Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
337 FXHW60 PHFO 181937 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 937 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. && .SHORT TERM UPDATE... No changes to the forecast with the morning package. Thunderstorm activity has been limited to the offshore waters, but will be monitoring for the possible development over the Kona slopes this afternoon. && .PREV DISCUSSION... Issued at 353 AM HST Fri Sep 18 2026 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... No major changes to the forecast today, as we transition from a moist airmass with a vicinity trough to drier trades tomorrow. Today, look for VFR to MVFR cloud decks and isolates showers to pass through the region. Stronger showers will likely bring a reduction in visibility as well, perhaps down to MVFR at times. There is a non-zero chance for isolated thunderstorms as well over Kauai and the Kona slopes of the Big Island; however, chances are generally around 15% or less and confidence is too low to add to the TAF. Finally, breezy trades persist today 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. Will need to evaluate to see if this will be allowed to end on time or need to be extended. As of 0830 AM HST, no other AIRMETs are in effect. && .MARINE... Trade winds will strengthen slightly to fresh to locally strong levels through the weekend and persist 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 tonight, and will likely continue into next week for these waters. Surf along south-facing shores will remain above average through the weekend. The current long-period south-southwest swell will peak today, then gradually decline through the weekend as the next south swell fills in late Saturday through Monday, possibly nearing advisory criteria. Moderate surf along east-facing shores will persist over the next several days as be dominated by the trades as fresh to locally strong trades continue. Surf along north-facing shores will remain flat to tiny, with a minor bump possible early next week. && .HFO WATCHES/WARNINGS/ADVISORIES... 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. && $$ UPDATE...M Ballard DISCUSSION...Thomas AVIATION...KR MARINE...Farris