FORECAST DISCUSSION

Synopsis

23 Nov 2017 02:17:00 GMT: Gusty trade winds are forecast tonight through the Thanksgiving Holiday Weekend. Showery conditions over windward and mountain areas may occasionally spill into adjacent leeward sections. A more moist trade wind pattern may set up late in the weekend and early next week.

Discussion

23 Nov 2017 02:17:00 GMT: A gusty and blustery Thanksgiving Day is in store for much of the forecast area. A 1033 mb surface high centered far northwest of the state is moving slowly eastward and strengthening. Thus have begun to tighten the pressure gradient over are area this afternoon. This trend will continue over the next 24 hours with winds peaking Thanksgiving afternoon and evening. Guidance suggests winds will reach wind advisory levels in a few areas such as the island of Lanai, and the leeward Kohala area from the crest of the mountains down to the leeward Kohala coast, from Waikoloa north to near Hawi. These areas will experience wind gusts approaching 50 mph starting overnight tonight and continuing through tomorrow, and a wind advisory has been posted. During the peak of the winds Thanksgiving afternoon, sustained winds will reach 30 mph or more in many of these areas. Elsewhere across the state, winds will be gusty as well with gusts reaching 35 to 45 mph in many areas overnight and during the day tomorrow. The stable airmass remains over the state tonight with dry air capping a somewhat showery mixed layer below. These showers are affecting mainly windward areas tonight, and will probably see these again tonight. Some isolated showers spilling over into the leeward areas of smaller islands are anticipated due to the increasing wind flow. This typical trade wind shower pattern is expected to continue through much of the weekend, although models show a more moist, 2 inch precipitable water, airmass developing over the eastern half of the state Sunday into Monday. This appears in response to an upper level trough which dives southward over the state, bringing colder air aloft to the western portion of the state and increasing the subtropical jet across the eastern half of the state. Although this remains highly uncertain, if the models persist in this forecast pattern, the result will be a chance of heavier showers, especially for windward Big Island Sunday into Monday. There remains even less certainty on whether this moisture would interact with instability across the smaller islands.

Aviation

23 Nov 2017 02:17:00 GMT: High pressure northwest of the State will keep a moderate to breezy trade wind flow in place tonight, with the trades strengthening to locally windy levels on Thanksgiving Day. Bands of clouds and showers will move into windward areas through the period, with showers also reaching leeward areas from time to time. Some brief MVFR cigs/vsbys will be possible in windward areas as showers move through, but predominantly VFR conditions are expected at the terminals through 23/18Z. AIRMET Tango remains in effect for moderate low level turbulence over and downwind of the mountains of all islands. The AIRMET is expected to continue through Thanksgiving Day. AIRMET Sierra may be needed for mountain obscuration later tonight.

Marine

23 Nov 2017 02:17:00 GMT: As expected, the warning level northerly swell has come in a bit bigger than guidance indicated at Buoys 51101 and 51001. Made a minor tweak upward to the forecast grids for swell late tonight into Thanksgiving to account for this, and hopefully this is enough. Also adjusted the surf height range for north facing shores a bit. Surf at these heights will tend to break on the outer reefs. A Marine Weather Statement is also out to cover possible harbor surges for N-facing harbors. The swell and surf will peak Wed night into Thanksgiving Day, then slowly decline Thu night and Friday. This incoming swell, combined with strengthening trades, will make for hazardous conditions to small craft across most of our coastal waters for the Thanksgiving holiday, and into the holiday weekend. Some of the high resolution guidance shows winds approaching gale force in the Alenuihaha Channel on Thu, so will keep an eye on this. The trades may ease off slightly on Fri but it will still be quite windy. Yet another NNE swell is forecast to arrive late in the weekend and continue into early next week. Most of the swell energy is forecast to pass E of the state, but it still appears likely to produce advisory-level surf on N-facing and some E-facing shores.

HFO Watches/Warnings/Advisories

23 Nov 2017 02:17:00 GMT: High Surf Warning until 6 AM HST Friday for Niihau, Kauai Windward, Kauai Leeward, Oahu North Shore, Oahu Koolau, Olomana, Molokai, Maui Windward West, Maui Leeward West, Maui Central Valley, Windward Haleakala, Big Island North and East. Wind Advisory until 10 PM HST Thursday for Lanai, Big Island North and East, Kohala. Small Craft Advisory until 6 PM HST Friday for all Hawaiian waters,

Footer

23 Nov 2017 02:17:00 GMT: Discussion...Brenchley Aviation...Jelsema Marine...R Ballard

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