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Hawaiian Islands Synoptic Discussion and Guidance

819
FXHW60 PHFO 122000
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Thu Dec 12 2019

.SYNOPSIS...
Light trade winds and a few windward showers will prevail over
most islands today. Strong high pressure passing north of the
islands will bring increasing trade winds tonight and Friday, with
locally breezy trades persisting into Saturday. The strengthening
trade winds will bring passing clouds and showers to windward
areas, but a front stalling just northwest of the islands will
bring increased rainfall to Kauai later today through Friday.
High clouds are expected to pass over the islands late tonight
through Saturday.

&&

.DISCUSSION...
One last day of light trade winds is in store for most of the
state. A stalled front located roughly 50 miles northwest of
Kauai bisects pair of surface highs centered about 1800 east-
northeast and 1200 northwest of the state. As a result, the
pressure gradient over the islands remains rather weak and is
driving gentle to locally moderate easterly trade winds. As the
high to the northwest advances eastward, we could see stronger
northeasterly trade winds building around Kauai this afternoon,
with little change elsewhere. Modest shower activity will persist
along windward slopes from the Big Island to Oahu, while nearby
moisture from the front may fuel additional showers on Kauai. An
upper level trough digging toward the state is triggering heavy
showers and thunderstorms along the front this morning, and while
it is not out of the question that a heavy shower could develop on
Kauai this afternoon, chances remain low at this time.

Trades will ramp up considerably tonight and Friday as high
clouds thicken. The somewhat strong surface high currently sitting
northwest of the state will pass by to the north, as the front
stalled just northwest of Kauai begins to fall apart. Expect
stronger trades to initially build around Kauai tonight, then
state-wide on Friday as the front further erodes. Enhanced
moisture will continue to be pooled just north of Kauai, while the
upper level trough moves overhead. The greatest instability and
potential for heavy showers and thunderstorms will remain north
and west of Kauai, but we will monitor closely. Elsewhere, a
typical pattern of windward rainfall will prevail, with a few
showers blowing over to leeward areas. The approaching upper level
trough will also tap high level moisture and spread thickening
high clouds over the islands.

Breezy trades will slowly subside during the weekend as high
clouds thin. The surface high will move off to the northeast, and
its trailing ridge north of the islands will be weakened,
allowing trades to back off to moderate strength by Sunday. The
upper level over the state will creep eastward and slowly fill as
a mid level ridge strengthens. As a result, expect an increasingly
stable pattern of mainly windward showers and decreasing high
clouds.

Another shot of stronger trades is due Monday and Tuesday as
another high pass north of the state. A ridge aloft will
strengthen, maintaining a rather stable weather pattern with
showers remaining focused on windward slopes.

&&

.AVIATION...
East or northeast moderate trades will persist today as a weak
front remains stalled to the northwest of Kauai. Thus, scattered
showers will mainly affect windward locations of all islands. As
moisture from the front approaches the state this evening, expect
precipitation coverage to increase over Kauai. MVFR conditions
may develop with periods of mountain obscuration.

No AIRMETs are in effect at this time.

&&

.MARINE...
A large northwest swell will maintain surf above warning levels
through tonight. A High Surf Warning is in effect for north and
west facing shores of Niihau, Kauai, Oahu, and Molokai as well as
for north facing shores of Maui. A High Surf Advisory is in effect
for the leeward Big Island coast. The surf is currently peaking
this morning and will slowly subside into Saturday. Another
northwest swell may produce advisory level surf early next week.

A full moon high tide earlier this morning and the large NW swell
has brought some overwash along coastal areas. We have received
reports of water overwashing onto Kamehameha highway near
Laniakea early this morning. With the tides much lower this
afternoon and the swell slowly subsiding, we should see less
coastal overwash by this afternoon.

A front moving SE over the NW offshore waters will stall tonight
NW of Kauai. A new high will pass north of Kauai tonight and
Friday, driving a boost in northeast winds to near gale force
north of the front. Waters around Kauai will see the strongest
winds tonight and Friday. Strong trades will also develop over
the typically windy coastal waters Friday. As the high continues
eastward, trades will weaken over the western end of the island
chain by Saturday, but the locally strong trades over the eastern
waters will continue through the weekend and into next week.

Surf along east facing shores is currently below the seasonal
average. A boost in the trades later today and Friday will lead
to higher surf along east facing shores of all islands, but
especially for Kauai where a High Surf Advisory may be needed for
east facing shores. Some increase in surf is also expected from a
northeast swell entering Hawaiian waters Friday and Saturday. The
trade swell will gradually ease this weekend.

Due to the large northwest swell, a Small Craft Advisory is in
effect for all windward waters as well as the leeward waters of
Kauai and Oahu, including the Kaiwi Channel through today.
Starting tonight, trade winds will strengthen and will cause winds
to reach advisory levels for the typical windy areas around Maui
County and the Big Island. Strong northeast winds will also
strengthen around Kauai County tonight into Friday. A Small Craft
Advisory has been posted for all Hawaiian Waters except Maui
County Leeward starting tonight and continuing through early
Saturday morning due to the return of strong trades and the large
northwest swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Friday for north and west facing
shores of Niihau, Kauai, Oahu, and Molokai, and for north facing
shores of Maui.

High Surf Advisory until 6 AM HST Friday for Kona.

Small Craft Advisory until 6 AM HST Saturday for all waters except
Maui County Leeward.

&&

$$

DISCUSSION...Wroe
AVIATION...Thomas
MARINE...Kino

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