Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
155 FXHW60 PHFO 230700 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 900 PM HST Sun Sep 22 2024 .SYNOPSIS... Moderate trade winds will prevail through 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. Locally breezy trades and more stable conditions will return on Tuesday and persist into next weekend. && .DISCUSSION... Currently at the surface, a ridge of high pressure is located around 950 miles north of Honolulu, with broad but weak troughing present between the ridge and the Hawaiian islands. Aloft a closed upper low can be seen in water vapor imagery around 150 miles to the north and northeast of Oahu and Kauai respectively. The broad surface troughing over and north of the state has relaxed the pressure gradient a bit, with generally moderate trade winds blowing across the island chain this evening. Infrared satellite imagery partly to mostly cloudy conditions, with radar imagery showing scattered showers moving in with the trades and some decaying daytime heating driven showers over leeward areas. Main short term focus is on rain chances and trade wind trends. The broad trough of low pressure over and north of the state will gradually dampen out and shift westward during the next couple days. Moderate trades will prevail through Monday with localizes seas breezes developing in some of the more sheltered leeward areas Monday afternoon. The trades will increase a tad Monday night and hold at moderate to locally breezy levels Tuesday through Friday. The trades may ease a bit next weekend as the ridge to the north weakens. As for the remaining weather details, the airmass will remain unstable with the upper level low near the western islands tonight through Monday night. This will allow some more robust showers to move in on the trades and produce brief downpours. Additionally, some locally heavy showers could develop in leeward areas with the assistance of localized sea breezes during the afternoon and early evening hours before fizzling out overnight. Mid-level ridging building in from the east will bring a more stable airmass to the islands Tuesday through Thursday. This should result in a return to typical trade wind weather featuring mainly windward and mauka showers. Another disturbance aloft could bring an increase in trade wind shower and intensity Friday through next weekend. && .AVIATION... Moderate trades will continue through tomorrow and usher in low clouds and showers to windward and mauka areas, with a few showers occasionally spilling over to the leeward side. Sea breezes will develop along some wind-protected areas tomorrow afternoon, leading to some low cloud and shower development over those areas. MVFR conditions can be expected in any showers. Otherwise, VFR should prevail. No AIRMETs are in effect. AIRMET Sierra may be needed if shower coverage becomes more extensive overnight. && .MARINE... High pressure generally north of the islands will drive moderate to locally fresh trades through Tuesday morning. High pressure northwest of the islands will strengthen through the middle of the week. This will produce fresh to locally strong trades from Tuesday through the remainder of the week. This will likely require a Small Craft Advisory for the windier waters surrounding Maui County and Big Island by Tuesday evening. Surf along north and west-facing shores will remain very small through Monday. A small to moderate size, long to medium period northwest swell (330 degrees) is timed to arrive Tuesday from a passing North Pacific cold front. This swell will increase mid week north- facing shore surf to well above head high at peak locations. North shore surf is expected to remain well under the High Surf Advisory 15 foot thresholds. South-facing shore surf will remain small as a mix of small, medium period southwest swells pass through the next few days. Eastern shore waves will remain small in response to recently lightened trades. An upward trend in eastern chop will occur later Monday and last through the week in response to strengthened trades. && .HFO WATCHES/WARNINGS/ADVISORIES... None. && $$ DISCUSSION...Jelsema AVIATION...Farris MARINE...Blood