Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 222001

Area Forecast Discussion
National Weather Service Honolulu HI
1001 AM HST Wed Nov 22 2017

Gusty trade winds are forecast today 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.


A 1032 mb surface high centered far northwest of the state is
moving slowly eastward. The high is forecast to strengthen as it
drifts east, and this will tighten the pressure gradient over our
area over the next 24 hours. The result will be increasing
northeasterly trades today and tonight. Areas that are exposed to
this wind direction will see gusts approaching 30 mph this
afternoon. The pressure pattern and winds aloft for Thanksgiving
Day only increase the winds further, with gusts over 40 mph
possible in the windier areas of the state. Will need to consider
if winds are expected to get any stronger than this, which would
necessitate a wind advisory for these windy areas, such as Leeward
Kohala, Lanai, central and west Maui, or even elevated parts of
Oahu and Kauai.

Otherwise, a stable airmass remains over the state today with dry
air capping a somewhat showery mixed layer below. These showers
are affecting mainly windward areas today, and will probably see
these again tonight. Isolated shower spillover to leeward areas
of smaller islands are also 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 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.
Even less certainty on whether this moisture would interact with
instability across the smaller islands.


High pressure northwest of the State will keep a moderate to
breezy trade wind flow in place through 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 today
or tonight.


Even as local seas are slowly dropping below the 10-foot Small
Craft Advisory (SCA) criteria, the next large N swell is
approaching on its heels. Starting to see long-period forerunners
at NOAA buoy 51101. 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.

Although surf has probably come below advisory level temporarily
on the N- (and some E-) facing shores, surf along these shores is
expected to rise rapidly this afternoon into the evening as the
new large N swell arrives. This swell is forecast to produce
warning-level surf on N- and exposed E-facing shores, and a High
Surf Warning remains in effect. 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.

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.


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.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Pailolo
Channel-Alenuihaha Channel-Big Island Windward Waters.

Small Craft Advisory until 6 PM HST Friday for Maalaea Bay-Big
Island Southeast Waters.



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