Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 161946
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
945 AM HST Tue Jan 16 2018

.SYNOPSIS...
Moderate trade winds and mostly dry weather will continue today. A
strong high passing far north of the islands will bring
strengthening trade winds beginning tonight, with windy conditions
Wednesday and Thursday. The strong trade winds are expected to bring
an increase in windward showers, with just a few showers
occasionally spreading leeward. Winds will ease toward the end of
the week, and may become quite light over the weekend.

&&

.DISCUSSION...
Island weather remains rather benign this morning, as a deep-layer
ridge continues to support a stable and dry atmosphere, evidenced by
overnight soundings indicating PWAT near 0.9" and a subsidence
inversion based between 4000` and 5000`. A surface ridge just N of
Kauai is supporting gentle to moderate ENE trade winds that are
escorting scattered low clouds over windward portions of the
islands, with radar detecting isolated light showers near the Big
Island. Further afield, a 1029 mb surface high is centered far to
the NW, while a cold front 400 miles NW of Kauai lies between the
islands and the high.

The weak surface ridge will continue to drift slowly N today, with
trade winds continuing to gradually strengthen in response. Limited
rainfall is expected, even as the trades deliver a few windward
clouds. The high to the distant NW will strengthen as it tracks
steadily E along ~35N, peaking near 1037 mb when it passes N of the
islands Wednesday night. This will lead to increasing trade winds
beginning tonight, with windy conditions Wednesday and Thursday
likely necessitating a Wind Advisory for at least portions of the
state.

Moisture associated with the nearby front is expected to remain N of
the islands as it stalls, and a mid-level ridge will continue to
support a stable atmosphere through midweek. However, an increase in
low-level moisture and greater orographic forcing supplied by the
strong winds should lead to an increase in trade wind showers,
especially by Thursday. The strong trade winds may blow a few stray
showers leeward, but dry conditions will prevail.

The high will track steadily E through the week, and will be far NE
of the islands by Friday. Forecast models continue to indicate that
low pressure will develop NW of the islands (near Midway Atoll) by
the middle of the week, and then track toward the NE through the
weekend. The evolution of this feature is where forecast models
differ next weekend, leading to some forecast uncertainty. Although
differing on the specifics, models agree on the general large scale
pattern, which adds a little more confidence to the notion that
winds will ease as they veer to the ESE/SE Friday, potentially
becoming quite light next weekend as a front associated with the low
stalls NW of Kauai. Another high passing far N of the area may
bring moderate trade winds back by early next week.

&&

.AVIATION...
High pressure building north of the state will strengthen the trade
winds to moderate levels today and locally breezy levels tonight.
Fairly dry conditions are expected to remain in place through the
day. Some brief MVFR cigs will be possible across windward areas,
but prevailing VFR conditions are expected at all terminals. MVFR
cigs and/or vsbys will be more common in passing showers for
windward areas tonight, with VFR conditions expected in leeward
areas.

No AIRMETs are currently in effect. AIRMET Tango may be needed for
moderate low level turbulence over and downwind of the terrain late
this afternoon or tonight.

&&

.MARINE...
A High Surf Advisory remains in effect through Wednesday for north
and west facing shores of the smaller islands exposed to the current
northwest swell. This swell is expected to subside through tonight,
however a new swell, from a low far north of Oahu, will boost surf
heights again on Wednesday.

A Small Craft Advisory (SCA) remains in effect for the typical
windier waters near the Big Island and Maui County. As winds and
seas are expected to increase tonight and tomorrow, all waters will
be under an SCA starting tonight. Winds could reach gale force in
the windiest areas, and those areas are under a Gale Watch starting
Wednesday morning.

The increasing trade winds will bring large, rough seas to east
facing shores, likely requiring a High Surf Advisory through the end
of the work week.

See the latest Oahu Surf Discussion (SRDHFO) for additional details
on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Wednesday for north and west
facing shores of Niihau Kauai Oahu Molokai and for north facing
shores of Maui.

Small Craft Advisory until 6 AM HST Friday for Big Island Leeward
Waters.

Small Craft Advisory from 6 PM this evening to 6 AM 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-Big Island
Windward Waters.

Gale Watch from Wednesday morning through late Thursday night for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel-Big Island Southeast
Waters.

Small Craft Advisory until 6 AM HST Wednesday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Southeast Waters.

&&

$$

Discussion...Birchard
Aviation...Kinel
Marine...M Ballard


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