898 FXHW60 PHFO 101408 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 408 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 1028 mb high is centered around 1650 miles northeast of Honolulu, while a trough of low pressure is located around 1000 miles west of Kauai. The gradient between these features is driving moderate trade winds across the unsheltered areas of the island chain early this morning. Infrared satellite imagery shows partly cloudy conditions in most areas, with a few pockets of more extensive cloud cover over windward locales. Radar imagery shows scattered showers moving into windward areas, with the greatest coverage moving into Oahu. Across leeward areas, a few isolated showers are making the trip over the terrain, but many locales are rain free at the moment. Main short term concern revolves around rainfall trends over the next few days. High pressure northeast of the state will keep a moderate trade wind flow in place today. 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 through Saturday as the weak surface trough slowly slides eastward across the state, with overnight land breezes and daytime sea breezes common in most areas. 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 low clouds and showers will continue to sag southward into the islands this morning, so expect scattered to numerous windward showers with a few leeward spill overs through the morning hours. This band of enhanced moisture will then linger over the area into Thursday, keeping windward areas fairly showery with some leeward convective shower development expected each afternoon in addition to the spill over showers from windward 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. The airmass will become increasingly unstable due to a sharp shortwave trough passing through the state. As a result, we should see an increase in convective shower activity beginning late Thursday night and continuing through Saturday, with the highest chances for rain expected from Oahu eastward to the Big Island from Friday afternoon through Saturday. Given the unstable airmass expected over the islands through this period, some locally heavy rainfall can't be ruled out. Additionally, the most unstable conditions appear to be moving through Saturday afternoon across Maui County and the Big Island, where a thunderstorm can't be ruled out either. A mention of heavy rainfall and thunder may need to be added to the forecasts across portions of the state once the details and timing become a bit more clear. The higher precipitable water airmass is expected to shift eastward and away from the islands Saturday night, with shower activity diminishing fairly quickly Saturday evening over the eastern end of the state. At the same time, a weakening cold front is forecast to be shifting southward through Kauai, bringing with it a band of showers. This front should continue to progress into Oahu and Maui County on Sunday, before stalling out and slowly dissipating over the eastern islands Sunday night into Monday. The coverage and intensity of the showers should diminish as the front progresses eastward down the island chain. Drier and cooler conditions should then build in following the frontal passage and continue through early next week, with shower activity primarily confined to windward areas. && .AVIATION... Thus far, VFR conditions prevail across the area. There are some small showers passing through favoring the windward and mountain areas of mainly the smaller islands. Contrary to the past few nights, windward Big Island has been mostly clear and dry. A change is forthcoming for mainly the smaller islands in the coming hours. Satellite imagery shows the leading edge to a band of clouds draped across the windward waters from the Kauai Channel eastward. Should this band hold, it will begin affecting Kauai around or shortly after sunrise. Since it is an elongated band, the weather with extensive MVFR conditions will keep Kauai under wraps for most of today. So, AIRMET Sierra is a strong likelihood for mainly the windward and mountain areas. This same band poses a threat to the other smaller islands as well. Forecast models suggest that the band will eventually sag south and into the remaining smaller islands starting later today. If this forecast pans out, expect AIRMET Sierra to be expanded to the other smaller islands. Windward Big Island may escape the bulk of this weather, although the lee side can expect another round of scattered afternoon heavy showers. Radar is detecting tops of the showers within this band at around 9k feet, and to 12 to 15k feet with the heavier showers. && .MARINE... An ASCAT pass overnight showed winds have dropped below the Small Craft Advisory (SCA) threshold. Thus, the SCA has been cancelled. High pressure sitting far northeast of the islands will continue to weaken through Thursday. A developing trough near or over the islands will disrupt the trades on Friday, followed by a weak frontal passage that will usher in gentle to moderate northerly winds late in the weekend. SCA strength winds are not expected to redevelop through early next week. East swell from distant tropical cyclone Sergio continues to hold steady this morning. PacIOOS buoys off of the Big Island, Maui, and Oahu suggest swell heights of 3 to 5 feet at 13 seconds, large enough to produce High Surf Advisory (HSA) level surf across east facing shores of those islands. The HSA is in effect through tonight based on Wavewatch III output. 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 continue to fill in throughout the day and is expected to peak at advisory levels later today into tonight. Due to the rising long-period swell, a 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...Jelsema AVIATION...Lau MARINE...Kino