Synopsis22 Sep 2017 19:57:00 GMT: Breezy trade winds will continue today, then gradually trend down over the weekend as the ridge of high pressure to the north weakens and a weak surface trough approaches from the east. A typical pattern of mainly windward showers will prevail, though interior and leeward Big Island will experience extensive afternoon cloud cover and a few showers each day.
Discussion22 Sep 2017 19:57:00 GMT: A 1030 mb high far northeast of Hawaii while a weak trough of low pressure positioned south of the islands. The pressure gradient between these two synoptic features continues to support breezy trade winds across the local area. Satellite-derived precipitable water imagery and upper air soundings at Lihue and Hilo reflected values around normal across much of the state for this time of year that ranged from 1.2 to 1.4 inches. The exception was near the southern half of the Big Island, where higher moisture values are observed due to the aforementioned surface trough to the south. 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 morning.
The latest short-term model guidance remains in good agreement and is lining up well with the current pattern and trends around the state. A general consensus supports breezy trade winds persisting today, then steadily trending down over the weekend as the high to the north weakens and the trough south of the Big Island continues westward. 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 1-1.3" range (near to below average) through the weekend.
For the extended (Monday through midweek), light to moderate trades are expected to persist across the islands due to a weakness within the ridge north of the state. Clouds and showers will likely favor windward locations. The GFS and ECMWF both depict a weak surface trough approaching the islands from the east by midweek. This combined with a slight increase in moisture, may be enough to bring the rain chances back to normal over windward locations.
Aviation22 Sep 2017 19:57:00 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.
Marine22 Sep 2017 19:57:00 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/Advisories22 Sep 2017 19:57:00 GMT: Small Craft Advisory until 6 PM HST this evening for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.
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