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Hawaiian Islands Synoptic Discussion and Guidance

618
FXHW60 PHFO 210159
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
359 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 afternoon as
northerly winds begin to increase ahead of an approaching front.
Afternoon soundings and recent aircraft data show that the
weakening ridge aloft overhead is maintaining a strong inversion
at 4500 to 5000 ft. A field of stable stratocumulus clouds has
been surprisingly persistent today, 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.

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.

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. 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...
High pressure will build in from the northwest as weakening cold
front moves into the Hawaiian Islands. Expect increasing north to
northeast winds tonight ahead of the front with strong
northeasterly trade winds peaking by Saturday afternoon.

Mostly VFR conditions are forecast with only isolated MVFR mainly
over windward airfields this evening. Early Saturday morning a
weak cold front moves down the Hawaiian Island chain from Kauai
Sat 12Z to the Big Island by Sun 02Z, expect scattered MVFR with
low ceilings and showers along the frontal boundary.

No Airmet is expected tonight. However strong winds behind the
cold front will produce moderate to severe turbulence requiring an
Airmet for the Hawaiian Islands on Saturday.

&&

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

North-northeast winds will increase tonight ahead of a front
approaching from the northwest. By late tonight, expect SCA
strength northeast winds to affect waters from Kauai and 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 as 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 through
the day on Sunday, 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.


&&

$$

Wroe

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office