Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
302 FXHW60 PHFO 211905 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 905 AM HST Sat Sep 21 2024 .SYNOPSIS... Moderate to locally breezy trade winds will ease Sunday and Monday as the surface ridge to the north weakens. Showers will favor windward and mauka locations, but a few may develop over leeward areas as a disturbance aloft moves overhead tonight and generates some isolated heavier rainfall. Breezy trades and more stable conditions are expected to return Tuesday. && .DISCUSSION... A somewhat stable, moderate to locally breezy trade wind flow will remain in place today. A surface ridge sitting about 1,000 miles to the north is driving the trades, and while this feature may begin to erode later today, expect winds to be close to yesterday`s strength. An upper-level trough dropping southward is sending thin high clouds over the state, and a weakening mid- level ridge overhead will maintain somewhat stable conditions today. A narrow and diffuse band of moisture moving within the trade winds will be moving onto windward areas. With the inversion slightly elevated near 10,000 feet this morning, a couple of heavier showers cannot be ruled out, but due to typical diurnal trends, only modest rainfall of a quarter of an inch or less is expected this afternoon. Little rainfall will occur over leeward areas, with the exception of afternoon clouds and spotty showers over the Kona slopes of the Big Island. Trade winds will drop a notch, and showers will likely be heavier Sunday and Monday. The surface ridge to the north will erode, causing trades to decline to moderate strength. Meanwhile, the upper-level low will stall over the western end of the island chain, causing 500 mb temps to drop to around -9C, steepening the lapse rate aloft, and significantly weakening the low-level inversion. The GFS and ECMWF show a lack of organized low-level moisture in the trade wind flow, which suggests that a general increase in shower activity is unlikely. However, showers that do develop could be locally heavy. A stronger and more stable trade wind flow is expected by midweek. The locally breezy trades will be driven by a tighter pressure gradient between the strengthening low-level ridge to the north and a pair of tropical disturbances passing several hundred miles south of the islands. The upper-level low fill and drift away to the west, and a ridge aloft building in from the east will bring stability. The GFS and ECMWF 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, we will monitor areas of tropical moisture that could clip the Big Island, but expect typical windward rainfall for most islands. && .AVIATION... Moderate to locally breezy trades will continue today with isolated to scattered showers favoring windward and mountain areas. With temperature inversion heights around 10,000 feet this morning, expect brief moderate to heavy downpours in showers and some spillover into leeward areas. An upper level low is anticipated to move over the state tonight into tomorrow, which will further enhance shower development (increase intensity, but not necessarily coverage) and trade winds will weaken slightly. MVFR conditions can be expected in heavier showers with isolated IFR conditions possible. VFR should prevail outside of shower activity. No AIRMETs are in effect. AIRMET Sierra may be needed for mountain obscuration for windward locations during the overnight hours through this weekend. && .MARINE... The trade winds will ease during the next couple days as a ridge to the north of the islands weakens. Winds have already dropped below Small Craft Advisory (SCA) thresholds and are expected to remain below these levels through Monday. As a result, the SCA has been cancelled. The trades should re-strengthen back to moderate and locally strong levels Monday night and hold steady in this range through the remainder of the upcoming work week. Surf along north and west facing shores will remain very small through Monday. A moderate sized medium to long-period northwest swell build Monday night and Tuesday, peak Tuesday night into Wednesday out of the northwest, then fade Thursday through next Saturday as it shifts out of a more north-northwest direction. Surf along south facing shores will remain small through the weekend, with mainly a mix of background long-period southwest and short-period southeast swells moving through. An upward trend is possible Tuesday through midweek due to a slightly larger southwest swell arriving. Surf along east facing shores will remain small today, then decrease Sunday and Monday as the trades lower locally and upstream of the islands. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Wroe AVIATION...Farris MARINE...Jelsema