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Hawaiian Islands Synoptic Discussion and Guidance

463
FXHW60 PHFO 170643
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Tue Jul 16 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 this evening,
as a surface high persists far north of the state. An upper level
trough in the island vicinity has led to passing showers being
enhanced. Bulk of the clouds and showers are affecting windward and
mountain areas, though winds are strong enough to carry some of the
clouds and showers to the lee areas of the smaller islands. Evening
convection is affecting lee areas of the Big Island at this time,
though these will slowly dissipate as the night progresses. Latest
satellite imagery revealed a little more low clouds over the
windward coastal waters at this time, with a slight break in the
cloud coverage then an extended line of low clouds further east.
In all, expect trade wind weather to continue through rest of the
evening into early Wednesday morning.

The upper level trough in the island vicinity will slowly move
northwest through Wednesday. As such, airmass in the island vicinity
will become more stable. Latest forecast solutions hinted a slight
decrease in moisture across the area Wednesday. Meanwhile, the
aforementioned high will persist far north through the rest of the
week, and in fact will strengthen slightly in the next couple of
days. The airmass may still be unstable enough for a slight chance
of an upslope thunderstorm to develop over the Big Island late
Wednesday afternoon. Shall take a closer look at this before
updating the forecast on this matter. Forecast models then hinted
an increase in moisture for the islands Thursday. Therefore, breezy
trade wind weather will prevail for the next few days, with possible
slightly less clouds and showers on Wednesday.

The high to our north is expected to slowly weaken by the weekend
into early next week. This points a slight downward trend in trade
wind speeds for the 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 1032 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 the windward and mauka areas with
isolated showers passing over onto the leeward side. Periods of MVFR
conditions may develop along the slopes tonight due to low clouds
and SHRA creating mountain obscuration.

AIRMET Tango remain in effect below 7000 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 through
tonight and Wednesday, 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, but the
fetch will be short through Wednesday, so wave heights will remain
small. East shore surf will get a boost Thursday and Friday as the
upstream fetch of trade winds increases. Surf is expected to stay
just below advisory levels along east facing shores Thursday and
Friday and slowly ease 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