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Hawaiian Islands Synoptic Discussion and Guidance

075
FXHW60 PHFO 160145
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
345 PM HST Sat Dec 15 2018

.SYNOPSIS...
High pressure north of the state will provide mainly dry and
pleasant conditions along with moderate trade winds through the
weekend. Rather dry conditions will continue into early next week,
although the trade winds will diminish in association with an
approaching front. This front will move down the island chain late
Tuesday through Wednesday, bringing an increase in showers along
with cooler and breezy conditions. Rather dry and cool conditions
will then continue for the tail end of next week.

&&

.DISCUSSION...
Currently at the surface, a 1023 mb high centered around 350
miles north-northwest of Kauai, is driving moderate to breezy
trade winds across the island chain this afternoon. Visible
satellite imagery shows partly cloudy skies in most areas, with
a few pockets of more extensive cloud cover in windward locales
as well as leeward sections of the Big Island. Radar imagery
shows very little, if any, shower activity statewide, with nothing
more than a few sprinkles impacting windward and mauka areas.
Main short term concern continues to revolve around the incoming
large northwest swells which are addressed in the marine section
below.

A ridge of high pressure will hold in place to the north of the
island chain through the weekend, keeping a moderate to locally
breezy trade wind flow in place. The ridge will then begin to
break down on Monday as a cold front approaches from the
northwest, easing the trade winds across the state. The winds will
remain rather light Monday night and Tuesday, particularly across
the western islands where land and sea breezes will be common.
Moderate and locally strong northerly winds will then spread down
the island chain from late in the day Tuesday through Wednesday.
High pressure building in behind the front will shift the winds
around to more typical east-northeasterly trade wind direction
Wednesday night, with moderate to locally breezy trades continuing
through Thursday. Late in the week and into next weekend, the
trades will likely ease once again as another cold front
approaches from the northwest.

Winds at the Big Island summits have been hovering just below
Wind Advisory levels all day. The 600 mb winds are forecast to
increase tonight and hold through the morning hours on Sunday,
which should push wind speeds into the advisory range. As a
result, a Wind Advisory is now in effect for the Big Island
summits through Noon on Sunday.

As for weather details, dry and stable conditions will hold in
place through Monday, keeping showers isolated and limited to
windward and mauka areas. Moisture will increase a bit Monday
night into early Tuesday across the eastern end of the state, so
we could see an uptick in windward showers here before the trades
cut off in association with the approaching front. The front is
expected to push down the island chain from northwest to
southeast late Tuesday through Wednesday, bringing an increase in
low clouds and showers to windward slopes and coasts favored
under a northerly to north-northeasterly wind direction. The
showers should first begin to affect Kauai late Tuesday, then
spread to the other islands Tuesday night. We should see
conditions dry out fairly quickly behind the front Tuesday night
and Wednesday, although windward sections of the Big Island could
see showery conditions continue through the day on Wednesday. In
addition to the showers, the front is expected to bring cooler
conditions to the area, with high temperatures quite possibly not
reaching the 80 degree mark on Wednesday. Fairly dry and cool
conditions should then prevail statewide Wednesday night through
the end of the work week, with mainly isolated to widely scattered
light windward and mauka showers.

&&

.AVIATION...
A deep layer high pressure ridge north of the Hawaiian islands
will bring moderate trades to the TAF sites this evening and into
tonight. However, drier air advecting into the region should keep
shower coverage to a minimum. Isolated SHRA will likely only
affect the windward slopes, bringing brief periods of MVFR
conditions. Otherwise, VFR is expected to prevail elsewhere.

AIRMET Tango remains in effect for TEMPO moderate low level
turbulence southwest of all island mountain chains.

&&

.MARINE...
High pressure north of the state will maintain moderate to locally
strong trade winds through Sunday night. An approaching front
will shift the high further east, resulting in a slight decrease
in trade winds for Monday. The front is expected to begin
impacting the northwest waters around Kauai Tuesday, and spread
down the island chain reaching the waters around Big Island
Tuesday night. Strong north winds are expected with and behind the
frontal passage. High pressure is expected to return northeast of
the state towards the end of next week.

Forerunners from a large long-period northwest swell are
beginning to arrive along select north and west facing shores. The
swell seems to be slightly behind schedule (3 to 5 hours), but we
do still expect the surf to rapidly rise tonight and spread
southeast across the Hawaiian Islands. A High Surf Warning (HSW)
for north and west facing shores of Niihau, Kauai, Oahu, Molokai,
and north facing shores of Maui and a High Surf Advisory for north
facing shores of the Big Island is in effect through Sunday
night. Also a Small Craft Advisory (SCA) is in effect for all
Hawaiian waters due to seas exceeding 10 feet. The large northwest
swell is expected to peak Sunday, and then subside some on
Monday. An even larger northwest swell will build late Monday,
peaking well above warning levels Tuesday night. Exposed coastal
waters will once again rise to over 10 feet as these swells come
rolling through. Expect the current SCA to once again be expanded
to most, if not all the coastal waters for the first half of the
new week as the new swell arrives.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Surf Advisory from midnight tonight to 6 AM HST Monday for
Big Island North and East.

Wind Advisory until noon HST Sunday for Big Island Summits.

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

Small Craft Advisory until 6 AM HST Monday 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.

&&

$$

DISCUSSION...Jelsema
AVIATION...Thomas
MARINE...Kino

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



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