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Hawaiian Islands Synoptic Discussion and Guidance

745
FXHW60 PHFO 250644
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
844 PM HST WED AUG 24 2016

.SYNOPSIS...
Very humid conditions with the chance for some enhanced showers
will continue over the western islands tonight into early
Thursday. Tropical moisture will then move west of the area by
Thursday afternoon bringing some relief to the humid conditions as
drier trade winds move in from the east. Breezy trade winds and
much more comfortable conditions are then expected Friday through
early next week, with showers focused over the windward slopes and
mountains. The weather may get a bit more active by mid to late
next week, as there is some indication that a tropical cyclone
could approach the island chain from the east.

&&

.DISCUSSION...
Currently at the surface, a trough of low pressure is located
around 500 miles to the west of Kauai, while a 1039 mb high is
centered well to the north-northeast of the state. Infrared
satellite imagery shows variably cloudy skies in place across the
state. Meanwhile, radar imagery shows scattered showers across
Oahu, with very little shower activity noted elsewhere. Main
short term concern for tonight revolves around rain chances.

Rest of tonight,
The trough off to the west of the state will weaken and edge
further westward, as deep layered ridging builds to the north of
the island chain. This will allow trade winds to get established
over the eastern islands by morning. MIMIC total precipitable
water imagery shows a band of higher moisture between the Big
Island and Oahu, with values between 2.0 and 2.2 inches, with some
slightly drier air over Kauai and a notably drier airmass upstream
of the Big Island. This band of higher moisture should shift
slowly westward through the night, with Oahu and Kauai within the
deepest tropical airmass through the overnight hours. As a result,
some enhanced showers will be possible overnight across the
western islands, with more of a trade wind shower pattern
developing over eastern sections of the state.

Thursday,
Deep layer ridging will continue to build north of the area,
allowing the trade winds to fill back in from east to west across
the island chain. Trades should return to Oahu during the morning
and to Kauai during the afternoon, bringing some relief to the
oppressively humid conditions. The band of deeper moisture will
remain over Oahu and Kauai during the morning, so some enhanced
showers will remain possible. By afternoon however, the band of
tropical moisture will begin to erode as it pushes further west,
with a typical trade wind shower pattern overspreading the entire
state.

Thursday night through next Wednesday,
High pressure to the north of the state will keep a moderate to
locally breezy trade wind flow in place through early next week,
with clouds and showers focused primarily over windward and mauka
locales. Drier air will continue to move into the area on the
east-northeasterly trades, with noticeably more comfortable
humidity levels expected Friday through early next week. Attention
then turns to the tropics, as both the 00z GFS and 00z ECMWF both
show a pair of tropical cyclones moving westward toward the island
chain by mid to late week. It's still way to early to tell what,
if any, impacts could be seen here in Hawaii, but we will
certainly be keeping a close eye these systems and the latest
model trends.

&&

.AVIATION...
VFR will be the prevailing flying condition during the overnight
hours with a strengthening pressure gradient and a drier and more
stable air mass continuing to spread across the area from the
east. Moderate trades will be in place by the end of Thursday. In
the mean time, there are small, isolated pockets of instability
still lingering around resulting in very isolated heavy showers
with mainly MVFR conditions. No AIRMETs expected however.

&&

.MARINE...
High pressure will build well north of the state tonight, with
moderate to locally fresh trades returning to eastern islands
tonight, and into the western islands Thursday. As a result, small
craft conditions are expected to develop over the windier waters
around Maui and the Big Island Thursday, with winds remaining at
advisory levels through at least Thursday night and likely
continuing into the weekend. Short period trade wind swell will
increase surf a notch along eastern facing shores Friday and into
the weekend. Otherwise, no significant surf is expected.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory from 6 AM Thursday to 6 AM HST Friday for
Maalaea Bay-Pailolo Channel-Alenuihaha Channel.

&&

$$

DISCUSSION/MARINE...Jelsema
AVIATION...Lau

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



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