Oahu, Molokai, and Maui.

Mid-level ridging gradually builds as the week progresses leading to increasingly stable trades.

AVIATION

Issued 301 PM HST Sun Feb 22 2026

A few SHRA and some isol TSTMS are ongoing across mostly Maui and the Big Island this afternoon. The trough that moved north is expected to move back southward through the state tonight and tomorrow, bringing another round of SHRA and TSTMS with a return of breezy and gusty trade winds. IFR/MVFR conds possible within SHRA, otherwise VFR should prevail.

AIRMET Sierra is not in effect at this time but may be needed later tonight as the trough moves southward.

AIRMET Tango remains in effect for mod upper level turb across the entire state.

MARINE

Issued 301 PM HST Sun Feb 22 2026

A trough stretching from a surface low north of Maui across Oahu to south of Kauai will produce gentle westerly winds south of the trough, while fresh to strong easterly winds prevail north of the trough. Scattered heavy showers and isolated thunderstorms can be expected near the trough. The trough will be pushed slowly southward by a strong North Pacific surface high tonight and Monday. Fresh to strong trade winds north of the trough will spread over waters north of the Big Island, with the strongest winds initially around Kauai and Oahu requiring a Small Craft Advisory (SCA). Winds should ease slightly Monday night and Tuesday, but a large north-northeast swell will produce seas in excess of the 10 foot SCA threshold over most waters into Tuesday night. Moderate to locally fresh trades are expected on Wednesday, giving way to southeasterly winds Thursday or Friday as a front approaches.

Very small surf will persist across most shores through tonight, followed by a building north-northeast (010-030 deg) swell on Monday. This long period swell will build rapidly during the afternoon and will produce breaking waves at High Surf Warning levels across most north and east facing shores during the peak Monday night and Tuesday. Due to the direction of the large swell, exposed west facing shores of West Maui, western Molokai, and North Kohala on the Big Island will likely experience High Surf Advisory waves. A Coastal Hazard Message has been issued to highlight these conditions. The north-northeast swell will gradually decline Tuesday night through Friday. In addition to this swell, a small pulse of overlapping west-northwest swell is expected Monday through Wednesday.

Due to the direction of the large north-northeast (010-030) swell, some overwash will be possible along low-lying coastal roads and infrastructure in windward areas, and hazardous conditions and heavy surges are expected in north facing harbors, mainly in Hilo and Kahului. A Marine Weather Statement has been issued to highlight this threat.

Very small wintertime surf will continue along south facing shores this week.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch through Monday afternoon for Niihau, Kauai, Oahu, Molokai, Lanai, Kahoolawe and Maui.

High Surf Warning from noon Monday to 6 PM HST Tuesday for north and east facing shores of Niihau, Kauai, Oahu, Molokai, Maui and the Big Island.

High Surf Advisory from 6 PM Monday to 6 PM HST Tuesday for west facing shores of Molokai, Maui and the Big Island.

High Wind Warning until 6 AM HST Monday for the Big Island summits of Mauna Kea and Mauna Loa.

Small Craft Advisory until 6 PM HST Tuesday for Kauai Channel- Kauai Leeward Waters-Kauai Northwest Waters-Kauai Windward Waters.

Small Craft Advisory from 6 AM Monday to 6 PM HST Tuesday for Alenuihaha Channel-Big Island Windward Waters-Kaiwi Channel-Maui County Windward Waters-Oahu Windward Waters.