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Hawaiian Islands Synoptic Discussion and Guidance

087
FXHW60 PHFO 121331
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
331 AM HST Wed Dec 12 2018

.SYNOPSIS...
Windy and wet weather will continue through Thursday as a strong
high pressure ridge lingers to the north of the Hawaiian Islands.
Wind speeds will weaken and showers will diminish on Friday as the
high slowly drifts east. Expect light winds and continued dry
sunny weather from Saturday through Monday as the ridge breaks
down further ahead of a cold front approaching the islands from
the northwest. Moderate to breezy trade winds and showers will
return on Tuesday and Wednesday as the front sweeps through the
island chain.

&&

.DISCUSSION...
Our windy and wet trade wind pattern continues through Thursday
due to a strong surface ridge of high pressure north of the
state. Instability in the lower atmosphere is due to a combination
of strong low level mixing and an trough in the upper levels
providing a large scale lift to the clouds upstream of the
Hawaiian Islands. A band of unstable open cell cumulus clouds shown
on the infra-red satellite picture looks kind of like a centipede
moving across the state from east to west. These showers will pass
through each island in waves with a bit of enhancement during the
overnight and early morning hours. The wind advisory was cancelled
this morning as wind speeds will remain below advisory thresholds.
Additional wind advisories are possible on Thursday as the surface
high builds and the wind speeds increase.

The high pressure surface ridge to the north of the islands will
weaken starting on Friday resulting in decreasing wind speeds as
the surface high center begins to drift eastward. Strong downward
vertical motions (or subsidence) will develop from Thursday
afternoon into Friday due to a strengthening upper level ridge
building over Hawaii. The increase in subsidence aloft from this
upper ridge will effectively bring the stability hammer down,
lowering the height of the trade wind inversion to around 5000
feet, decreasing the thickness of the cumulus clouds, and
producing much less tropical rainfall in a more stable environment.

Additional weakening of the surface ridge will begin this weekend
as a cold frontal trough approaches the Hawaii region from the
northwest. Expect light trade winds on Saturday becoming
primarily land and sea breezes with continued dry weather forecast
with interior afternoon clouds over each island on Sunday and
Monday. The 850 MB winds will shift to a more southeasterly
direction on Sunday moving the smaller islands into the lee of the
Big Island relative to the changing wind flow.

Not a lot has changed in the weather pattern over the past 24
hours and we will likely keep the same trends in the forecast
grids through the morning forecast package update. Overnight we
reduced precipitation chances on Friday and Saturday as stronger
subsidence aloft decreases precipitation chances. Models remain
in good agreement through Monday with slight differences in the
arrival timing of the shallow cold front passing through the
islands on Tuesday and Wednesday. The cloud heights associated
with this front remain on the shallow side, which translates to
less shower activity. However, increasing low level winds will
help to wring out showers along northern and windward slopes of
each island as the frontal zone passes by.

&&

.AVIATION...
A strong surface high north northeast of Hawaii will maintain a
tight pressure gradient across the region late this afternoon and
into tonight, fueling a breezy to windy trade wind pattern.
AIRMET Tango remains in effect for all areas below 9000 feet
south and west of mountain ranges due to expected TEMPO moderate
turbulence. For Lanai, the Alenuihaha Channel, and water south of
the Big Island, surface winds greater than 30 KT are forecast
through this afternoon and will likely continue tonight.

Moisture caught up in the trade wind flow will bring scattered
showers to mainly the windward slopes and mountain areas over the
next 24 hours. Isolated MVFR conditions in passing showers could
become more widespread across Kauai or Windward Big Island toward
morning.

&&

.MARINE...
Strong high pressure northeast of the islands is driving strong
trade winds across the marine area early this morning. The trades
are expected to ease slightly today as a cold front passes by
well to the north of the state. Strong trades will then return
Thursday and Thursday night as a new strong high builds north of
the area. The high will weaken as it settles slowly southward
Friday through early next week, gradually weakening the trade
winds.

An overnight ASCAT pass indicated that the trade winds have
dropped just below Gale force, so the Gale Warning has been
cancelled for the Alenuihaha and Pailolo Channels as well as
Maalaea Bay. A Small Craft Advisory (SCA) is now in effect for all
of the Hawaiian coastal waters through Friday due to a
combination of strong winds and elevated seas. Another round of
Gales is still possible Thursday and Thursday night for the
Alenuihaha and Pailolo Channels as well as Maalaea Bay, although
the latest model runs have trended slightly weaker with the new
high building north of the state.

The current northwest swell will continue to build this morning,
then peak this afternoon into tonight just below the High Surf
Advisory (HSA) level. A smaller northwest swell is expected
Thursday night through Saturday, followed by a new, large,
northwest swell which will build late Saturday, and peak late
Saturday night and Sunday. This swell will likely produce warning
level surf along north and west facing shores. Another even larger
northwest swell is then expected to build late Monday or Monday
night, bringing another round of warning level surf to north and
west facing shores Tuesday through the middle of next week.

A HSA is in effect through Friday for east facing shores of
Kauai, Oahu, Molokai, Maui and the Big Island due to short period
choppy surf produced by the strong trades. Surf should lower
below advisory levels over the weekend as the trades diminish.

&&

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

Small Craft Advisory until 6 PM HST Friday for all Hawaiian
waters-

&&

$$

DISCUSSION...Bohlin
AVIATION...Bedal
MARINE...Jelsema

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

Current Conditions
Temp		1.2 C
RH		12 %
Wind		NE 14 mph
Road		Open (4x4)