824 FXHW60 PHFO 110712 CCA AFDHFO Area Forecast Discussion...CORRECTED National Weather Service Honolulu HI 912 PM HST Wed Oct 10 2018 .SYNOPSIS... Light to moderate trades will hold into Thursday, then become light and variable late Thursday through Saturday as a cold front approaches from the northwest. Increasing moisture combined with a sufficient amount of instability and light winds ahead of this front will translate to warm and muggy conditions with increasing rain chances Friday through Saturday. A few thunderstorms will remain possible along and ahead of the front. A gradual drying trend is expected from west to east as the front advances down the island chain and light to moderate northerly winds fill in. A return of a more typical trade wind pattern is anticipated Tuesday through midweek as high pressure builds north of the state. && .DISCUSSION... Recently animated water vapor imagery showed an shortwave trough extending southwest just east of the Date Line and an upper low positioned north-northeast of the islands. The latest surface analysis showed a cold front associated with this shortwave trough extending southwest across the central Pacific to Midway and a weak trough out ahead of it several hundred miles west of the state. Satellite-derived precipitable water (PW) imagery depicted a plume of deep tropical moisture pooling northward along and ahead of it, mostly west-southwest of the islands. Here locally, light to moderate trades are holding in place south of a ridge of high pressure. Radar showed decent shower coverage over windward and mauka locations due to a batch of moisture that has filled in from east to west. Peak rainfall totals over the past six hours are coming in from windward Big Island (1-1.5 inches) and around an inch for windward Oahu. Expect this trend to hold into Thursday. Short-term (through Saturday) guidance remains in good agreement and supports an amplifying upper pattern evolving over the northern Pacific as the aforementioned shortwave trough digs southeastward toward and over the region. The attendant cold front and surface trough, currently near and east of Midway, are forecast to progress eastward toward the islands Thursday through Friday night, then advance down the island chain over the weekend. Trades will ease late Thursday with a return of light and variable winds by Thursday night. The pool of deep tropical moisture (2+ inch PWs) lurking off to the southwest of the islands is forecast to lift northward over the islands Friday through Saturday as the front approaches. This moisture combined lowering upper heights and the approaching front will be enough to support heavy showers come Friday afternoon, as sea breezes become established. Although the better instability and forcing are likely to remain north of the islands, a few thunderstorms can't be ruled out through this time. For the extended period (Sunday through midweek), guidance reflects a gradual drying trend in the wake of the front from west to east. Light to moderate northerly winds will fill in over the western end of the state beginning Sunday, then quickly shift to a more typical trade wind pattern Tuesday through midweek as high pressure builds north of the state. Rain chances may hang around Sunday toward the Big Island, where the front will stall and dissipate. && .AVIATION... Light to moderate trade winds will persist tonight and Thursday, with the trades then easing to light levels Thursday night. Clouds and showers will favor windward and mauka areas, but showers will reach leeward communities at times as well. The showers will bring MVFR cigs/vsbys to windward areas at times, with predominantly VFR conditions expected in leeward locales. AIRMET Sierra is in effect for mountain obscuration across north through east sections of Kauai, Oahu, and the Big Island. These conditions could hold in place through much of the night, and may need to be expanded to include Maui and Molokai as well. && .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, 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. The new long-period south-southwest swell is expected to peak tonight and gradually fade Thursday through Saturday. An HSA is in effect for south facing shores of all Hawaiian Islands through tonight, but it may have to be extended through Thursday depending on the buoy reports overnight. This swell will slowly fade late Thursday through Saturday. The current northwest swell will continue to slowly decline tonight. 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...Gibbs AVIATION...Jelsema MARINE...Kino