Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
292 FXHW60 PHFO 181353 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 353 AM HST Fri Sep 18 2026 .SYNOPSIS... A surface trough, along with moisture associated with the remnants of Norbert will continue to move westward across the state today, bringing more enhanced showers and a slight chance of thunderstorms to mainly the western end of the state. Some thunderstorm activity will also be possible this afternoon over the Kona slopes on Big Island. 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. && .DISCUSSION... Latest IR satellite loop shows convection with frequent lightning roughly 60 miles southwest of Maui. This activity is due in part to deeper moisture associated with a north to south oriented surface trough draped over the western end of the state and weak instability aloft stemming from a digging upper level trough. These features could help to bring at least some enhanced showers to the islands today, especially over windward and mauka locations. Kauai, located closest to the departing surface trough, may see the best chance of receiving heavy showers. Atmospheric instability generated by diurnal heating could lead to more showers and a slight chance of thunderstorms this afternoon over the Kona slopes on Big Island. That being said, overall instability and precipitable water values are forecast to be less than yesterday, so confidence of occurrence is slightly deflated. High resolution models, such as the WRF ARW and WRF FV3, also indicate very little in the way of widespread heavy precipitation today. Thus, it seemed warranted to cancel the Flood Watch for all islands. If any flooding does occur, but it would likely be an isolated event. 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 nights or early morning hours along windward areas. Tuesday through Thursday, there is yet another potential push (by way of moderate trades) of deeper moisture from the remnants of Tropical Depression 15-E. The GFS indicates precipitable water values could peak out between 2" and 2.25" across the southeast half of the state, while the ECMWF shows between 2" and 2.5". These values are far above seasonal normals. A short wave passing just south of the islands may provide additional lift and instability for more widespread and heavier showers. && .AVIATION... A trough in the vicinity of the Hawaiian Islands, in tandem with a resident moist air mass, will likely produce return periods of MVFR decks and accompanying showers. Brief periods of moderate intensity rain may lower visibilities down into MVFR, as well. Breezy trades will persist through the day with the typical light and variable breezes within wind sheltered areas. AIRMET SIERRA will remain in effect this morning for island-wide windward exposure mountain obscuration near or above 2,500 ft. As of 3 AM HST, no other AIRMETs are in effect. && .MARINE... Moisture from the remnants of Norbert and a trough aloft will bring a few thunderstorms through this 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 today 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... Fire weather risk will remain low today 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 today. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Saturday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Windward Waters- Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...Thomas AVIATION...Blood MARINE...Walsh FIRE WEATHER...Thomas