Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
435 FXHW60 PHFO 221334 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 334 AM HST Sun Sep 22 2024 .SYNOPSIS... Trade winds will decrease through Monday as a surface ridge to our north weakens. Rainfall will favor windward and mauka locations but a few showers, some heavy, may develop across leeward areas as an upper disturbance moves by. Breezy trades and more stable conditions are expected to return on Tuesday. && .DISCUSSION... A surface ridge far north of the main Hawaiian Islands is driving moderate trades across local waters this morning. An upper low north of Kauai will dig southward over the next few days, causing the inversion to lift and weaken. As a result, mainly windward showers may become a bit heavier than the past few days. A narrow and diffuse band of moisture in the trades will likely bring higher shower coverage from Maui to Oahu this morning. Expect slightly weaker trades and heavier showers through Monday. Expect stronger and more stable trade wind flow by midweek. Locally breezy trades will be driven by a tighter pressure gradient between the strengthening surface ridge to the north and a pair of tropical disturbances passing several hundred miles south of the islands. The upper low will fill and drift away to the west, and a ridge aloft building in from the east will bring stability. Models differ on timing but show a pocket of moisture in the building trades passing over the islands late Monday and Tuesday. This will likely be too late for enhanced rainfall and is expected to produce typical windward showers. Through the rest of the work week, tropical moisture could clip the Big Island, but expect typical windward rainfall for most islands. && .AVIATION... Moderate trade winds with scattered showers favoring windward and mountain areas. As an upper level low approaches from the north today, brief moderate to heavy downpours will occur with some spillover into leeward areas. AIRMET Sierra posted for tempo mountain obscurations above 2500 feet for windward portions of Oahu, Molokai, Maui and Big Island. Brief mountain obscurations, MVFR cig/vis may occur elsewhere within the passing showers. AIRMET Tango is in effect for tempo moderate turbulence between FL200 and FL400 over the islands tonight. This AIRMET will likely be needed through the morning. && .MARINE... Fresh trade winds will ease slightly today as a cold front passing through the far northern Central Pacific basin weakens the ridge north of the island chain. Wind speeds will remain below Small Craft Advisory levels through Monday. The trade winds will strengthen back to moderate and locally strong levels from Monday night on through the rest of the week. Surf along north and west facing shores will remain tiny through Monday. A moderate, long-period northwest swell will arrive on Tuesday and push surf heights along north and west facing shores well above seasonal average, below High Surf Advisory levels, Tuesday night and Wednesday. This swell will decline as it shifts out of the north-northwest on Thursday, then fades on Friday. Surf along south facing shores will remain just below seasonal average levels as an inconsistent southwest swell moves through the Hawaii region. A reinforcing southwest swell is expected late Monday into Wednesday. As trade winds decline, east shore surf will remain small during the next few days with slight increasing trends by Tuesday night or Wednesday. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Powell AVIATION...Foster MARINE...Bohlin