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Hawaiian Islands Synoptic Discussion and Guidance

062
FXHW60 PHFO 160457 AAA
AFDHFO

Area Forecast Discussion...UPDATED
National Weather Service Honolulu HI
657 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...
Currently at the surface, a 1020 mb high is centered around 1100
miles northwest of Kauai, with ridging extending east-southeast
to a location roughly 300 miles north of the island chain.
Meanwhile, remnant moisture associated with a dissipated front is
strung out over mainly Maui County this afternoon, and is edging
slowly southward. Satellite imagery shows partly cloudy skies in
place across Kauai, with partly to mostly cloudy conditions over
the rest of the state. Meanwhile, radar imagery shows scattered to
numerous showers over windward areas from Oahu to the Big Island
with a few showers drifting into leeward areas of the smaller
islands as well. Across the Big Island, scattered showers have
also developed as a result of daytime heating, with one
thunderstorm firing over the Kau District. Across Kauai, mainly
rain free conditions are present, with perhaps an isolated shower
over the windward slopes. Main short term concern revolves around
rainfall trends over the next couple of days.

A weak ridge of high pressure will hold in place to the north of
the island chain through next weekend, keeping a light to
moderate trade wind flow in place. The GFS and ECMWF remain
consistent showing the remnant moisture associated with a
dissipated front hanging up over Maui County and the Big Island
through the rest of the work week, keeping a showery trade wind
pattern in place here, particularly for windward areas.
Meanwhile, the models are also in good agreement showing a drier
airmass holding in place across Oahu and Kauai, and this should
result in light mainly windward and mauka showers, primarily
during the overnight and early morning hours. The models diverge a
bit over the weekend, with the ECMWF more aggressive in cutting
off an upper low over or just east of the Big Island, while the
GFS shows a progressive upper trough moving eastward away from
the island chain. The ECMWF solution would support deeper moisture
and more showery weather continuing over the eastern end of the
state, while the GFS solution would keep drier conditions in
place. Given the uncertainty in the forecast, will lean the
weekend forecast in favor of climatology, with showers favoring
windward and mauka locales.

&&

.AVIATION...
A cloud band marking the remnants of a dissipated front continues
to move slowly southeastward across Maui County this afternoon.
Moisture from this feature is producing widespread MVFR
conditions across windward terrain on Maui, Molokai, and Lanai,
where AIRMET Sierra for mountain obscuration remains posted.
Conditions will slowly improve from west to east overnight as the
cloud band moves southeastward to windward Big Island, where
mountain obscuration is expected, possibly into the day on
Tuesday. Kauai and Oahu will experience typical trade wind
conditions with isolated MVFR conditions limited to windward
slopes.

AIRMET Tango for upper-level turbulence remains in effect over
the island chain between FL260 and FL400. This AIRMET will likely
remain in effect through the evening.

&&

.MARINE...
A large NNW swell will build late tonight and Tuesday, continue
through Wednesday, then slowly diminish Thursday and Friday. A
High Surf Advisory has been issued for exposed N and W facing
shores of most islands in anticipation of rising surf on Tuesday.
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. After this swell diminishes. a smaller NNW
swell over the weekend is expected to produce surf heights below
advisory levels.

With open ocean swell heights expected to be near 8-9 feet with
the first swell, combined seas in some zones will be near 10 feet
Tuesday and Tuesday night, thus requiring a Small Craft Advisory.
This will likely be issued later tonight.

Surf will remain elevated, but below advisory levels, along S
facing shores through Tuesday. A potent S swell arriving Friday is
expected to peak Saturday into Sunday, bringing high surf that
could potentially produce surf near the warning level of 15 feet
along S facing shores at its peak. 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...
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.

&&

$$
SYNOPSIS...Donaldson
DISCUSSION...Jelsema
AVIATION...Wroe
MARINE...Birchard

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

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