Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
310 FXHW60 PHFO 220630 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 830 PM HST Sat Sep 21 2024 .SYNOPSIS... Trade winds will decrease Sunday and Monday as the 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 locally breezy trades across local waters this evening. Models show winds should begin to gradually decrease tonight as the ridge weakens and drifts southward. An upper low north of Kauai has had minimal effect on local weather so far, but as it digs southward tonight the inversion will 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 overnight. Expect slightly weaker trades and heavier showers Sunday and 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 that could clip the Big Island, but expect typical windward rainfall for most islands. && .AVIATION... Moderate to locally breezy trades will weaken slightly heading into Sunday with scattered showers will favoring windward and mountain areas. With temperature inversion heights expected to remain elevated and weaken heading into Sunday as an upper level low approaches from the north, expect brief moderate to heavy downpours in showers with some spillover into leeward areas. MVFR conditions possible heavier showers. AIRMET Sierra may be needed for mountain obscuration for windward locations during the overnight hours. AIRMET Tango is in effect for tempo moderate turbulence between FL200 and FL400 over the islands tonight. This AIRMET will likely be needed through Sunday morning. && .MARINE... Fresh trade winds will ease slightly as a cold front passing through the far northern Central Pacific basin weakens the ridge north of the island chain. passing ridge to the north of the islands weakens. Winds will remain below Small Craft Advisory levels through Monday. The trade will strengthen back to moderate and locally strong levels from Monday night onward through the remainder of the upcoming work 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 along north and west facing shores well above seasonal average, though below High Surf Advisory levels, Tuesday night and Wednesday. This swell will decline as it shifts out of the north-northwest on Thursday, then fade on Friday. Surf along south facing shores will remain just below seasonal average over the next couple of days as an inconsistent southwest swell moves through. A reinforcing southwest swell is expected late Monday into Wednesday. With trade winds on a decline, east shore surf will be well below seasonal average during the next few days with increasing trends by Tuesday night or Wednesday. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Powell AVIATION...Foster MARINE...Bohlin