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Hawaiian Islands Synoptic Discussion and Guidance

395
FXHW60 PHFO 151925
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
925 AM HST Mon Oct 15 2018

.SYNOPSIS...
A weak area of high pressure building to the north of the state
will keep a light to moderate trade wind flow in place through the
week. Remnant moisture associated with a dissipated front will
sag slowly southward through Oahu and into Maui County today,
then linger over Maui County and the Big Island tonight through
the remainder of the week. This will keep a showery trade wind
pattern in place for windward sections of Maui County and the Big
Island through much of the week, while fairly dry trade wind
weather prevails across Kauai and Oahu.

&&

.DISCUSSION...
Currently at the surface, a 1020 mb high is centered around 1200
miles northwest of Kauai, with ridging extending east-
southeastward roughly 300 miles north of the island chain.
Meanwhile, remnant moisture associated with a dissipated front is
strung out over Oahu and Molokai this morning, and is shifting
slowly southward. Satellite imagery shows partly to mostly cloudy
skies in place across the state. Meanwhile, radar imagery shows
scattered to numerous showers over windward areas from Kauai to
Maui with a few showers drifting into leeward areas as well.
Across the Big Island, most areas are rain free at the moment,
with a few isolated showers brushing the windward coast. Main
short term concerns revolve around rainfall trends and the
potential for thunderstorms this afternoon.

Today,
Light to moderate trade winds will prevail across the islands
today as a ridge of high pressure remains anchored to the north of
the state. Meanwhile, the band of clouds and showers associated
with an old front will sag slowly southward through the day, and
linger over Maui County through the afternoon. As a result, we
should see improving conditions across Kauai and Oahu as the day
progresses, but a few showers will continue to affect windward
areas. Showery conditions are expected to overspread all windward
sections of Maui County this afternoon, with some of the showers
also drifting into leeward locales. Across the Big Island, daytime
heating and sea breezes will lead to scattered shower development.
Additionally, some jet forcing will move overhead this afternoon
as well, and this could allow a thunderstorm or two to develop.
As a result, we have elected to add a slight chance of
thunderstorms to the forecast for all areas of the Big Island this
afternoon.

Tonight through Sunday,
A weak ridge of high pressure will hold in place to the north of
the island chain through the period, 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
lingering over Maui County and the Big Island through the work
week, keeping a showery trade wind pattern in place here.
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 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 away from the island chain. The
ECMWF solution would support deeper moisture and more showery
weather over the eastern end of the state, while the GFS solution
would keep drier conditions in place. Given the uncertainty in
the forecast over the weekend, will favor 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 the central portion of the
island chain. Moisture from this feature remains hung up along
windward terrain from Molokai to Kauai this morning, and with MVFR
conditions widespread in those areas, AIRMET Sierra for mountain
obscuration is in effect. Conditions are expected to improve
through the day on Kauai and Oahu, while mountain obscuration is
expected to develop on Maui and Lanai this afternoon and tonight.
On the Big Island, weak flow and lingering instability will
produce afternoon clouds and showers, along with a slight chance
of an afternoon and evening thunderstorm. The cloud band will
likely reach windward Big Island tonight, raising the possibility
of mountain obscuration.

AIRMET Tango for upper-level turbulence remains in effect over
the island chain between FL260 and FL420. This AIRMET will likely
remain in effect over the eastern end of the state this morning.

&&

.MARINE...
A weak high pressure cell far NW of the islands will move quickly
E, supporting moderate to locally fresh ENE trade winds through
Wednesday. High-resolution model guidance indicates wind speeds
may get to Small Craft Advisory (SCA) levels in the Alenuihaha
Channel on Tuesday, but confidence remains low, and no SCA is
currently posted. 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 today and tonight may
bring a brief period of instability, with a slight chance of
thunderstorms added to forecast for waters S and E of the Big
Island.

A large NNW swell is expected to arrive tonight and peak late
Tuesday, then gradually diminish Wednesday and Thursday. Peak surf
heights will require a High Surf Advisory for exposed N and W
facing shores, and this will likely be issued later today or
tonight. With open ocean swell heights expected to be near 8
feet, combined seas in some zones will be near 10 feet Tuesday and
Tuesday night, thus requiring a SCA. A smaller NNW swell over the
weekend is expected to produce surf heights below advisory
levels.

Surf will remain elevated along S facing shores the next couple
of days, but below advisory levels. However, a larger S swell
next weekend is expected to produce advisory-level surf along S
facing shores of all islands, with the swell expected to begin
arriving on Friday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Jelsema
AVIATION...Wroe
MARINE...Birchard

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

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