618 FXHW60 PHFO 110205 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 405 PM HST Wed Oct 10 2018 .SYNOPSIS... An area of clouds and showers will pass over the area through Thursday. Gentle to moderate trade winds will keep most of the shower activity across windward areas, but showers will reach leeward areas at times as well. The trades will ease Thursday through Saturday as a trough of low pressure develops over the western islands and shifts slowly eastward across the state. Deep moisture will spread northward along and ahead of this trough, bringing some unsettled weather to the islands Friday through Saturday. A weak cold front will then move into Kauai Saturday night and progress southeast down the island chain Sunday and Monday. Drier and cooler conditions are expected behind the front for early next week. && .DISCUSSION... Currently at the surface, a 1026 mb high is centered around 1600 miles northeast of Honolulu, while a trough of low pressure is located around 700 miles northwest of Kauai. The gradient between these features is driving gentle to moderate trade winds across the state this afternoon. The trade winds are ushering in an area of clouds and showers that will linger over the islands until Thursday. Visible satellite shows mostly cloudy skies over most of the islands with some breaks in the clouds over leeward Oahu and Molokai. Radar show scattered to numerous showers impacting windward slopes, with scattered showers moving over the mountains onto leeward sides at times. The trades will weaken on Thursday as an approaching upper trough begins to develop weak surface troughing over the western islands. Nighttime land breezes and daytime sea breezes will become widespread Thursday night through Friday night as the trough slowly moves eastward. Deep moisture will also pool to the south of the islands Thursday night with precipitable water values climbing into the 1.6 to 1.9 inch range. This deeper moisture will then spread northward over Oahu, Maui County and the Big Island Friday and Friday night, then shift eastward mainly over the Big Island on Saturday, then east of the state by Saturday night. Along with the moisture, the airmass will also become more unstable due to the shortwave trough passing through the state Friday and Saturday. Temperatures aloft will not be particularly cold, but enough to interact with the moisture and produce heavier showers or thunderstorms. A slight chance of thunderstorms and locally heavy rainfall has been added to the forecast from Friday afternoon (western half of the state) spreading eastward to the Big Island on Saturday. A Flood Watch will be considered as this event gets closer. Following the exit of the surface trough, a deep closed low digging southward in the vicinity of 32N 155W is expected to drive a weak cold front through Kauai Saturday night, into the central islands on Sunday, with the front then stalling out and slowly dissipating over the eastern islands Sunday night into Monday. A band of clouds and light to moderate showers will accompany the front. In advance of the front light south to southwest winds will likely develop on Saturday, with winds swinging around to the north at light to moderate levels following the frontal passage through Monday night. Drier and cooler conditions will build with the north winds on Monday. && .AVIATION... Light to moderate trade winds will persist through the evening and nighttime hours, and VFR will predominate. However, showery low cloudiness upwind from the isles will mainly affect windward and mauka sections with brief periods of MVFR conditions. No AIRMETs are currently in effect. && .MARINE... High pressure sitting far northeast of the islands will continue to weaken through Thursday. A developing trough near the islands will disrupt the trades on Friday, followed by a weak frontal passage that will usher in gentle to moderate northerly winds through the weekend. SCA strength winds are not expected to redevelop through early next week. However, there is a chance for thunderstorms over the coastal waters Friday into Saturday with the trough. East swell from distant Hurricane Sergio is holding steady this morning, large enough to continue High Surf Advisory (HSA) level surf across east facing shores of Oahu, Maui, and the Big Island through tonight. The east swell is expected to gradually decline Thursday and Friday. A new long-period south-southwest swell will continue to fill in and peak at advisory levels into tonight. An HSA is in effect for south facing shores of all Hawaiian Islands through tonight. This swell will slowly fade Thursday through Saturday. The current northwest swell will continue to slowly decline over the next couple of days. A smaller reinforcing swell is expected on Thursday and Friday from former Typhoon Kong-rey. A potentially larger north-northwest swell is due late Sunday and Monday. && .HFO WATCHES/WARNINGS/ADVISORIES... High Surf Advisory until 6 AM HST Thursday for Niihau-Kauai Windward-Kauai Leeward-Oahu South Shore-Waianae Coast-Oahu Koolau-Olomana-Molokai-Lanai Makai-Kahoolawe-Maui Windward West- Maui Leeward West-Maui Central Valley-Windward Haleakala-Leeward Haleakala-Kona-South Big Island-Big Island North and East. && $$ DISCUSSION...Foster AVIATION...Kinel MARINE...TS