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Hawaiian Islands Synoptic Discussion and Guidance

782
FXHW60 PHFO 210133
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
333 PM HST Wed Sep 20 2017

.SYNOPSIS...
Moderate to locally strong trades will continue through Thursday,
thanks to persistent high pressure to our north. Wind speeds will
decrease slightly by this weekend as the main high drifts
eastward. Clouds and showers will favor windward and mauka areas,
mainly at night and during the morning hours.

&&

.DISCUSSION...
A 1032 mb high far north of the main Hawaiian Islands continues to
drive moderate to strong trade winds across local waters this
afternoon. Skies have cleared a bit since this morning as an area
of showery low clouds moved west of the island chain. Radar shows
very little shower activity, with only isolated showers noted
west of Kauai and Niihau. Since skies look rather clear upstream,
we have adjusted the gridded database for clearer and drier
conditions tonight and Thursday.

Water vapor loop shows an upper low about 800 miles northeast of
the islands. While this feature raised inversion heights and
allowed shower-bearing clouds to build a bit taller as late as
this morning, afternoon soundings continue to show a stable
airmass and only 1.2 to 1.6 inches of PW. Models show the upper
low and associated upper trough will deepen and dig southward over
the next few days, but remain northeast and east of the the
island chain. In short, at this time we expect only minimal shower
enhancement from this upper trough through the forecast period.

Models show the high to our north drifting eastward and weakening
over the next few days. The trailing ridge will be pushed closer
to the islands by a cold front, but the front itself will not
reach us. Weaker high pressure will cause trade winds to decrease
a bit across local waters by this weekend. However, with no
disruptions from intervening surface troughs or fronts forecast
into next week, we expect continued trade flow and a trade wind
weather distributions of clouds and showers to continue through
the forecast period. Total rainfall will be light.

&&

.AVIATION...
Moderate to locally breezy trade winds will continue tonight and
tomorrow, with clouds and showers favoring the windward and
mountain areas. Rainfall maxima will occur overnight and in the
early morning hours. Overall conditions should remain
predominately VFR, with MVFR likely in passing showers. AIRMET
SIERRA for mountain obscuration can not completely be ruled out
later tonight and will be issued as needed.

AIRMET TANGO is posted for low level turbulence over and
immediately south through west of the mountains.

&&

.MARINE...
A ridge of high pressure far north of the state remains in place
and is strong enough to keep trade winds above Small Craft
Advisory (SCA) criteria in the Pailolo and Alenuihaha Channels,
Maalaea Bay, and the waters south of the Big Island. The ridge is
forecast to weaken slowly over the next several days which will
allow conditions to drop below SCA levels. An approaching cold
front far to the northwest will push the ridge toward the state
over the weekend and further weaken the trades.

The persistent moderate to fresh trade winds over and upwind from
the main Hawaiian Islands will continue to produce rough surf
along the east-facing shores through Friday. As the trades
diminish during the weekend, surf along the east-facing shores
will trend down.

For the north- and west-facing shores, a small northwest swell is
expected to arrive on Thursday and persist through Saturday.
South-facing shores should just see small background surf from the
southeast and south-southwest directions through the weekend.

&&

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

&&

$$

DISCUSSION...Powell
AVIATION...Eaton
MARINE...Kodama

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



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