Synopsis23 Sep 2017 01:58:00 GMT: Trade winds will gradually trend down over the weekend as the ridge of high pressure to the north weakens and a weak surface trough slowly approaches from the northeast. Mainly windward clouds with light passing showers will prevail into early next week, with a possible boost in shower activity around the middle of next week.
Discussion23 Sep 2017 01:58:00 GMT: A weakening 1028 mb high is currently positioned far northeast of the area this afternoon and driving moderate trade winds over the islands. An upper low northeast of the area is also inducing a weak surface trough, though it is having little impact on our weather now.
Satellite-derived precipitable water imagery and upper air soundings at Lihue and Hilo reflected values between 1.2 to 1.4 inches. Visible satellite shows a few windward and mauka clouds with mostly sunny skies leeward. Radar shows very little precipitation blowing in with the trade winds this afternoon.
Trade winds are expected to trend down to gentle to moderate breezes over the weekend as the high to the far northeast weakens and the surface trough edges closer. Mostly dry and stable trade wind weather will continue each day with clouds and showers favoring windward locations. Rainfall accumulations will be light, with precipitable water values holding in the 1 to 1.3 inch range (near to below average) through the weekend and Monday.
The GFS and ECMWF both depict the surface trough approaching the islands from the east by midweek. The weak trough, combined with a slight increase in moisture, may be enough to bring the rain chances back to normal over windward locations Tuesday and Wednesday. Meanwhile, trade winds will remain in the gentle to moderate range until the weak surface trough reaches the state the middle of next week.
Aviation23 Sep 2017 01:58:00 GMT: High pressure north-northeast of the State will keep a moderate trade wind flow in place through tonight. Clouds and showers will favor windward and mauka areas, with isolated spillover showers trickling over into leeward areas from time to time. Brief MVFR cigs/vsbys in showers, otherwise prevailing VFR conditions expected all areas.
AIRMET Tango remains in effect for low level turbulence over and downwind of the terrain of all islands. This AIRMET will likely remain in place through at least the evening hours.
Marine23 Sep 2017 01:58:00 GMT: Fresh to locally strong trades should gradually weaken later today, decreasing wind speeds into the light to moderate range through the weekend. Trade winds will continue to bring rough surf along east facing shores today, easing this weekend.
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 is expected by Sunday and could lead to a slight increase in surf. For the extended forecast, strong to gale-force southwest winds associated with a low in the Tasman Sea may lead to another small southwest swell Tuesday through midweek.
A small northwest swell will continue into the weekend before easing. Another small reinforcement from that direction is expected to fill in around Sunday night through the middle of next week.
HFO Watches/Warnings/Advisories23 Sep 2017 01:58:00 GMT: None.
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