Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 221325 CCA
AFDHFO

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
325 AM HST Fri Sep 22 2017

.SYNOPSIS...
Breezy trade winds will hold through the day today, then gradually
trend down over the weekend as the ridge of high pressure to the
north weakens and a weak surface trough approaches from the east. A
typical pattern of mainly windward showers will prevail, though
interior and leeward Big Island will experience extensive afternoon
cloud cover and a few showers each day.

&&

.DISCUSSION...
The latest surface analysis showed a ridge axis associated with
1032 mb high pressure extending west-southwest across the central
Pacific north of the state and a weak trough of low pressure
positioned south of the islands. The pressure gradient between
these two synoptic features continues to support breezy trades
across the local area. Satellite-derived precipitable water
imagery and upper air soundings at Lihue and Hilo reflected values
around normal across much of the state for this time of year that
ranged from 1.2- 1.4". The exception was near the southern half
of the Big Island, where higher moisture values were shown
(1.4-1.6"), likely due to the aforementioned surface trough to the
south and an upper trough extending southwest across the Big
Island.

The latest short-term model guidance remains in good agreement and
is lining up well with the current pattern and trends around the
state. A general consensus supports breezy trade winds persisting
today, then steadily trending down over the weekend as the ridge to
the north weakens and the trough south of the Big Island continues
westward. Mostly dry and stable trade wind weather will continue
each day with clouds and showers favoring windward locations.
Rainfall accumulations will be light, with precipitable water values
holding in the 1-1.3" range (near to below average) through the
weekend.

For the extended (Monday through midweek), light to moderate trades
are expected to persist across the islands due to a weakness within
the ridge north of the state. Clouds and showers will likely favor
windward locations. The GFS and ECMWF both depict a weak surface
trough approaching the islands from the east by midweek. This
combined with a slight increase in moisture, may be enough to bring
the rain chances back to normal over windward locations.

&&

.AVIATION...
High pressure north-northeast of the State will keep a moderate
trade wind flow in place through tonight. Clouds and showers will
favor windward and mauka areas, with a stray shower trickling
over into leeward areas from time to time. Brief MVFR cigs/vsbys
will be possible in windward areas as trade showers move through,
but predominantly VFR conditions are expected at the terminals
through 12Z Saturday.

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

&&

.MARINE...
Fresh to locally strong trades should gradually weaken beginning
today, putting wind speeds into the light to moderate range
through the weekend. Trade winds will bring rough surf along east
facing shores today.

Surf along south facing shores will remain small through early next
week, with mainly background southeast and southwest swell energy
expected. The next potential long-period pulse out of the
southwest from the Tasman Sea will be by Sunday, which should
lead to a slight increase in surf. For the extended forecast,
strong to gale-force southwest winds associated with a low in the
Tasman Sea may lead to another small southwest swell Tuesday
through midweek.

A small northwest swell will continue into the weekend before
easing. Another small reinforcement is expected to fill in around
Sunday night through the middle of next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

Discussion...Gibbs
Aviation...Jelsema
Marine...Powell



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