textproduct: Honolulu

SYNOPSIS

Weakening trade winds will produce light rainfall over mainly windward areas tonight. Low pressure will develop north of the state on Wednesday, causing winds to shift out of the south and allowing sea breezes to produce a few interior showers in the afternoon. Deep southerly winds will draw tropical moisture over the portions of the island chain Wednesday night through at least Thursday, bringing some potential for flooding rainfall. The low will weaken several hundred miles northwest of Kauai Friday and Saturday then lift away on Sunday, though a few heavy showers may linger through the weekend under light winds. Light trade winds are expected to return early next week.

UPDATE

Land breezes developed late last night as region winds declined, leading to mostly clear conditions early this morning. Southerly winds are beginning to develop as of mid morning, with little to no rainfall expected through midday. Sea breezes will dominate this afternoon and will produce a few interior showers as the atmosphere destabilizes. Flood Watch goes into effect this evening for Kauai and Oahu. See below for more details on the unfolding flood threat.

PREV DISCUSSION

Issued at 436 AM HST Wed Oct 7 2026 Stable and light ESE flow prevails beneath an inversion averaging about 7.5kft over the islands at press time this morning. Winds will continue veering through today becoming SSE by this evening. This is occurring in response to an approaching mid-level trough presently approaching Kauai from the NW. At this time, a frontal wave is developing in advance of this trough while frequent thunderstorms are noted in the base of the trough where 700-500mb lapse rates are firmly in excess of 7C/KM and 500mb are around -14C. As today progresses, cyclogenesis will focus near 30N/160W, or about 550 miles due north of Kauai as additional energy focuses in the base of the trough. In the near term, emerging sea breezes against a background of light southerlies will tend to focus afternoon clouds and showers over northern portions of the islands where scattered coverage is anticipated during peak heating.

By tonight, attention turns toward the northward progress of the tropical disturbance near 15N/162W, or about 600 miles SSW of Kauai. This disturbance is embedded within a broader tropical airmass characterized by surface dewpoints well into the 70s, PWAT values in excess of 2", and is surrounded by shower activity. Pockets of thunderstorms are also noted near its core where conditional instability is is apparently sufficient for moderately intense updrafts in spite of warm 500mb temperatures around -4C and sub 6C/km lapse rates. Southerly flow in advance of the developing extratropical cyclone north of the area will draw this feature north and shear it across the islands tonight into Thursday. The presence of thunderstorms at this time increases confidence in their inclusion in the official forecast seeing as how instability will only increase with northward extent as 500mb temps over Kauai & Oahu will turn at least 2-4C colder than what the system is currently experiencing. In addition, fact that convection is already fairly widespread south of the islands suggests the incoming atmosphere will be primed for shower and isolated thunderstorm activity in spite of the resident hostile stable/dry environment which should in turn be displaced rather easily.

Hi-res guidance suggests showers, some heavy, will become increasingly widespread between sunset and midnight tonight. At the same time, the digging trough will result in outstanding right entrance jet dynamics parked over Kauai late tonight through much of Thursday. At the least, this will favor steady, broad ascent from Maui County through Kauai in maintenance of increasing shower coverage as moisture steadily deepens. However, while differing on exact location, the EC and GFS both allow a band of particularly intense rainfall to develop along this developing jet-front feature, though the front will be washed out at the surface subsequently limiting low-level convergence. Nonetheless, any corridor of enhanced confluent flow around Kauai or Oahu should be more than sufficient to trigger banding of deep convection, heavy rain, and isolated thunderstorms. The heaviest rain is anticipated from around sunrise Thursday through Thursday evening focused mainly over Kauai and Oahu. At this time, it appears that once the increasingly strung out tropical disturbance, or remnant thereof, advances through the area, shower coverage should decrease considerably Thursday night into Friday. With that said, right entrance dynamics will persist over the western end of the state through at least Friday as the upper trough closes off and stays nearly stationary NW of Kauai. This should sufficient for showers to capitalize on rich low-level moisture and produce pockets of heavy showers Thursday night into early Friday night, especially over leeward zones.

A Flood Watch has been issued for tonight through Thursday for Kauai and Oahu. Given the potential for flow-parallel bands of convection to develop, there is concern for training of heavy showers. Rain rates could also be intense at times given the combination of forcing and deep tropical moisture. Subsequent shifts will evaluate the potential for an expansion to Maui County pending additional data.

Mid-level ridging becomes increasingly prominent beginning Friday night with light to moderate trades and increasing stability making a push into the area for the weekend. Should trades remain sufficiently light, then afternoon sea breezes will still have plenty of moisture to work with for afternoon showers over island interiors this weekend potentially leading to pockets of nuisance flooding during peak heating.

AVIATION

Light southeast winds expected through the forecast period due to a surface trough just west of Kauai County. Increasing SHRA and a few isolated TSTMs are possible through Thursday as the surface trough moves even closer. IFR/MVFR conds likely within heavier SHRA and TSTMs. VFR prevails elsewhere.

No AIRMETs are in effect.

MARINE

Gentle to moderate south to southeast winds will continue over the next several days as a low pressure system develops north of Hawaii. Enhanced showers with the potential for a few thunderstorms are forecast for the western half of the state through Thursday. A surface trough will linger near the area through the weekend and early next week keeping potential for variable winds and unsettled weather.

South swell will maintain into next week with pulses of long period reinforcement arriving Thursday and Saturday. Surf will stay below advisory levels along south facing shores.

Small to moderate surf along north and west facing shores will continue through the rest of the week as swell energy is reinforced by the slowly moving surface low northwest of the Hawaiian Islands. A larger swell will arrive around Tuesday, bringing north and west shore surf to just below advisory thresholds. Surf along east facing shores will remain small as local winds remain fairly weak.

FIRE WEATHER

Issued at 436 AM HST Wed Oct 7 2026 Light to moderate ESE winds veer to SSE as the day progresses. Fire weather risk diminishes through Friday as southerly winds bring increased moisture and humidity. The inversion range from 6 kft to 8 kft through the evening and will deteriorate this evening as deeper moisture arrives.

HFO WATCHES/WARNINGS/ADVISORIES

Flood Watch from 6 PM HST this evening through Thursday afternoon for Central Oahu-East Honolulu-Ewa Plain-Honolulu Metro-Kauai East-Kauai Mountains-Kauai North-Kauai South-Kauai Southwest-Koolau Leeward-Koolau Windward-Niihau-Oahu North Shore- Olomana-Waianae Coast-Waianae Mountains.