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Hawaiian Islands Synoptic Discussion and Guidance

088
FXHW60 PHFO 170140
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
340 PM HST Tue Jul 16 2019

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

&&

.DISCUSSION...
At the surface, a 1030 mb high is centered far north of the
state. This places the islands in a moderate to breezy trade wind
environment. An elongated upper level trough in the vicinity of
the state, is seen in water vapor imagery. Satellite imagery shows
an area of enhanced moisture embedded in the trades, which has
caused some increase in shower activity across portions of the
state including a few thunderstorms over the leeward slopes of the
Big Island due the vicinity of the upper trough. We expect this
area of enhanced moisture to move through the area overnight,
bringing with it some passing showers for windward and mauka areas
with the trade winds sufficiently strong enough to blow a few
showers over to select leeward areas at times. Thunderstorm
activity will likely taper off this evening as daytime heating
over the slopes diminishes.

The upper trough is forecast to weaken and lift out to the north
and west later Wednesday through Friday. This will stabilize the
airmass with showers becoming less prevalent. Meanwhile, the
surface high far to our north will strengthen a bit causing a
slight uptick in trade winds speeds.

As we head into the weekend and on into early next week, the high
to our north will slowly weaken with a slight downward trend in
trade wind speeds. The above mentioned trough will sag back
southward, closer to the state. This may cause for a slight uptick
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...
High pressure north of the island chain continues to bring
moderate to breezy trades to the state. Clouds and showers
continue to favor windward and mauka sections of the islands,
however, a few thunderstorms have popped up on leeward slopes of
the Big Island this afternoon. These may result in occasional MVFR
conditions, but VFR conditions are expected to dominate.
Thunderstorm activity will decrease this evening and clouds are
expected to clear out during the overnight hours, so the issuance
of AIRMET Sierra is not anticipated at this time.

AIRMET Tango remains in effect for south through west areas of
all islands below 7000 feet for tempo moderate turbulence.

&&

.MARINE...
High pressure far north of the state will maintain fresh to strong
trades through Friday. The trades are expected to ease slightly
over the weekend, as high pressure to the north weakens. A Small
Craft Advisory (SCA) is posted for most waters between Oahu and
the Big Island. For the remaining waters not in the SCA, winds
will remain borderline with fresh trades in these areas. We will
continue to monitor winds across the coastal waters and additional
zones may be added to the SCA if needed.

The current south swell will continue to slowly subside through
mid-week, with only small background south and southwest swells
expected through the weekend. Locally strong trade winds will
produce choppy waves along east facing shores, but the fetch will
be short for the next couple of days 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 weaken
slightly.

&&

.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 from 6 PM this evening to 6 PM HST
Wednesday for Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters.

&&

$$

DISCUSSION...Burke
AVIATION...Dye
MARINE...Kino

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



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