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Hawaiian Islands Synoptic Discussion and Guidance

460
FXHW60 PHFO 171345
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Wed Jul 17 2019

.SYNOPSIS...
Trade wind weather will continue through the remainder of the week
into early next week, thanks to high pressure far north of the
area. Clouds and passing showers will favor windward and mauka
areas. Leeward sections of the Big Island will see clouds and
showers mainly during the afternoon and evening hours.

&&

.DISCUSSION...
Locally breezy trade winds prevail across the islands early this
morning as a surface high persists far north of the state. An upper
level trough in the island vicinity is still bringing some airmass
instability in the island vicinity, especially over the western
islands. Early morning Lihue sounding pretty much confirmed that,
depicting a somewhat moist and unstable airmass, with low level
moisture extending to about 8 thousand feet. Early morning Hilo
sounding depicted a more stable and drier airmass, with low level
moisture extending to about 6 thousand feet. Low clouds embedded in
the trade winds are passing over the islands at times, bringing some
showers to mainly windward and mountain areas. Winds are strong
enough to carry some of the clouds and showers to the lee areas of
the smaller islands.

The upper level trough in the island vicinity will move northwest
today. As such, airmass in the island vicinity will become more
stable. Latest forecast solutions still hinted a slight decrease in
moisture across the area today, though satellite imagery does show
lines of low clouds just east of the islands. These lines of clouds
will bring more showers to the area, at least through the rest of
the morning. Meanwhile, the aforementioned high will persist far
north through the rest of the week, and strengthen slightly through
Thursday. Overall, breezy trade wind weather will prevail across the
islands through the next few days. The airmass may still be unstable
enough for a slight chance of an upslope thunderstorm over the Big
Island this afternoon. Forecast models hinted an increase in
moisture for the islands Thursday. Hence the islands may see a
slight increase of passing clouds and showers then.

The high to our north is expected to slowly weaken by the weekend
into early next week. This points to a slight downward trend in
trade wind speeds for that period. Furthermore, the aforementioned
upper level trough may sag back southward, closer to the state. This
may lead to a slight uptick in shower activity, mainly for windward
and mauka areas. Daytime heating will continue to allow for clouds
and showers to form over the leeward Big Island slopes each
afternoon and evening.

&&

.AVIATION...
A 1031 mb high north of the Hawaiian Islands will generate breezy to
occasionally windy easterly trades for the next 24-hours. Expect
scattered showers to affect mainly windward and mauka areas with
isolated showers passing over and onto the leeward side.

Periods of MVFR conditions could develop along the windward slopes
through early this morning, as low clouds and SHRA may create
mountain obscuration.

AIRMET Tango remain in effect below 8000 ft for areas South thru
West of mountain ranges due to tempo moderate turbulence.

&&

.MARINE...
High pressure north of the state will keep fresh to strong trades in
place through Friday. The trades are expected to ease slightly over
the weekend, as high pressure to the north weakens. A Small Craft
Advisory (SCA) remains in effect for most waters between Oahu and
the Big Island. Fresh trades are expected for the remaining waters
not in the SCA, with winds likely flirting with SCA levels at times.
We will continue to monitor observational and model trends over the
next few days, and expand the SCA to additional marine zones if
needed.

The current south swell will continue to slowly subside today and
tonight, with only small background south and southwest swells
expected Thursday through the weekend. A bit of a boost in south
shore surf is possible early next week, as a pair of overlapping
southwest swells move through. Locally strong trade winds will
produce choppy waves along east facing shores into the weekend. East
shore surf will get a boost Thursday and Friday as the upstream
fetch of trade winds increases. Surf is expected to stay below
advisory levels along east facing shores Thursday and Friday and
slowly lower over the weekend as trades ease.

&&

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

Small Craft Advisory until 6 PM HST Friday for Oahu Leeward
Waters-Kaiwi Channel-Maui County Windward Waters-Maui County
Leeward Waters.

&&

$$

DISCUSSION...Hui
AVIATION...Thomas
MARINE...Jelsema

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



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