Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 210615
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
815 PM HST Fri Jan 20 2017

.SYNOPSIS...
Northeasterly winds will increase tonight as a front approaches
the state from the northwest. The front will reach Kauai late
tonight, move across the smaller islands Saturday, then stall
near or just south of the Big Island from late Saturday night into
early next week. Very strong and potentially damaging northeast
trade winds are expected along and behind the front this weekend,
with winds easing early next week.


&&

.DISCUSSION...
Rather dry and stable conditions remain in place this evening as
northerly winds begin to increase ahead of an approaching front.
The field of stable stratocumulus clouds near the state has been
surprisingly persistent and is keeping skies rather cloudy, and
the developing low level northerly flow appears to be generating
enough low level convergence to squeeze out a few light showers.
Afternoon soundings and recent aircraft data show that the weakening
ridge aloft is maintaining a strong inversion at 4500 to 5000 ft
over the state.

North to northeast winds will steadily increase overnight ahead
of the approaching front. Breezy northeast winds will develop over
Kauai, and possibly as far east as Maui, with the front expected
to reach the Garden Isle in the predawn hours. The somewhat
shallow front will be accompanied by rainfall over Kauai`s north
and east facing slopes, and although the ridge aloft will be
steadily eroded by the broad upper level trough driving the front,
only a modest increase in windward rainfall is expected from Oahu
to the Big Island tonight.

Strong, gusty, and potentially locally damaging northeast trade
winds remain the main concern this weekend. The weak front will
move across the smaller islands on Saturday then to the Big
Island Saturday evening, followed by a somewhat strong 1033 mb
surface high setting up north of the state. This will produce a
strong low level pressure gradient along and behind the front as
the inversion is expected to rapidly strengthen and lower back to
around 4000 ft. The GFS and ECMWF are both showing winds around
and just below the inversion increasing to 40 to 45 kt by
Saturday evening. This scenario often creates periods of strong,
gusty, and locally damaging winds over and downwind of terrain
when the inversion is based near or just above the mountains. A
High Wind Watch has been issued for all islands on Saturday
afternoon and Saturday night. Winds will begin a gradual decline
on Sunday but could remain windy in some areas, especially on the
Big Island and Maui. Regarding rainfall, expect modest showers
along the front to be focused along northern and windward slopes,
with a few showers spilling over to leeward areas. There are no
changes at this time to the previous forecast philosophy for this
weekend.

Winds will decrease and veer out of the east to east-southeast on
Monday and Tuesday as the surface high to the north is pushed off
to the east. The GFS and ECMWF are hinting at some of the
dissipated frontal moisture drifting back up over windward
portions of the state during this period. This effect is most
prominent in the GFS, which shows a disruption in the trade winds
on Tuesday. Otherwise, trade winds and a typical pattern of
windward showers should prevail through much of the work week.


&&

.AVIATION...
A weakening cold front will reach the Hawaiian Islands overnight.
Mostly VFR conditions will continue across the state at this time,
with only isolated MVFR ceiling mainly over the windward sites. As
the frontal cloud band advances across the island chain overnight,
expect widespread MVFR ceiling with showers conditions to reach
Kauai later this evening, then through rest of the island chain on
Saturday. Northerly winds will be increasing ahead of the approaching
front over the area, with strong northeasterly trade winds peaking
behind the front by Saturday afternoon.

No AIRMET is expected at this time. However strong winds behind the
cold front will produce moderate to severe turbulence requiring an
AIRMET for the Hawaiian Islands probably by Saturday. Low clouds
along the frontal band may be extensive enough to bring AIRMET
mountain obscuration along the windward areas.


&&

.MARINE...
A mix of declining west-northwest and reinforcing northwest swell
will produce marginal high surf along most north and west facing
shores overnight. These swells are also producing border line
hazardous combined seas around 10 ft in some areas, and a Small
Craft Advisory (SCA) remains in effect overnight.

North-northeast winds will increase overnight ahead of a front
approaching from the northwest. By late tonight, expect SCA
strength northeast winds of 25 kt or greater to affect waters
from Kauai to Maui. Winds will increase significantly on Saturday
as the front moves down the island chain, and a Gale Warning has
been posted for all waters. The Gale Warning may persist for
waters around the Big Island and Maui on Sunday, while strong
easterly winds will likely require an SCA elsewhere.

Large, short-period seas well in excess of 10 ft will accompany
the strong to gale force trade winds behind the front. These seas
are expected to produce high surf along east facing shores by
Saturday afternoon. A High Surf Advisory has been issued for these
shores from noon Saturday through Sunday afternoon, but it will
likely need to be extended through Monday as the trade winds and
seas diminish.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Wind Watch from Saturday afternoon through late Saturday
night for all islands.

High Surf Advisory until 6 AM HST Saturday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala-Kona-Kohala.

High Surf Advisory from noon Saturday to 6 PM HST Sunday for
Niihau-Kauai Windward-Oahu Koolau-Olomana-Molokai Windward-Lanai
Makai-Maui Windward West-Windward Haleakala-Big Island North and
East.

Small Craft Advisory until noon HST Saturday for all Hawaiian
waters.

Gale Warning from noon Saturday to 6 AM HST Sunday for all
Hawaiian waters.


&&

$$

DISCUSSION...Foster
AVIATION...Hui
MARINE...Jacobson



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