Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 201400
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Wed Sep 20 2017

.SYNOPSIS...
A moderate to locally strong trade wind flow will continue for the
rest of the work week as high pressure remains to the NE of the
islands. The trades will become a tad lighter over the weekend
as the high drifts further away from the islands, and the
associated surface ridge approaches the islands from the NW.
Showers will continue to favor windward and mauka areas, with a
stray shower reaching leeward areas from time to time. Showers
will be most prevalent during the night and morning hours.

&&

.DISCUSSION...
A 1031 mb surface high centered 1330 miles NNE of Oahu is
generating a mostly moderate trade wind flow across the main
Hawaiian Islands this early Wednesday morning. A large patch of
clouds over the central islands, has caused a slight deviation to
the normal trade wind forecast. Infrared satellite imagery shows a
large patch of clouds covering Maui and Molokai, and is advancing
onto Oahu. Meanwhile, Kauai is so far sitting on mostly fair
skies. There have been some passing showers on Oahu, favoring
mainly the windward and mountain areas. Satellite and radar loops
are also picking up a band of showers lined up across the leeward
coastal waters, some 50 miles south of Kauai and Oahu. The band
is moving straight west, and will pass harmlessly south of Niihau.

Just a few miles east of Hana, Maui, is an area of less clouds
and showers. It has been slow in reaching Hana, but we anticipate
an improving trend to start at any time. We kept Oahu under
clouds and showers as this patch of clouds makes it way westward.
Any improvement will be this afternoon or late morning, over on
the town side. This cloud mass, should it hold together, will
reach Kauai shortly after sunrise, and persist through a good
part of the day. Clearing won`t probably come until this evening.
As for the Big Island, the clouds and showers have had some
impact, resulting in scattered showers. Scattered to broken cloud
coverage lies upwind of Cape Kumukahi, stretching east for a
couple of hundred miles. Most of the clouds will likely stay south
of Cape Kumukahi and ride parallel to the Kau Coast.

The upper low NE of the Big Island may have weakened the low level
inversion over the eastern half of the island chain. This has
allowed low level moisture to extend to 16k feet over the Big
Island, and 10k feet over Maui. This air mass may have supported
the showers over on the lee side of Big Island yesterday
afternoon. With little change being noted through Thursday as far
the inversion is concern, we expect more of the same for leeward
Big island for the next couple of afternoons.

The upper low settles in some 500 miles NE of the Big Island on
Friday, and lingers there into the weekend. Little change is
noted with the inversion, that is it remain from around 8k feet
over Kauai, sloping to 14k feet over the Big Island. The precip
pattern noted by the GFS, shows a generally diurnal pattern, with
relatively light amount into the weekend.


&&

.AVIATION...
High pressure north-northeast of the state will keep a moderate to
breezy trade wind flow in place today and tonight. Some MVFR
cigs/vsbys will be possible in windward and mauka areas through
mid morning, with PHTO, PHMK and PHLI the most likely TAF sites
to see conditions drop below VFR levels. Otherwise, prevailing VFR
conditions are expected through 12Z Thursday.

AIRMET Tango remains in effect for low level turbulence over and
downwind of the terrain of all islands. The AIRMET will likely
remain in effect through the day today.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of Oahu, Molokai, Maui and the Big Island. The
AIRMET may need to be expanded to include Kauai over the next
couple of hours. Conditions should start improving across the
central islands as early as late this morning, but remain over
Kauai for a good part of today.


&&

.MARINE...
Small craft advisory conditions are forecast to continue through
Thursday due to strong trade winds across the Pailolo and
Alenuihaha Channels, Maalaea Bay and the waters south of the Big
Island. Trades are expected to drop below small craft criteria
Thursday night into Friday as high pressure north-northeast of the
state shifts eastward and weakens. Light to moderate trades are
expected over the upcoming weekend, with the exception being
across the typically windier channel waters where fresh breezes
will remain possible.

A combination of moderate to fresh breezes locally and upstream of
the islands south of high pressure will continue to generate rough
surf along east facing shores into Friday. Surf along east facing
shores will trend down over the upcoming weekend as the trades
weaken.

Surf along south facing shores will remain small through the
weekend with mainly background southeast and south-southwest
swell energy expected. For the extended, strong- to near gale-
force southwest winds associated with a low that is currently
tracking across the Tasman Sea may lead to another small south-
southwest (210-220 deg) swell early next week.

A small northwest (310 deg) swell associated with broad low pressure
centered over the central Pacific just east of the Date Line is
expected to fill in locally Thursday and hold into the weekend
before easing. A recent ASCAT pass showed a decent sized batch of 25
to 35 kt winds focused toward the islands within the 310 deg band.
Peak seas from an altimeter pass within this batch of northwest
winds ranged from 15 to 18 ft, which lines up well with the latest
wave model guidance. Small surf will be expected from this source
Thursday through Saturday, before dropping Sunday along north and
west facing shores.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Thursday for Maalaea Bay,
the Pailolo and Alenuihaha Channels, and waters south of the
Big Island.

&&

$$

DISCUSSION...Lau
AVIATION...Jelsema
MARINE...Gibbs



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