Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
631 FXHW60 PHFO 220111 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 311 PM HST Sat Sep 21 2024 .SYNOPSIS... Moderate to locally breezy trade winds will ease for Sunday and Monday as the surface ridge to the north briefly weakens. Rainfall will favor windward and mauka locations, but a few showers, some heavy, may develop across leeward areas as a disturbance aloft moves overhead. Breezy trades and more stable conditions are expected to return on Tuesday. && .DISCUSSION... A moderate to locally breezy trade wind flow will begin to ease and destabilize tonight. A surface ridge sitting nearly 1,000 miles to the north is driving the trades, which have stubbornly remained breezy today. The winds should begin to gradually fall off this evening as the ridge starts to weaken as drift southward. An upper-level low centered a couple hundred miles north of Kauai has so far had minimal effect on local weather aside from some thin high clouds, but as the low aloft digs southward tonight, the inversion will lift and weaken. As a result, mainly windward showers should become a bit heavier than the past few days, and a narrow and diffuse band of moisture in the trades will likely bring higher shower coverage from Maui to Oahu overnight. 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 weaken slightly heading into tomorrow and isolated to scattered showers will favor 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 can be expected in heavier showers with isolated IFR conditions possible. VFR should prevail outside of shower activity. AIRMET Tango is in effect for tempo moderate turbulence between 12,000 and 40,000 feet this afternoon as the upper level low approaches from the north. This AIRMET will likely be needed through Sunday morning. AIRMET Sierra may be needed for mountain obscuration for windward locations during the overnight hours through this weekend, depending on if the shower coverage becomes extensive enough to warrant it. && .MARINE... Fresh trade winds will ease slightly tonight and Sunday 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. The trades should 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 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. An increase in trade winds will produce larger east shore surf near seasonal average by Tuesday night or Wednesday. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Wroe AVIATION...Farris MARINE...Wroe