Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
465 FXHW60 PHFO 151959 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 959 AM HST Sun Sep 15 2024 .SYNOPSIS... Breezy trade winds and typical showers within this trade regime, with higher rain amounts confined to windward exposures and upper elevations, will be the general theme this week. A slug of higher moisture passing in from the east tonight will increase shower coverage and possibly periodic heavy rainfall primarily over windward Big Island and Maui County. && .DISCUSSION... A large area of surface high pressure approximately 1,400 miles from Oahu will remain anchored in nearly the same location through the week. The pressure gradient from this high will remain tight enough to support areawide breezy to locally gusty trades through terrain gaps the next several days. The high will weaken late this week as will the resultant trades. Precipitation will primarily be focused along windward areas with the bulk of the rain falling during the overnight hours. Nighttime into early morning windward mauka showers will occasionally pass over the ridge tops and wetten leeward slopes or provide leeward sprinkles. Higher mid level moisture over the eastern offshore waters (a greater than 2 inch pwat air mass) is noted in a wide swath of warmer low to mid level clouds moving toward the state this morning. This area of deeper moisture and associated mid level trough will pass across Big Island and windward Maui County tonight into early Monday morning. The depth of this higher moisture increases the probabilities that various windward Big Island and Maui County communities may experience bouts of brief heavy rain tonight. While the moisture increase over the smaller islands is not expected to be as deep, a higher frequency of (predominantly windward) showers is likely. Once mid level troughing washes out west of the state Tuesday, typical trade wind weather will return. Less mid to late week diurnal rainfall is expected (in both frequency and areal coverage) as a drier air mass advects in. The extended forecast going into the final third of the month is looking fairly wet. Forecast guidance has an upper low moving toward the state from the north northeast a week from now. While rain probabilities will be on the rise, the chances for heavy rain and isolated thunder (as upper levels cool a touch and mildly destabilize the regional atmosphere) are low due to the uncertainty of this low`s location or proximity to the islands. && .AVIATION... High pressure centered far north of the state will maintain moderate to breezy trades over the next few days. Clouds and showers will generally favor windward and mauka areas, though the temperature inversion heights will remain elevated enough to allow occasional showers to pass over island terrain into leeward areas. Tonight, a plume of deeper moisture will move in from the southeast, affecting primarily east and southeast portions of the Big Island and Maui. MVFR conditions, and possibly isolated IFR, are expected in heavier showers. Elsewhere, VFR conditions are expected to prevail. With locally breezy trade winds prevailing across the state, AIRMET Tango remains in effect for low-level turbulence over and downwind of terrain. This AIRMET will likely continue through Monday. AIRMET Sierra remains in effect for tempo mountain obscuration across windward areas of the smaller islands this morning due to low clouds and showers. However, latest satellite trends suggest that this AIRMET will likely be expired later this morning. Mountain obscuration is forecast to return by this evening for at least the eastern end of the state. && .MARINE... Moderate to locally strong trade winds will hold into Wednesday, as a strong surface high meanders 1,000 to 1,400 nm north of the islands. A Small Craft Advisory (SCA) remains in effect for the typically windy waters around the Big Island and Maui County through Tuesday morning and will likely be extended into Wednesday. The high will weaken and move eastward late Wednesday and Thursday, leading to a slight decline in the trades. Surf across all shores will be below seasonal average through the week. Trade winds will drive short period seas of around 5 to 6 feet at 7 seconds into Wednesday, maintaining moderate east shore surf. A decrease in trade winds will cause east shore surf to decline Thursday and Friday. Moderate surf along south facing shores will slowly decline Monday and Tuesday as the current south swell drops. A small southwest pulse out of the Tasman Sea could produce a slight rise in surf Friday. Aside from trade wind wrap, surf along north facing shores will be minimal through Wednesday, and a tiny northwest swell will be possible Thursday or Friday. The combination of seasonally high astronomical tides and water levels running around 6 inches higher than predicted will produce localized coastal inundation each afternoon through Tuesday. A Coastal Flood Statement has been issued to highlight these conditions. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Tuesday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...Blood AVIATION...Vaughan MARINE...Wroe