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Hawaiian Islands Synoptic Discussion and Guidance

647
FXHW60 PHFO 240155
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
355 PM HST Mon Jan 23 2017

.SYNOPSIS...
High pressure northeast of the state will keep a light to moderate
trade wind flow in place through Thursday. The trades will
diminish Friday as a cold front approaches from the northwest. The
cold front will then sweep through the island chain over the
weekend, with breezy north to northeast winds expected Sunday
through early next week. Showers will favor windward and mauka
areas through the work week, with showers most prevalent through
Tuesday as an upper level disturbance moves overhead. A drier and
more stable airmass will then limit shower activity Wednesday
through Saturday. Showers will then increase in coverage over the
weekend as the cold front moves through the state, with a drier
trade wind shower regime returning for early next week.

&&

.DISCUSSION...
Currently at the surface, a 1029 mb high is centered around 1000
miles northeast of Honolulu, with light to moderate trades in
place across Kauai and Oahu, and moderate to locally breezy trades
over Maui County and the Big Island. Aloft, a broad southwest to
northeast oriented upper level trough is evident in water vapor
imagery, with the axis running through the central islands.
Visible satellite imagery shows mostly cloudy skies in place
across windward areas, with partly to mostly cloudy conditions in
leeward locales. Meanwhile, radar imagery and rain gauge
observations show numerous showers moving into windward and mauka
areas, with isolated showers reaching leeward sections of the
smaller islands from time to time. Main short term concern
revolves around rainfall trends.

Tonight and Tuesday,
High pressure will remain nearly stationary to the northeast of
the state, keeping a general light to moderate trade wind flow in
place through the period. A weakening front will stall out well
north of the island chain overnight however, and this may allow
for winds to slacken off enough to allow for localized land
breeze development. This will be short lived however, as the
trades quickly become re-established on Tuesday as this front
dissipates well north of the island chain. Remnant moisture
associated with an old front which moved through the area this
past weekend will continue to affect windward and mauka areas
through the period. Meanwhile the upper level trough axis will
slowly shift eastward across Maui County and the Big Island
tonight, then east of the Big Island on Tuesday. The deeper
moisture associated with the old front in combination with
elevated inversion heights resulting from the upper trough moving
through the area, will keep numerous to frequent showers affecting
windward and mauka areas through the period, with isolated showers
moving into leeward areas from time to time. Shower coverage and
intensity is expected to be greatest over the eastern end of the
state in closest proximity to the upper level trough.

Tuesday night through Friday night,
High pressure northeast of the islands will keep light to moderate
trade winds in place through Thursday. The trades will then
diminish Thursday night through Friday night as a cold front
approaches from the northwest, with land and sea breezes expected
in many areas. Aloft, mid and upper level ridging will build
eastward over the state Tuesday night and remain over the islands
through Friday night. As a result, we should see a decrease in
trade wind showers beginning Tuesday night, with a drier and more
stable airmass expected to overspread the state Wednesday through
Friday night. Showers are expected to be limited primarily to
windward and mauka areas through the period.

Saturday through next Monday,
The GFS and ECMWF are in good agreement showing a cold front
approaching Kauai from the northwest late Saturday afternoon,
then pushing southward down the island chain Saturday night and
Sunday. High pressure is then expected to build northwest of the
state Sunday night through early next week. A fairly dry and
stable pattern is expected to remain in place on Saturday, with
showers then increasing in association with the front Saturday
night and Sunday as it moves through the area. A drier trade wind
shower regime is then expected to return for early next week.

&&

.AVIATION...
Remnant clouds and showers from an old front will track over
windward sections of the islands tonight and Tuesday, resulting
in tempo mountain obscurations and MVFR cigs over most of these
areas tonight. AIRMET Sierra is in effect for tempo mountain
obscurations over the windward sections of Kauai, Oahu, Molokai,
Maui and the Big Island at this time. VFR conditions will prevail
for leeward areas with isolated mountain obscurations within
passing showers. No other AIRMETs are in effect or expected
through tonight.

&&

.MARINE...
A strong high northeast of the area is maintaining strong
easterly trade winds across some areas of the Hawaiian waters.
Also, in some areas seas remain above 10 feet. Please see the
latest Marine Weather Message, MWWHFO, for further details.

The strong trade winds and a short-period north swell continue to
produce surf above eight feet along east facing shores. This surf
will continue to trend downward and is expected to drop below
eight feet overnight.

On Tuesday a new northwest swell will enter the area forcing seas
to rise above 10 feet. As the seas spread across the Hawaiian
waters a new small craft advisory can be expected into Wednesday.
Also, surf along north and west facing shores will rise, most
likely prompting a new high surf advisory for Tuesday into
Thursday.

Another large northwest swell may produce advisory-level surf
along north and west facing shores of the smaller islands starting
Friday night and continuing into next weekend. There is a chance
this swell will require a high surf warning.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Tuesday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-Big Island North and East.

Small Craft Advisory until midnight HST tonight for Kauai
Windward Waters-Oahu Windward Waters-Kaiwi Channel-Pailolo
Channel-Big Island Leeward Waters.

Small Craft Advisory until 6 AM HST Tuesday for Maui County
Windward Waters-Alenuihaha Channel-Big Island Windward Waters-
Big Island Southeast Waters.

&&

$$

DISCUSSION...Jelsema
AVIATION...Foster
MARINE...Evans

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



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