Area Forecast Discussion
Issued by NWS Honolulu, HI
Issued by NWS Honolulu, HI
447 FXHW60 PHFO 200637 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 837 PM HST Thu Sep 19 2024 .SYNOPSIS... A fairly dry moderate to locally breezy trade wind pattern will continue through Friday afternoon. A weak disturbance moving through the Hawaii region this weekend will increase shower trends for all islands, especially over windward and mountain areas. Isolated thunderstorms are possible for Kauai and Oahu over the weekend due to a nearby upper low. Drier trends return from next week Monday onward. && .DISCUSSION... The satellite picture this evening shows a band of unsettled showers moving into the windward and mountain slope of the Big Island and Maui. Local radar imagery shows periods of moderate showers affecting both islands, mainly along the northern slopes. This shower band will continue to drift westward, affecting northern and eastern slopes of Molokai, Oahu and Kauai; through the early morning hours on Friday. Elsewhere, fairly stable conditions persist with just brief showers in the short term forecast. In the larger scale, a high pressure ridge remains far north of the Hawaiian Islands and this ridge will continue to produce moderate to locally breezy trade winds through Friday. Wind speeds will decrease slightly this weekend, as the ridge weakens in response to a cold front passing through the Central Pacific basin. Fairly strong subsidence (downward vertical motions) across the region will limit rainfall coverage through Friday afternoon. A weak low level trough riding in on the easterly trade winds will link up with an upper level low drifting into the islands from the north this weekend. These two systems will produce more unstable conditions statewide and increase shower trends statewide from Friday night through Sunday. Isolated thunderstorms are also possible over Kauai and Oahu this weekend. The development of any thunderstorms will highly depend on the strength of this developing upper low near the western islands this weekend. Drier conditions will return from next week from Monday onward as these low pressure systems diminish and move westward away from the state. Moderate to locally breezy trade winds continue with brief overnight to early morning showers possible over windward and mountain areas. && .AVIATION... Breezy trades will continue for the next few days. SHRA and low cigs should favor windward and mauka locations. Brief MVFR conds can be expected in any SHRA. VFR should generally prevail elsewhere. No AIRMETs are in effect. As we get a slight uptick in SHRA overnight, AIRMET Sierra for mtn obsc may be needed. && .MARINE... Fresh to strong easterly trade winds will continue through Saturday, then ease into the light to moderate range Sunday through the first half of next week as a weakness forms in the ridge to the north. The strongest winds are expected over the windier waters and channels around Maui County and the Big Island, where a Small Craft Advisory remains in effect through early Saturday. This may need to be extended through the day Saturday, before being lowered as the winds begin to trend down during the latter half of the weekend. Surf along north facing shores will trend up through the first week of fall, beginning next Tuesday through midweek in response to a gale forecast to develop near the Kurils Friday, racing eastward across the Date Line near the Aleutians this weekend, then to the Gulf of Alaska early next week. Guidance shows this northwest swell peaking locally Tuesday night into Wednesday out of the northwest, then fading Thursday into Friday as it shifts out of a more north-northwest direction. Surf along east facing shores will remain small and choppy into Saturday, then decrease Sunday into next week as the trades lower locally and upstream across the eastern Pacific. Surf along south facing shores will remain small through the weekend, with mainly a mix of background long-period southwest and short-period southeast swells moving through. An upward trend is possible Tuesday through midweek due to a slightly larger southwest swell arriving. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 AM HST Saturday for windier waters near Maui and the Big Island. && $$ DISCUSSION...Bohlin AVIATION...Walsh MARINE...Gibbs