Area Forecast Discussion
Issued by NWS Honolulu, HI

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874 FXHW60 PHFO 191339 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 339 AM HST Mon Apr 19 2021 .SYNOPSIS... An approaching front will disrupt the trade winds across much of the state today through Wednesday, resulting in a land and sea breeze pattern across the island chain. This light wind regime will focus showers over interior locations during the day and locations near the coast at night. Shower activity will remain limited today, with an increase in showers expected tonight and Tuesday over Oahu and Kauai as an upper level disturbance moves through. A thunderstorm or two is also not out of the question over the Garden Isle. Moderate to breezy trades and more typical trade wind weather will return Thursday and continue through next weekend, with showers favoring windward and mauka locales. && .DISCUSSION... Currently at the surface, a 1030 mb high is centered around 1650 miles northeast of Honolulu, while a cold front is located around 750 miles northwest of Kauai. Light winds have developed over the western islands, with land breezes present in most areas, while light to moderate east-southeasterly boundary layer flow now established over the eastern end of the state. Infrared satellite imagery shows mostly clear to partly cloudy conditions in most areas. Radar imagery shows a few showers moving into east and southeast facing slopes and coasts of most islands, although most of the shower activity remains offshore. Main short term concern revolves around the potential for some unsettled weather Monday night through Wednesday. A cold front approaching from the northwest and a building high to the far northeast, will keep southeasterly boundary layer flow in place through mid week. The Big Island terrain will block the stronger winds from reaching the smaller islands, allowing for widespread land and sea breeze development today through Wednesday. A new surface ridge is forecast to build to the north of the islands during the second half of the work week, bringing a return of moderate trade winds by Thursday, with the trades then ramping up to breezy levels Friday through the weekend. As for the remaining weather details, the land breeze and sea breeze pattern expected through mid week, will favor showers developing over interior locations during the day and areas near the coast at night. Models continue to advertise an increase in deep layer moisture lifting up from the south and over the western islands late Monday night through Wednesday. This will likely increase shower coverage over Kauai and Oahu during this time, with some locally heavy rainfall and even a thunderstorm or two possible over Kauai late Monday night and Tuesday as an upper level disturbance moves overhead. A more typical trade wind pattern will return Wednesday night and linger through the weekend, with showers favoring windward and mauka areas. && .AVIATION... A cold front approaching the islands from the northwest will weaken the ridge north of the islands, causing trade winds to transition to a light and variable wind pattern for most areas. These light large scale winds will allow local scale land and sea breezes to develop over each island. Converging sea breeze winds during the daylight hours will build clouds over island mountain and interior sections. A few showers may develop over these areas in the late afternoon to early evening time periods. Mostly VFR conditions are forecast with the possibility for Tempo BKN025 along the east to southeast slopes of the Big Island, and over island interior sections in the afternoon to early evening. No AIRMETs in effect and none are expected. && .MARINE... Winds have shifted out of the southeast overnight due to an approaching front an upper trough from the northwest. This shift will translate to a land and sea breeze regime along the coasts for most areas through the first half of the week. The exception will be over the windward waters of the Big Island along the Hamakua coast and around South Point where moderate to fresh southeast winds will continue. In addition to the winds, a few heavy showers and even a thunderstorm or two will be possible tonight through Tuesday night over the Kauai and Oahu waters as the front nears Kauai. A return of moderate to fresh easterly trades will be possible for all waters through the second half of the week. Surf along north facing shores will hold steady today due to a persistent short-to-medium period mix of north-northwest to north-northeast energy moving through. As this energy eases into Tuesday, a medium-period, moderate northwest swell from a nearby source should arrive and begin to build down the island chain Tuesday night through Wednesday. This will be a long-lived event that should hold into next weekend as it shifts out of a more north-northwest direction. Surf along south facing shores will remain up today, then trend down Tuesday through midweek as the south-southwest swell eases. A similar south-southwest swell will be possible later in the week. Surf along east facing shores will remain small through the week. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Jelsema AVIATION...Bohlin MARINE...Gibbs

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