textproduct: Honolulu

SYNOPSIS

Moderate trades this morning will gradually strengthen each day over the next several days with widespread strong trades expected by Thursday and very windy conditions possible on Friday. The main focus for the weather this week will be the tropical cyclone development southeast or south of the Big Island over the next few days. While the potential wind impacts remain uncertain, the threat for flash flooding will increase over the Big Island during the second half of the week.

DISCUSSION

This morning's satellite and radar imagery shows some scattered showers riding in with the moderate trades over windward and mauka areas. For today, while conditions are stable, we are seeing higher than normal moisture levels as seen in the 12Z soundings from Lihue and Hilo. This will allow for some brief downpours at a few windward and mountain areas. Otherwise, we should see a fairly typical trade wind day with breezy trades returning by later this afternoon.

This week's forecast remains very challenging and will be highly dependent on the formation of a tropical cyclone near the Big Island over the next several days. Latest Tropical Weather Outlook from the National Hurricane Center has a high chance of formation over the next 48 hours. While strong shear has been keeping the system from getting organized, the models do show a decrease of windshear over the next 24 to 36 hours. So the expectation is for the thunderstorms southeast of the Big Island to become better organized with a tropical depression likely to form over the next day or two.

Around Thursday, global models and the latest ensembles are showing the potential tropical cyclone drifting northward towards the Big Island as it reaches the western periphery of a subtropical ridge of high pressure. At this time, there is still high uncertainty on the strength and position of this potential tropical cyclone as it slowly drifts northward. While there is a great deal of uncertainty, there is a significant potential for a prolonged heavy rain event over windward and southeast Big Island due to the deep tropical moisture and weak steering flow. While there is a lot of moisture available, due to the high uncertainty on the intensity of the potential tropical cyclone, rainfall rates remain very uncertain. While tropical moisture could begin impacting windward and southeast Big Island as early as Wednesday, the heavier rain rates looks to be more Thursday into Friday, thus we held off on a Flood Watch for the Big Island for now. A Flood Watch will be issued for the Big Island eventually, as we get a better handle on the potential tropical cyclone development.

As for the potential wind impacts, while there is high uncertainty, there is a high likelihood of at least windy trades by Friday of this week with gusts of 50 mph to even 60 mph possible over the typical windy areas around the eastern half of the state. The potential for stronger wind gusts will be highly dependent on the tropical cyclone development.

AVIATION

Breezy trade wind will persist today. Starting tonight, attention will turn to a disturbance southeast of Hawaii as associated moisture gradually increases, especially across eastern and windward areas with periods of MVFR ceilings and visibility possible in heavier showers.

AIRMET Sierra is in effect for mountain obscuration and will likely be extended.

AIRMET Tango is in effect for low level turbulence downwind of island terrain and will likely be needed through the day.

MARINE

The gradient between high pressure to the northeast and an approaching trough that may develop into a tropical cyclone to the southeast will maintain moderate to locally strong trade winds through Wednesday. A Small Craft Advisory is in effect for the typical windy waters of Maui County and the Big Island through early Wednesday evening with the possibility of it being extended as well as expanded. Wind speeds will strengthen during the latter half of the week as the trough or potential tropical disturbance moves near the islands. Shower coverage will also increase, along with the potential for isolated thunderstorms, particularly across the eastern waters.

Surf along south facing shores will gradually drop heading into midweek as the current south-southwest swell fades. A slightly smaller, long-period south-southwest swell will fill in late this week into next weekend generated by a series of lows near New Zealand. West facing shores will experience a bit of the southerly wrap through most of the week.

Breezy trade winds will maintain moderate surf along east facing shores for the next several days. Surf along north facing shores will see a bump today into Wednesday as a small, medium- period north-northwest swell fills in from an Aleutian low. Then a small, long- period west-northwest swell generated by Typhoon Dujuan as it curves northeast near Japan, will build locally Friday into next weekend, increasing surf along north and west facing shores.

FIRE WEATHER

Issued at 333 AM HST Tue Sep 22 2026

Breezy trades will return by this afternoon and steadily strengthen each day over the next several days. With winds strengthening, moderate fire weather conditions will be possible this week mainly over drier leeward areas. The inversion is around 5000 feet this morning, but will rise over the eastern end of the state Wednesday as deeper moisture moves in.

HFO WATCHES/WARNINGS/ADVISORIES

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