textproduct: Honolulu

SYNOPSIS

Trade winds will continue to strengthen slightly tonight as Tropical Storm Moke moves generally from east to west, south of the islands. This system will deliver periods of rainfall to all islands from late tonight through Monday, with the heaviest rainfall forecast along windward and southeast slopes of the Big Island. A Flood Watch goes into effect for the Big Island at midnight Saturday night. A Wind Advisory is in effect for parts of the Big Island and Maui County through 6 PM HST Sunday. Drier trade wind weather will return Tuesday.

SHORT TERM UPDATE

This evening's IR satellite imagery loop shows an area colder cloud tops associated with Tropical Storm Moke southeast of the Big Island. However, the storm has become a bit more disorganzied compared to 24 hours ago. This deterioration appears to be due, at least in part, to persistent southwesterly vertical wind shear and dry mid-level air. According to the 5 PM HST NHC advisory, the center of Moke was around 360 miles south southeast of Hilo and moving west at 14 miles per hour. Trade winds have increased to moderate to locally strong this evening, and will continue to edge higher tonight into Sunday as Moke passes south of the state. The Wind Advisory looks on track for select wind prone zones in Maui County and the Big Island. Winds in these zones have been averaging sustained 25 to 30 mph with gusts 40 to 50 mph. A deeper push of moisture is still expected to reach eastern portions of the Big Island later tonight with shower coverage on the increase. Expect some heavier activity to become embedded in the showers, esspecially on Sunday. The Flood Watch for the Big Island looks to be in good shape, beggining later tonight and continuing through Monday afternoon. It will not take a significant amount of rain to cause flash flooding due to saturated grounds from recent heavy rain. The deeper push of moisture will then push accross the remaining smaller islands midday Sunday into Monday, but with less intense rainfall than expected on the windward and southeastern sides of the Big Island.

PREV DISCUSSION

Issued at 321 PM HST Sat Aug 22 2026 Satellite imagery shows Moku weakening this afternoon as dry air and vertical wind shear take a toll. Winds have not officially decreased, but it looks like that may be happening soon. It was centered around 380 miles SE of Hilo as to 3 PM HST. Otherwise, isolated light were moving across northern sections of the Big Island. Other areas were dry. Trade winds were averaging 10 to 20 mph with gusts 20 to 35 mph.

Depending on exactly if or how much Moke weakens over the next 12 or so hours, the surface pressure gradient is expected to tighten up and strengthen winds up to advisory criteria in some areas of Big Island and Maui County. Winds in the lee of the Kohala Mountains may approach wind warning criteria, and winds in the lee of the Ko'olau and Wai'anae mountains on Oahu may approach wind advisory level Sunday. These areas will be watched closely to see how the forecast evolves. Winds are forecast to begin a gradual weakening Sunday night into Monday, and a return to more typical trades is expected by Tuesday.

We have maintained the forecast of rainfall amounts. The latest GFS is trending slightly down, but the EC has trended up. Mesoscale models show a slightly later arrival time. There is sufficient uncertainty to make keeping things steady the most appropriate forecast at this time. Flooding rains remain possible on Big Island. Eastern Maui will be monitored closely. Other areas seem unlikely to experience flooding rains apart from isolated training effects that may develop.

A Flood Watch for the Big Island will go into effect tonight and continue through Monday afternoon. Rainfall totals of 5 to 10 inches, locally up to 15 inches are possible over the windward and southeast slopes of the Big Island. It will not take a significant amount of rain to cause flash flooding due to saturated grounds from recent heavy rain.

Moisture will remain above normal Monday as Moke moves west away from the area. Showers will continue, mainly windward and mauka. By Tuesday, drier air will move in from the east, and typical trade winds/showers are forecast for the remainder of the week.

AVIATION

Trade winds are continuing to strengthen across the state as Tropical Storm Moke approaches and passes south of the Big Island this weekend. Trades will continue to strengthen from east to west through Sunday. Showers will primarily impact windward and mountain locations providing occasional MVFR conditions to those areas. Windward Big Island could see periods of heavy rain and IFR conditions starting early Sunday morning from Tropical Storm Moke. VFR is expected to prevail elsewhere.

AIRMET Tango is in effect for low level turbulence downwind of terrain. AIRMET Sierra will likely be needed for windward Big Island when showers start moving into the area.

MARINE

Issued at 321 PM HST Sat Aug 22 2026 Trades are strengthening in response to Tropical Storm Moke passing south of the islands and the Small Craft Advisory remains in effect through the weekend. Breezy trades then persist well into next week as high pressure remains anchored along 30N.

Surf along E and SE shores gradually builds in response to strengthening trades and short period fresh swell emanating from Tropical Storm Moke. The direction of this swell will transition to S and then SW as Moke advances west. A High Surf Advisory (HSA) may be needed for the SE shores of the Big Island tonight into Sunday, but at this time it appears surf will fall shy of the HSA threshold.

Surf along S shores remains slightly below the summer average but still slightly elevated through the weekend due to a small, long period SW swell. A series of long period S swells arrive during the middle of next week bumping surf to around the advisory threshold.

A small medium to long period NW swell originating from Hurricane Lala provides elevated surf along N and W shores this weekend.

FIRE WEATHER

Issued at 321 PM HST Sat Aug 22 2026

Trade winds continue to increase tonight. The inversion will steadily weaken and erode as deep moisture associated with TS Moke moves into the area tonight into Sunday. Heavy rain is expected over windward and SE facing slopes of the Big Island on Sunday.

HFO WATCHES/WARNINGS/ADVISORIES

Wind Advisory until 6 PM HST Sunday for Big Island Interior-Big Island North-Big Island South-Big Island Southeast-Kahoolawe- Kohala-Lanai Leeward-Lanai Mauka-Lanai Windward-Maui Central Valley North-Maui Central Valley South-Maui Leeward West-Molokai Leeward South-Molokai North-Molokai Southeast-Molokai West-South Haleakala-Windward Haleakala.

Flood Watch from midnight HST tonight through Monday afternoon for Big Island East-Big Island Interior-Big Island North-Big Island South-Big Island Southeast-Big Island Summits-Kohala-Kona.

Small Craft Advisory until 6 PM HST Sunday for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters- Big Island Windward Waters-Kaiwi Channel-Kauai Channel-Kauai Leeward Waters-Kauai Northwest Waters-Kauai Windward Waters- Maalaea Bay-Maui County Leeward Waters-Maui County Windward Waters-Oahu Leeward Waters-Oahu Windward Waters-Pailolo Channel.