Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 220156
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
356 PM HST Thu Sep 21 2017

.SYNOPSIS...
Locally breezy trades will gradually weaken during the next couple
of days as surface high pressure far northeast of the state
erodes. 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 through at least
the weekend.

&&

.DISCUSSION...
A locally breezy and mainly stable trade wind flow persists this
afternoon. The trades are being driven by a somewhat strong 1033
mb high sitting 1600 miles northeast of the islands and its
associated ridge stretching about 650 miles north of Kauai. A
trough within the ITCZ remains north of 10N, and this is helping
to enhance the winds as the pressure gradient over the state is
somewhat tight. Mid level ridging is producing an inversion
between 6,000 to 7,000 ft according to the afternoon soundings
and aircraft data, and precipitable water within the trade wind
flow ranges from near September normal of almost 1.5 inches to as
low as 1.1 inches in dry slots. The result has been modest
rainfall along mainly windward slopes. The far southwestern edge
of an upper level trough remains over the islands. This feature is
having little effect other than pulling up some elevated moisture
near the Big Island.

Expect little change tonight. Locally breezy trade winds will
ease slightly, and modest shower activity will be focused over
windward slopes. Clouds and a few showers over leeward Big Island
will clear as nighttime land breezes kick in.

A gradual decrease in trade winds is due on Friday and Saturday as
the ridge currently 650 miles north of the state is pushed
southward. Moderate trade winds are forecast to persist through
the upcoming week. No big changes in the moisture content and
stability of the trade wind flow are noted in the near future. As
a result, showers will continue to be largely confined to windward
slopes. The upper level trough will meander in place through the
weekend, which will likely lead to continued afternoon cloud cover
and spotty showers over leeward and interior Big Island.

&&

.AVIATION...
VFR conditions prevail at all TAF sites and these conditions are
expected to continue this evening and on through the overnight hours.
There will likely be some brief MVFR conditions along windward slopes
at times as moisture riding in on the trades interacts with island
terrain.

AIRMET Tango remains in effect for low level turbulence over and
immediately south and west mountain ranges of all islands. The
AIRMET is expected to continue through the overnight hours and
possibly through the day Friday.

&&

.MARINE...
Fresh to locally strong trades will continue tonight driven by a
1032 mb high pressure far north-northeast of Hawaii. Trades
should gradually weaken over the next couple of days to the light
to moderate range by the end of the weekend. This pattern should
hold through the first half of the upcoming week. Before they
weaken, the trade winds will bring rough surf along east facing
shores. In the far southern offshore waters, thunderstorms will
occur along a trough along the Intertropical Convergence Zone
tonight, moving west of the offshore area by Friday.

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 possible by Sunday, which should lead to
a slight increase in surf. For the extended, strong to gale-force
southwest winds associated with a low that has been impacting the
Tasman Sea may lead to another small southwest (210-220 deg)
swell Tuesday through midweek.

A small northwest (310 deg) swell will continue into the weekend
before easing. Another small reinforcement is due is expected to
fill in around Sunday night through middle of next week.

&&

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

&&

$$
Public...Wroe
Aviation...Burke
Marine...Foster


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