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Temp6.8 C
RH65 %
WindSW 19 mph
RoadOpen (4x4)
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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 201933

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

Moderate to locally strong trades will continue through the work
week, thanks to persistent high pressure to our north. Wind speeds
will decrease slightly beginning 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.


A 1032 mb high far north of the main Hawaiian Islands continues to
drive moderate to strong trade winds across local waters this
morning. An area of broken low clouds embedded within this trade
flow lies across the island chain, with radar showing associated
moderate showers across windward portions of Kauai, Oahu, Molokai
Maui and the Big Island. We expect cloud and shower coverage will
decrease a bit this by afternoon as the cloud area mixes out and
slides west of us.

Water vapor loop shows an upper low about 800 miles northeast of
the islands. While this feature may have raised inversion heights
and allowed shower-bearing clouds to build a bit taller recently,
overnight soundings also show a stable airmass and only 1.2 to
1.5 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.


Moderate to locally breezy trade winds will continue today, with
clouds and showers favoring the windward and mountain areas.
AIRMET SIERRA for mountain obscuration is posted for the north
through east sections on the Big Island this morning. Conditions
should improve over the next few hours. Otherwise, expected
predominately VFR conditions.

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


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.


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




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