421 FXHW60 PHFO 121309 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 309 AM HST Mon Apr 12 2021 .SYNOPSIS... A slow moving trough aloft will maintain unsettled conditions over the eastern half of the state today. Northeasterly trade winds will gradually strengthen over Kauai and Oahu today, while lighter winds and lingering instability maintain chances for spotty showers and thunderstorms across Maui County and the Big Island. Moderate northeasterly trade winds and a typical pattern of windward and mauka showers will prevail Tuesday through Thursday, with an increase in showers possible over some areas Friday and Saturday. && .DISCUSSION... Have lowered the PoPs over Kauai and Oahu a little for the first 36 hours or so based on the overall model trend, and the lower precipitable water (PW) values moving over the western end of the state. Have boosted the winds a touch in the first couple of days in keeping with the general trend of the higher resolution models. Otherwise, the forecast remains largely unchanged. An upper level trough remains over/near the Big Island early this morning and will continue to bring some instability to the eastern half of the state into tonight. The chance for thunderstorms remains in the forecast for Maui County and the Big Island, with the Big Island being slightly more likely to experience thunderstorms in part due to the proximity to the upper trough. PW from the Lihue sounding at 2am was 0.76, while Hilo was just over an inch at 1.07. Satellite derived PW shows the PW is just a bit higher than both soundings over Oahu and Maui County this morning, however all in all, values are near or below normal for April. The upper level trough is bringing much colder than normal temperatures to the upper atmosphere over the islands, which is helping to fuel the instability that has sparked some of the heavier showers and thunderstorms. As the trough makes it way to the east, we do expect warmer temperatures to move in at the upper levels from the west, which will help to stabilize the atmosphere. A ridge to the north of the islands has already returned a moderate to locally breezy trade wind pattern to the western end of the state. Combined with the drier PW values moving in, expecting showers over Kauai and Oahu to be somewhat limited today, and focused over the typical windward and mauka areas. For Maui County and the Big Island, winds remain on the lighter side, which will likely lead to some localized land and sea breezes. This will in turn lead to some afternoon clouds and showers developing over leeward and interior locations. As the upper trough moves to the east, expect the trade winds to filter in to Maui County and the Big Island. Look for a statewide stable trade wind flow Tuesday. A lingering low pressure system far to the northeast, combined with the strengthening high to the far northwest will mean the trades will be little more northeasterly than usual, and winds will be strongest over the western end of the state. Precipitable water values will be near to slightly below normal, and dew points will remain in the upper 50s to around 60, suggesting modest rainfall over windward areas and a continued cool feel. Winds may back off on Friday and next weekend as the ridge to the north weaken. The GFS and ECMWF are showing a decayed front dropping near or over the northern half of the state, which favors Kauai and Oahu for wetter trade wind weather. && .AVIATION... Light to moderate northeast trades will prevail today and tonight, with a gradual strengthening trend expected. An upper level disturbance will keep some unsettled weather in place over Maui and the Big Island today, with some locally heavy showers and even a thunderstorm or two possible, mainly during the afternoon and early evening hours. Elsewhere, more typical trade wind weather can be expected, with clouds and showers favoring windward and mauka areas, and a stray shower reaching leeward locales from time to time. No AIRMETs are currently in effect. AIRMET Sierra may be needed later today for mountain obscuration across portions of the state. AIRMET Tango may be needed later this morning for moderate upper level turbulence over the western half of the state between FL290 and FL370. && .MARINE... A ridge north of the area will produce moderate northeast trade winds over Kauai and Oahu waters today and light to gentle northeast trade winds over Maui County and Big Island waters. A trough aloft has been producing locally heavy showers and thunderstorms over the Maui County and Big Island waters. The threat of thunderstorms will continue through tonight. Northeast winds will strengthen through mid-week as a high far northwest of the area moves east. Winds may reach Small Craft Advisory speeds in some areas. Winds will subside through the second half of the week as a front approaches from the north. A small northwest swell will peak tonight, then subside through Wednesday night. A moderate, shorter-period north swell expected Thursday may persist through next weekend. Lighter than normal trade winds will keep surf small along east facing shores through mid-week. The north swell will wrap in to some east facing shores later in the week. Swells from distant southern hemisphere storms will produce small to moderate surf along south facing shores. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...M Ballard AVIATION...Jelsema MARINE...Donaldson