000 FXHW60 PHFO 210651 AFDHFO Area Forecast Discussion National Weather Service Honolulu HI 851 PM HST Wed Sep 20 2017 .SYNOPSIS... A high pressure system located far NNE of the islands will continue to generate moderate to locally strong trades through Thursday. Lighter trades will come about starting Friday as the high drifts further away from the area, and its trailing ridge shifts closer to the islands from the north. Still, the trade wind regime will prevail with clouds and showers favoring the windward and mauka areas. The showers will be most frequent during the nights and mornings. && .DISCUSSION... A 1032 mb high far NNE of the main Hawaiian Islands continues to produce moderate to locally strong trade winds across the area this evening. Satellite imagery shows a narrow band of showers nearing the windward shores of the smaller islands, especially Oahu to Maui. Expect some scattered windward showers for these three islands within the next hour. Showers from this same band will affect windward Kauai in an hour or two. Cloud and shower distribution further upwind is scattered at best now with a couple of large breaks. We expect a slight increase to these trade showers as the night progresses but we do not expect anything out of the ordinary. There is no organized cloud masses that will significantly boost the overnight showers like last night. In brief, it is more or less a typical trade wind pattern this evening. The windward and mountain areas will absorb most of these showers, leaving the lee side dry. Weather radar is picking up a couple of moderate to heavy showers over Kailua-Kona Town now. There has been scattered showers along the entire stretch of the Kona coast prior to these showers We would not rule out a few pop up showers along the coast lasting into the early morning hours of Thursday. The Big Island is cloudier than usual, thanks to a patch of mid level clouds between 15k and 20k feet. Water vapor imagery shows a shortwave trough extending from the upper low, located 700 miles NE of the Big Island, through Oahu. The trough is moving southeastward and should kick out the mid level clouds later tonight or Thursday morning. Satellite derived moisture content or precipitable water (PW) was between 1.3 and 1.6 inches this afternoon as per soundings from Lihue and Hilo, with the highest over on the Big island. The outlook is for no significant change for the next few days. The latest GFS solution keeps the upper low where it is now through Friday evening. This suggest little change to the current marine layer, that is it slopes from 7k feet over Kauai to 10k feet over Maui to 16k feet over the Big Island. The upper low weakens on Saturday. And this will allow for mid level ridging to build across the Big island, which in turn lowers the marine layer to 13k feet over the weekend. As noted above, we are looking at slightly weaker trades beginning Friday. This is due to the surface high moving further away from the area and its associated ridge being a bit closer to the islands. It looks like the strength of the trades will ward off widespread daytime onshore sea breezes. And this mean, limited afternoon clouds to perhaps the leeward shores like the Waianae Coast on Oahu. In summary, trades will continue through the weekend although at tad weaker starting Friday. Embedded trade showers will continue to favor the windward and mountain areas, especially nights and mornings. Plenty of sun all around but especially along the southern shores. .AVIATION... High pressure north-northeast of the State will continue to drive a moderate to breezy trade wind flow across the islands through Thursday. Clouds and showers will favor windward and mauka areas, with a stray shower reaching leeward locales from time to time. Predominantly VFR conditions are expected at the terminals through 06Z Friday, but some brief MVFR cigs/vsbys will be possible in windward areas as trade showers move through. AIRMET Tango remains in effect for low level turbulence over and downwind of the terrain of all islands. The AIRMET will likely continue through the day on Thursday. AIRMET Sierra may be needed for mountain obscuration across portions of the island chain later tonight. && .MARINE... Fresh to locally strong trades associated with 1033 mb high pressure north-northeast of the state across the eastern Pacific will hold through Friday before trending down over the upcoming weekend and early next week. Strongest winds are expected across the Pailolo and Alenuihaha Channels, Maalaea Bay and the waters south of the Big Island through this time. Trades should reach the light to moderate range by the end of the weekend as high pressure shifts eastward and weakens in response to a frontal boundary passing far north of the state. This pattern should hold through the first half of the upcoming week (potential for land/sea breezes for some leeward areas setting up Monday through Tuesday). A combination of moderate to strong breezes locally and upstream of the islands south of high pressure will continue to generate rough surf along east facing shores into Friday. Surf along east facing shores will trend down over the upcoming weekend as the trades weaken locally and upstream. Guidance suggests a weak or broad surface trough developing east of the islands Friday due to a broad upper low centered northeast of the state, which will weaken the easterlies upstream of the state and lower the trade wind swell. Surf along south facing shores will remain small through early next week with mainly background southeast and southwest swell energy expected. The next potential long-period pulse out of the southwest from the Tasman Sea will be possible by Sunday, which should lead to a slight increase in surf. For the extended, strong- to gale-force southwest winds associated with a low that has been impacting the Tasman Sea over the past 24 hrs may lead to another small southwest (210-220 deg) swell Tuesday through midweek. A small northwest (310 deg) swell associated with broad and complex low pressure that has been centered over the central Pacific near the Date Line over the past couple of days is expected to fill in locally Thursday and hold into the weekend before easing. The latest surface analysis showed 989 mb low centered near the Aleutians around the Date Line with a decent pocket of strong- to near gale- force winds focused within the 320 to 330 deg band relative to the islands. This may end up reinforcing what is left of this weekend's 310 deg energy by Monday, before completely fading Tuesday through Wednesday. As a result, a slight increase in surf is expected along north and west facing shores beginning Thursday, which should hold into early next week before dropping off. && .HFO WATCHES/WARNINGS/ADVISORIES... Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay, Pailolo and Alenuihaha Channels, and waters south of the Big Island. && $$ Lau/Jelsema/Gibbs