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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 271357

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Thu Oct 27 2016

Trade winds will prevail across the main Hawaiian Islands through
early next week, becoming stronger in the next few days due to
a building high pressure to our north. Passing low clouds and
showers carried by the winds will affect mainly windward and
mountain areas. An upper level trough is forecast to move near the
state over the weekend, and may result in wet trade wind weather
into early next week.


Much of the low clouds associated with a weak cold front has cleared
the state early this morning, though both satellite imagery as well
as radar data still indicated some low clouds and showers in the
island vicinity, especially over windward Big Island and Maui. A
strong surface high pressure will move to far north of the state
today, with pressure gradient tightening a bit across the area. This
will result in an increase of trade winds for the islands. Early
morning Lihue and Hilo soundings continue to depict a stable
airmass, with low level moisture extending from about 7 to 8
thousand feet. Latest satellite imagery revealed there is a slight
break in the low clouds in the island vicinity at this time, though
more low clouds are present further upstream northeast of the state.
Those clouds will reach the islands throughout the day, bringing
some showers at times. The coverage of these clouds across the state
will be much less than Wednesday. Much of these clouds and embedded
showers will affect mainly windward and mountain areas, though some
will reach the lee areas of the smaller islands under the increasing

The aforementioned high pressure system far to the north will weaken
but drop southward a bit and will eventually be replaced by another
high from the west by late Saturday. This next high will follow a
similar path, as it will weaken and sag south as well. The end
result of these synoptic features maneuvering will be the return of
breezy trade winds to the state for the next few days. Upper level
ridging will initially keep a cap on showers through Friday as the
airmass in the vicinity remains stable. Rainfall amounts will
therefore be limited. An upper level trough is then expected to move
near the island chain over rest of the weekend into early next week.
In turn, airmass will become more unstable across the areas,
allowing more active showers to develop. Thus, expect wet and breezy
trade wind weather for the islands over the weekend into early next
week, with passing showers reaching the lee areas of the smaller
islands at times.

A mid-level ridge may build back in the island vicinity around
middle of next week. Furthermore, the high pressure to the north
will weaken at the same time. This will lead to trade winds
returning to more moderate levels with less active showers.


A band of enhanced moisture will bring widespread MVFR cigs/vsbys to
the Big Island for the next few hours, with improving conditions
expected around daybreak. Otherwise, predominantly VFR conditions
are expected to prevail in all areas through much of the day.
Reduced cigs/vsbys may develop over windward sections of the islands
once again late this afternoon or early this evening, as another
band of enhanced moisture moves into the area.

AIRMET Sierra remains in effect for mountain obscuration across
windward sections of the Big Island. Conditions should improve here
later this morning.

AIRMET Tango will likely be needed for low level turbulence downwind
of the mountains of all islands later this morning, due to
strengthening trade winds.


As the trade winds strengthen today, the areal coverage of the Small
Craft Advisory (SCA) for the typical windy coastal zones around Maui
County and the Big Island will be expanded. In fact, overnight ASCAT
data indicated SCA level winds were already present over windward
and leeward Oahu coastal waters as well as Kaiwi channel. Winds are
expected to increase further tonight such that the SCA coverage will
be for all coastal waters. The breezy conditions will prevail
through the next few days, though the winds may ease a tad for a
couple of coastal zones by early next week.

A series of small to moderate north and northwest swells will fill
in over the weekend and again toward the middle of next week, but
surf heights are expected to remain below advisory levels. A long
period east swell is possible this weekend from the distant
Hurricane Seymour in the Eastern Pacific. Furthermore, east facing
shores will see an increase in rough and choppy surf under the
strengthening trades to possibly near advisory level in the next few


Small Craft Advisory until 6 PM HST Saturday for Oahu Windward
Waters-Oahu Leeward Waters-Kaiwi Channel-Maalaea Bay-Pailolo
Channel-Alenuihaha Channel-Big Island Leeward Waters-Big Island
Southeast Waters.

Small Craft Advisory from 6 PM this evening to 6 PM HST Saturday
for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Kauai Channel-Maui County Windward Waters-Maui County
Leeward Waters-Big Island Windward Waters.




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