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Hawaiian Islands Synoptic Discussion and Guidance

199
FXHW60 PHFO 160703
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
903 PM HST Mon Oct 15 2018

.SYNOPSIS...
A shower band over Maui will spread south slowly tonight. Remnants
of the band will linger near the Big Island through the week. A
surface ridge will remain north of the area through early next
week.

&&

.DISCUSSION...
Surface analyses show a 1019 mb surface high about 1100 miles
northwest of Kauai near 34N 170W. The high is forecast to move
east around 25 mph. A ridge extending southwest from the high will
persist through the weak and maintain northeast to east trade
winds over the main Hawaiian Islands. Showery low clouds, remnants
of a frontal band, extend east northeast from over Maui County
and the Big Island of Hawaii. The trade winds will push these
clouds over windward areas of Maui County and the Big Island.
Water vapor loops show a northeast to southwest trough aloft about
100 miles northwest of the islands. Southwest winds aloft
associated with the trough are carrying broken to overcast high
clouds over Maui County and the Big Island. The trough aloft is
also making the atmosphere less stable over the Big Island with
deeper moisture there. Showers remain more active on the Big
Island with some showers building over interior and leeward slopes
during the afternoon tomorrow. As the trough aloft lifts north
and weakens tomorrow, the atmosphere over the Big Island will
become more stable.

From Wednesday through next weekend the surface ridge will
persist north of the main Hawaiian Islands and maintain moderate
to locally strong northeast to east trade winds over the area.
Ridging aloft will keep the atmosphere fairly dry and stable, so
showers will be focused over mainly windward areas with no heavy
rainfall expected. Leeward areas will remain mostly dry.

&&

.AVIATION...
A band of low clouds and showers associated with the remnants of
a front continues near Maui early this evening. This feature is
producing widespread MVFR conditions across windward terrain on
Maui, where AIRMET Sierra remains posted for mountain obscuration.
Conditions will slowly improve from west to east across Maui
County through early Tuesday morning as the cloud/shower band
eventually shifts southeastward to the windward Big Island. AIRMET
Sierra, which is also posted for mountain obscuration along the
northeast through east facing slopes of the Big Island, may
possibly be required into the daytime hours on Tuesday. Elsewhere,
Kauai and Oahu will experience a rather typical trade wind
weather pattern with isolated MVFR conditions limited to windward
facing slopes.

AIRMET Tango for moderate upper-level turbulence remains in
effect over the island chain between FL280 and FL400.

&&

.MARINE...
A large NNW swell will build down the island chain late tonight
into Tuesday, peak late Tuesday through Wednesday, then gradually
ease Thursday through Friday. Offshore buoys to the northwest are
already reflecting small long-period energy associated with the
forerunners, so, the arrival time projected by the latest wave
model guidance appears to be on track at this point. A combination
of the large swell and increasing wind waves generated locally by
fresh to strong trades will produce advisory level seas over most
waters Tuesday through Wednesday.

Surf along exposed north and west facing shores will quickly rise
through the day Tuesday, likely exceeding the high surf advisory
thresholds by noon for Kauai and Oahu, then Maui County by Tuesday
evening. Surf may get to advisory levels along N facing shores of
the Big Island Tuesday night and Wednesday, and the advisory may
need to be expanded later. If the swell surges above the predicted
peak, marginal warning-level surf will be possible Tuesday night
into Wednesday, especially for Kauai and Oahu. After this swell
diminishes late in the week, a smaller NNW swell over the weekend
is expected to produce surf heights below advisory levels.

Surf will remain elevated, but below advisory levels, along S
facing shores through Tuesday, then fade through midweek. A
potent S swell arriving Friday is expected to peak Saturday into
Sunday, bringing surf near the warning-level of 15 feet along S
facing shores at its peak. This source is already filling in at
the Samoa buoy, which will help refine the forecast locally over
the coming days. See the latest collaborative surf discussion
(SRDHFO) for more details.

A weak high pressure cell far NW of the islands will move quickly
E, supporting moderate to locally fresh ENE trade winds through
Wednesday. The high is expected to dissipate NE of the islands
Wednesday night and Thursday as a front passes to the N, leading
to lighter trade winds. Fresh to strong NE winds are expected over
the weekend, supported by a new high pressure cell centered far
NW of the islands. A shortwave trough moving over the area tonight
may bring a brief period of instability, with a slight chance of
thunderstorms in the forecast for waters SE of the Big Island.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory from 9 AM Tuesday to 6 PM Wednesday for all
coastal waters except Maui Leeward Waters, Big Island Waters and
Big Island Southeast Waters.

High Surf Advisory from 9 AM Tuesday to 6 PM Wednesday for north
and west facing shores of Niihau, Kauai, Oahu and Molokai and
north facing shores of Maui.

&&

$$
DISCUSSION...Donaldson
AVIATION...Houston
MARINE...Gibbs

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

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