Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
852 FXHW60 PHFO 180625 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 825 PM HST Thu Sep 17 2026 .SYNOPSIS... Moisture associated with the remnants of Norbert will continue to move from east to west across the state through Friday, bringing more widespread shower activity, especially for windward and mauka locations. Some showers will be heavy at times with a slight chance of thunderstorms. A Flood Watch remains in effect for all the main Hawaiian islands through early Friday morning. Drier air will move back in this weekend behind the departing trough. Moisture from the remnants of Tropical Depression 15-E may bring another round of enhanced trade showers by the middle of next week. && .SHORT TERM UPDATE... Latest IR satellite imagery loop showed deep convection, associated with a north to south oriented surface trough near Maui County, slowly drifting westward over Big Island and its adjacent southwest waters. During the past several hours, thunderstorms where focused over the South Hilo District with rain rates generally ranging from 1 to nearly 3 inches per hour. Rises were noted on the Wailuku River gauge and Honolii stream gauge, but have since leveled off and begun to lower. A Flood Watch remains in effect for all islands through Friday morning as the trough, along with its accompanying deep moisture and instability, continues to move slowly westward. The forecast appeared on track, so no updates were needed this evening. && .PREV DISCUSSION... Issued at 404 PM HST Thu Sep 17 2026 Afternoon satellite and radar imagery shows clusters of deeper convection south of the islands and east of Oahu. An upper level trough and associated moisture from former tropical cyclone Norbert will continue to move west across the state into Friday. This band of moisture is now extending as far west as the windward side of Oahu. Weak to moderate instability will continue the threat of heavier showers and thunderstorms, especially on the windward and mauka locations and the Kona slopes on the Big Island. Thunderstorms with rain rates in excess of 1 inch per hour were already observed on the Big Island earlier this afternoon. A Flood Watch remains in effect for all main Hawaiian Islands through 6 AM Friday. The trough will slowly depart to the west of the state on Friday, but some lingering moisture may be enough for heavier showers to stick around near Kauai. Weak instability developing in the afternoon could lead to another round of enhanced showers and isolated thunderstorms Friday afternoon over leeward slopes of the Big Island. Saturday through Monday, moderate trades will help filter drier air back into the lower levels for more isolated to scattered coverage of showers during the evening into the night on the windward areas. Tuesday through Thursday, there is yet another potential push of deeper moisture from the remnants of Tropical Depression 15-E. Global ensembles are in generally good agreement with ensemble average precipitable water in excess of 2" sometime Tuesday through Thursday. A short wave passing just south of the islands may provide additional lift and instability for more widespread and heavier showers. && .AVIATION... A slow moving easterly trough, the remnants of TC Norbert, is pulling in a more moist air mass and will be impacting the islands through Friday. This will result in a continued greater frequency of showers with brief periods of moderate intensity rain. Decks will tempo lower into IFR/MVFR categories during times of more intense rainfall. Typically breezy trades will persist through the period with wind-sheltered area breezes remaining light and variable. AIRMET SIERRA remains in effect for island-wide mountain obscuration near or above 1,500 ft. AIRMET ZULU is in effect for icing within a FL170-280 layer to account for the earlier late afternoon thunderstorm activity along Big Island`s Mauna Kea windward slopes. && .MARINE... Moisture from the remnants of Norbert and a trough aloft will bring a chance of thunderstorms through tomorrow morning. Trade winds will strengthen slightly through this weekend and maintain through at least the first half of next week. A Small Craft Advisory is posted for the typical windy waters around Maui County and Big Island through Friday night, and will continue into next week for these waters. A south swell will fill in early Friday and surf will reach near advisory levels by Saturday. Surf along east-facing shores will maintain over the next several days as the east swell from Norbert declines, but it replaced by chop from the building trade winds. Flat to tiny surf along north facing shores is expected through the forecast period. && .FIRE WEATHER... Issued at 816 PM HST Thu Sep 17 2026 Fire weather risk will remain low through Friday due to moderate to breezy winds and increased relative humidity and moisture from the remnants of Norbert. Showers will decrease by this weekend. Inversion heights will be elevated or very weak. && .HFO WATCHES/WARNINGS/ADVISORIES... Flood Watch until 6 AM HST Friday for Big Island East-Big Island Interior-Big Island North-Big Island South-Big Island Southeast- Big Island Summits-Central Oahu-East Honolulu-Ewa Plain- Haleakala Summit-Honolulu Metro-Kahoolawe-Kauai East-Kauai Mountains-Kauai North-Kauai South-Kauai Southwest-Kipahulu- Kohala-Kona-Koolau Leeward-Koolau Windward-Lanai Leeward-Lanai Mauka-Lanai South-Lanai Windward-Maui Central Valley North-Maui Central Valley South-Maui Leeward West-Maui Windward West- Molokai Leeward South-Molokai North-Molokai Southeast-Molokai West-Molokai Windward-Niihau-Oahu North Shore-Olomana-South Haleakala-South Maui/Upcountry-Waianae Coast-Waianae Mountains- Windward Haleakala. Small Craft Advisory until 6 AM HST Saturday for Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters- Big Island Windward Waters-Maalaea Bay-Pailolo Channel. && $$ SHORT TERM UPDATE...JT DISCUSSION...SS AVIATION...Blood MARINE...Walsh FIRE WEATHER...SS