FORECAST DISCUSSION

Synopsis

18 Aug 2017 01:30:00 GMT: Moderate to locally breezy trade winds will gradually diminish, becoming light to moderate this weekend. The trade winds will deliver periods of clouds and showers to windward areas, with showers most active during nights and mornings. An increase in trade wind showers is possible early next week, with potential for a few afternoon leeward showers as winds diminish.

Discussion

18 Aug 2017 01:30:00 GMT: A relatively stable trade wind weather regime will continue into the weekend, with passing clouds and showers favoring windward slopes and coasts, especially during nights and mornings. Visible imagery shows limited low clouds directly upstream of the islands while water vapor imagery depicts a dry NW flow aloft over the region associated with a ridge aloft centered SW of the islands. A weak low aloft located about 750 miles NE of the islands near 31N 152W is having little effect on the stability of the island atmosphere, with afternoon soundings depicting a strong subsidence inversion right around 7000 feet. Trade wind speeds are expected to gradually and slightly diminish the next couple of days, as a surface high far NE of the islands drifts E while a low-level trough (the remnants of former east Pacific Tropical Storm Jova, which was spawned by the remnants of Atlantic Hurricane Franklin after it crossed Mexico) moves WNW and passes between the islands and the high center. This feature is currently 1150 miles E of the islands near 18N 138W. Meanwhile, in the GFS solution, the development of a bubble high to the NW of the islands is expected to help support a light to moderate trade wind flow through the weekend. ECMWF is weaker with the high to the NW and therefore more significantly weakens winds Sunday and Monday. Current wind forecast is a blend of the two solutions, and the PoP forecast allows for the potential of a few afternoon convective showers over leeward areas. The mid-level low to the NNE of the islands will very move slowly W the next couple of days, and is not expected to provide significant instability in the short term. It will however help to ensure that trade winds drop a few windward showers as the subsidence inversion will remain sufficiently high to support shower development, but sufficiently low to suppress cloud tops to below 8000 feet or so. An area of disturbed weather about 650 miles SE of the Big Island continues to be monitored for potential development of a tropical depression. Associated shower and thunderstorm coverage has recently diminished significantly, and it is looking less likely that a tropical depression will form. See Tropical Weather Outlooks issued by the Central Pacific Hurricane Center for updates and details. Forecast models differ in their handling of the pressure and moisture fields associated with the area of disturbed weather to the SE, especially early next week. An increase in moisture riding in on the trades during the Monday-Tuesday time frame is the general expectation, with latest model runs trending toward a somewhat later arrival time, while also trending slightly drier as the bulk of the moisture remains S of the islands. The low aloft and its associated destabilizing cool pocket of mid-level air is expected to be closer to the islands at the same time, and there is a chance this combination leads to the development of heavy showers. Confidence is low as to 1) the timing and the amount of low-level moisture that arrives on the trade wind flow, and 2) how weak the trade wind flow will become this weekend and early next week as the remnants of Jova pass N of the islands. By mid-week the low aloft will move W away from the area and deep-layer ridging building toward the area from the NE will bring increasing trade winds and stability.

Aviation

18 Aug 2017 01:30:00 GMT: A high pressure ridge north of the islands will maintain moderate to locally breezy trade winds for another day or so. Winds are sufficiently strong enough to maintain AIRMET Tango for mechanical low level turbulence to the lee of the mountains on all islands. Winds are expected to start to decline tonight and Friday, so expect conditions to improve by this weekend. VFR conditions are expected to persist across the area, though there will be some clouds and showers focused over the windward sides of the islands.

Marine

18 Aug 2017 01:30:00 GMT: Surface high pressure far northeast of the islands will maintain moderate to locally strong trades winds. This will continue to result in Small Craft Advisory (SCA) conditions for the typically windy waters adjacent to the islands of Maui County and the Big Island. The SCA for these areas remains in effect through Friday afternoon. The high is expected to continue drifting eastward during the next few days, and a slight downward trend in trade wind speeds is expected from Friday night through this weekend. A small, long period south-southeast swell is expected to peak late Friday before gradually subsiding this weekend. A small reinforcing long-period southwest swell is forecast to reach the islands Saturday, and continue into early next week. Surf is expected to trend down along east facing shores this weekend as the trades weaken slightly. Surf will likely remain below the High Surf Advisory criteria along all shorelines through the middle of next week. A period of above normal high tides is expected through this weekend. The water levels associated with these tides are forecast to be slightly lower than those observed in July. In addition, no large swells are expected during the next several days. There could still be some localized coastal flooding from these tides. See the Special Weather Statement, SPSHFO, for more details.

HFO Watches/Warnings/Advisories

18 Aug 2017 01:30:00 GMT: Small Craft Advisory until 6 PM HST Friday for Maalaea Bay, Pailolo Channel, Alenuihaha Channel, Big Island Leeward Waters, Big Island Southeast Waters.

Footer

18 Aug 2017 01:30:00 GMT: DISCUSSION...Birchard AVIATION...M Ballard MARINE...Powell

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