841 FXHW60 PHFO 102000 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 1000 AM HST Wed Oct 10 2018 .SYNOPSIS... A band of clouds and showers will sag southward into the islands today and linger over the area through Thursday. Light 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, particularly from Oahu eastward to the Big Island Friday through Saturday. A cold front along with a weakening band of showers will then move into Kauai Saturday night and progress southeast down the island chain Sunday into early next week. 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 1550 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 moderate trade winds across the unsheltered areas of the island chain late this morning. Visible satellite shows partly to mostly cloudy skies over Kauai and Oahu, with clouds over windward sections of Molokai and Maui. The rest of the state is mostly sunny this morning. Radar shows passing showers focused over the windward sides of the smaller islands. Leeward sides of the smaller islands are also seeing a few passing showers as they move across the mountain tops. These clouds and showers are associated with a band of moisture moving in with the trade winds from upstream. This area of moisture will continue to move in and linger over the islands into Thursday. The trades will ease on Thursday as an approaching shortwave trough begins to develop weak surface troughing over the western islands, allowing for localized sea breezes in the more sheltered areas. The trades will become disrupted statewide Thursday night as the weak surface trough slowly slides eastward across the state, with overnight land breezes and daytime sea breezes common in most areas. Deep moisture will begin to 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 Maui County and the Big Island on Saturday, and east of the state by Saturday night. The airmass will also become increasingly unstable due to the shortwave trough passing through the state Friday and Saturday. Thunderstorms or locally heavy rain could occur during this period. A deep closed low digging southward in the vicinity of 32N 160W is expected to drive a 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. In advance of the front light south to southwest winds will likely develop, with winds swinging around to the north and northwest at light to moderate levels following the frontal passage. We should see a slow return to trade winds with north- northeasterly winds early in the week transitioning back to east- northeasterly trades heading into the middle of next week. A band of clouds and light to moderate showers will accompany the front. Drier and cooler conditions should then build in Monday, with shower activity primarily confined to windward areas. && .AVIATION... Light to moderate trade winds will persist through the day, and VFR will predominate. However, showery low cloudiness upwind from the isles will mainly affect windward and mauka sections from Kauai to Maui with brief periods of MVFR conditions, though some leeward areas may see spotty showers by the afternoon hours as well. On the Big Island, afternoon heavy showers are likely again over leeward portions of the isle. 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. 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 is starting to register on the PacIOOS buoys off of Barbers point and Lanai. This swell is expected to fill in today and peak at advisory levels later today into tonight. An HSA has been issued 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