763 FXHW60 PHFO 111348 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 348 AM HST Thu Oct 11 2018 .SYNOPSIS... Light to moderate trades will hold today, then become light and variable tonight 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 a 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 near 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 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 over the past 24 hrs. Peak rainfall totals over the past few hours ranged from a quarter to half of an inch, mostly over windward Oahu and windward Big Island. Expect this trend to hold through the early morning hours today. 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 today through Friday night, then advance down the island chain over the weekend. Trades will ease through the day with a return of light and variable winds by tonight. The pool of deep tropical moisture (2+ inch PWs) lurking off to the southwest 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 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 today. The trades will then ease to light levels tonight with land breeze developing in many areas. 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. Additionally, some MVFR cigs/vsbys will be possible as convective showers develop over interior and mountain areas of the islands this afternoon into this evening. 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 into the early morning hours, with some improvement possible by late morning. Convective shower development could lead to mountain obscuration this afternoon as well across portions of the islands. && .MARINE... High pressure sitting far northeast of the islands will continue to weaken through today. 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. PacIOOS Hilo and Mokapu buoys continue to show a mix of tradewind swell and the slowly fading east swell from former Hurricane Sergio. Surf will remain elevated along east facing shores through today then gradually decrease tonight through Friday. NOAA buoys south of Hawaii continue to show long-period energy in the 18-20 second band out of the southwest direction. Surf along south facing shores is expected to peak today and gradually decrease through rest of the week. The High Surf Advisory (HSA) has been extended for south facing shores of all Hawaiian Islands and east facing shores of Big Island, Maui, Molokai, Oahu and Kauai through today. A small reinforcing northwest swell is expected to fill in today through Friday from former Typhoon Kong-rey. A potentially larger north-northwest swell is due late Sunday and Monday. A much larger north-northwest swell is possible next Tuesday and will likely reach advisory levels. && .HFO WATCHES/WARNINGS/ADVISORIES... High Surf Advisory until 6 PM HST this evening 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