076 FXHW60 PHFO 100648 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 848 PM HST Tue Oct 9 2018 .SYNOPSIS... High pressure northeast of the state will keep a moderate trade wind flow in place through Wednesday, with showers favoring windward and mauka areas. The trades will ease Thursday through Saturday as a cold front approaches from the northwest, and this will allow for daytime sea breezes and overnight land breezes in most areas. Showers will favor interior and mauka areas during the day, and coastal areas at night. A cold front along with a weakening band of showers will then move into the island chain Saturday night through early next week. Drier and cooler conditions are expected early next week behind the front. && .DISCUSSION... Currently at the surface, a 1029 mb high is centered around 1600 miles northeast of Honolulu, while a trough of low pressure is located around 950 miles west of Kauai. The gradient between these features is driving moderate trade winds across the island chain this evening. Infrared satellite imagery shows several cloud layers moving through the islands, with generally partly to mostly cloudy conditions statewide. Radar imagery shows scattered showers developing just upstream of the islands with a few of these showers moving onshore into windward areas. Elsewhere, mainly rain free conditions prevail at the moment, although a few showers continue to affect leeward sections of the Big Island. 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 through Wednesday. The trades will ease on Thursday as an approaching shortwave trough begins to develop weak surface troughing over the western islands, allowing for the 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. Tonight through Thursday, A band of low clouds and showers will move into the islands tonight, so expect an increase in showers particularly across windward areas as the night progresses. This band of enhanced moisture will then linger over the area through Wednesday night and possibly 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. Thursday night through Saturday, 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 as well 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. The light low level steering flow should keep showers confined mainly to locations near the coast during the overnight hours both Thursday night and Friday night, and over the interior and mountain areas during the afternoon and evening hours Friday and Saturday. Given the unstable airmass expected over the islands through this period, some locally heavy rainfall can't be ruled out, particularly from Oahu eastward to the Big Island. Additionally, the most unstable conditions appear to be on Saturday afternoon across Maui County and the Big Island, where a thunderstorm can't be ruled out either. Saturday night through next Tuesday, 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 follow the frontal passage during the early part of next week, with shower activity primarily confined to windward areas. && .AVIATION... Trade winds will continue to flow through the main Hawaiian Islands through at least Wednesday. VFR will be the prevailing flying condition tonight through Wednesday morning. There will be some scattered showers with brief MVFR conditions, favoring the windward and mountain areas, but there will a shower or two that will venture deep into the lee areas of the smaller islands. We are looking at prevailing tops of the showers at 8k feet, with higher tops at 12k feet in the heavier showers. The exception to this is the Kona side of the Big Island. It has been an active afternoon with scattered heavy showers, and even a thunderstorm. So far, this evening, isolated showers continued mainly along the shoreline with tops to 20k feet. We anticipate these isolated, pop up heavy showers, to continue overnight along its shoreline and adjacent waters. AIRMET Sierra is not expected overnight. The air mass is not stable enough to support low level turbulence despite local areas of strong trades. So AIRMET Tango is not expected. Showery, cloudy conditions may move through the islands Wednesday afternoon and evening, in association with a band of clouds and showers. Lee areas of the smaller islands may get wet as swell. AIRMET Sierra, is a strong possibility right now. The Kona side may once again experience scattered heavy showers and an outside chance of a thunderstorm Wednesday afternoon. && .MARINE... Moderate to locally strong trade winds will decrease tomorrow. A 1029 mb surface high sitting far northeast of the islands will gradually weaken on Wednesday and Thursday, leading to the decline in the trades. The Small Craft Advisory (SCA) in effect for the typically windy waters around the Big Island and Maui will likely be dropped by Wednesday morning. A developing trough near or over the islands will disrupt the trades by late Thursday or Thursday night, followed by a weak frontal passage that will usher in gentle to moderate northerly winds during the weekend. SCA strength winds are not expected to redevelop through early next week. East swell from distant Hurricane Sergio continues to hold steady this evening. 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 Wednesday night based on Wavewatch III output. The east swell is expected to gradually decline Thursday and Friday. North and south facing shores will see surf through much of the week. A northwest swell is peaking at 4 to 6 feet 12 seconds today. This swell will decline over the next couple of days, followed by a smaller reinforcing swell on Thursday and Friday. A potentially larger north-northwest swell is due late Sunday and Monday. Small to moderate southwest swells will persist through much of the week, though a larger pulse tomorrow could produce south shore surf near the advisory level. && .HFO WATCHES/WARNINGS/ADVISORIES... High Surf Advisory until 6 AM HST Thursday for Oahu Koolau- Olomana-Molokai Windward-Maui Windward West-Windward Haleakala- South Big Island-Big Island North and East. Small Craft Advisory until 6 AM HST Wednesday for Maalaea Bay- Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island Southeast Waters. && $$ DISCUSSION...Jelsema AVIATION...Lau MARINE...Kino