textproduct: Honolulu

SYNOPSIS

Moderate to locally breezy trade winds will continue tonight, with periods of showers favoring windward mountain areas. An upper level low will briefly move over the region on Friday, enhancing threats for locally heavy rainfall and isolated thunderstorms. While widespread heavy rain is not anticipated, pockets of heavy rain with localized flooding impacts are possible through Friday night. Wetter than normal conditions will linger into Sunday.

SHORT TERM UPDATE

The satellite imagery this evening shows an extensive band of unstable low level cumulus clouds builds into the islands from the southeast direction. A large band of frequent thunderstorms continues to fire along the eastern flank of an upper low roughly 300 miles north of Kauai. This upper low will continue to drift south, towards Kauai and Oahu tonight, and some of these thunderstorms may reach the western islands by Friday morning. Elevated threats for advisory level flooding with water ponding on area roadways by Friday. We remain less confident on warning level flood events, therefore we continue to keep the Flood Watch in reserve until this system gets closer to the islands and we can see more smaller scale (mesoscale) convective heavy shower development relative to the islands.

Large scale global weather models continue to show the center of the upper low reaching the island of Kauai around noon HST on Friday, then the low kicks out slowly to the east and weakens on Sunday, where enhanced showers will continue to produce a wet trade wind weather pattern across the state.

The Kilauea volcano on the Big Island continues to erupt in a fountain of lava this evening, trade winds are blowing most of the ashfall (tephra) from the volcano area towards the southwest into the Kau District. A Special Weather Statement (SPSHFO) for this event was issued this afternoon due to limited ashfall impacts in a trade wind weather pattern.

Updated the overnight to early Friday morning forecast this evening to increase clouds and showers along this developing low level cloud band spreading across the island chain.

PREVIOUS DISCUSSION

Issued at 358 PM HST Thu May 14 2026 High pressure far north of the state continues to maintain breezy trade winds across the region this afternoon, with clouds and showers focusing primarily over windward and mauka areas. These locally breezy trades will continue into the evening hours, but are anticipated to weaken to more moderate levels overnight.

An upper level low that can currently be seen on water vapor imagery entering the northern offshore waters this afternoon is expected to continue digging southward towards the island chain tonight. Latest global guidance remains in good agreement that this upper low will arrive near Kauai in the pre-dawn hours Friday morning, then move over the Garden Isle during the day with temperatures aloft at 500mb anticipated to be as cold as -15C. This will lead to increasing instability over the state, and the strength of the upper level low will induce a surface trough, serving as a trigger for heavy showers and isolated thunderstorms.

Given that the upper low is expected to be centered over Kauai at its closest pass, the best forcing is expected to be over Oahu and parts of Maui County, where guidance is showing over 1000 J/kg of CAPE by Friday afternoon. Of particular note, latest QPF guidance has trended upward, and the official forecast has been adjusted accordingly, but with great uncertainty. In particular, some guidance is hinting at the possibility of a plume of moisture developing from Molokai towards Oahu from late morning into the afternoon within a slightly more ESE wind regime. Should this occur, there is potential for heavy rain to train over portions of Oahu, which could lead to flooding concerns. This scenario in a few hi-res members that make up the National Blend of Models (NBM) is likely the reason for an unusually large discrepancy between the QPF 50th percentile for Oahu on Friday and the mean, with the mean being significantly higher. Meanwhile, Rapid Refresh Forecast System (RRFS) members seemingly range from not much notable rain over the islands on Friday, to interior and leeward afternoon convection development, to pluming and potential training of heavy rain and thunderstorms leading to flooding. These scenarios are similar (though to a lesser degree) for Kauai and the islands of Maui County as well, and slight deviations of the main upper low could shift convective focus east or west accordingly. In summary, the QPF forecast for Friday has significant and unusual uncertainty. There likely will be heavy rain over some locations, and given lighter low-level winds, showers could be slow moving with the potential for anchoring on island terrain. However, not confident enough at this point to issue a Flood Watch given the mesoscale effects that could be at play and the lack of a clear focus or consensus, but it is certainly worth keeping an eye on for flooding impacts. At this time, have opted instead to continue the heavy rain and thunderstorm mentions in the forecast.

This weekend, the upper level low will begin to drift further away from the state, which should decrease the threat of thunderstorms. While the instability decreases over the weekend, low level moisture will increase from the southeast with precipitable water values approaching 2 inches by late Saturday over windward Big Island. Periods of heavy rain will be possible over windward and southeast Big Island this weekend, with some enhanced showers possible elsewhere across the state. By next week, a more typical trade wind pattern looks to return to the region.

AVIATION

Moderate to locally breezy trades. Low cigs and SHRA possible along windward and mauka locations with some spillover possible into leeward areas. MVFR conds in any heavier SHRA but VFR should prevail. An upper low near the state may provide the potential for periods of +RA and isol tstms, especially over western side of the state. Conds may deteriorate to IFR/MVFR in those locations.

AIRMET Sierra is in effect for mtn obsc for windward locations of all islands due to clds and SHRA.

AIRMET Tango is in effect for low-level turb lee of terrain due to the breezy trades.

MARINE

High pressure far north of the islands will maintain moderate to locally fresh trades through this evening. An upper level low and its surface reflection of a surface trough north of Kauai this afternoon will turn weakening winds more east southeast from tomorrow into the weekend. The Small Craft Advisory (SCA) has been extended for the windier zones of Maui County and Big Island through 6 AM HST Friday morning. The close proximity of the upper low, along with disturbances coming up around the low from the south southwest, will increase the threat for widespread rain with periods of locally heavy rain and isolated thunderstorms across the entire island chain and surrounding waters tomorrow.

A small, moderate period north northwest (320-330 degree) swell will continue its peak this evening and produce near to slightly above head high surf along many north-facing shores. This swell will gradually fade from late tonight through the weekend.

A mix of small, medium to long period south swells will maintain small surf along south-facing shores into the weekend. A series of gale lows passing south of New Zealand should send a series of small south swells to the Hawaiian Islands through next week. Surf should lift to near summer averages around Sunday and this will hold through the week. Locally moderate to fresh trades will maintain rough and choppy surf along east-facing shores.

Tides will peak around 2.5 ft MLLW this weekend. Combined with ongoing trades and a small south swell bump, water levels will peak near 3.0 ft early next week. Minor overwash of low lying coastal areas will be possible during times of high tide.

HFO WATCHES/WARNINGS/ADVISORIES

Small Craft Advisory until 6 AM HST Friday for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters- Maalaea Bay-Pailolo Channel.