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Hawaiian Islands Synoptic Discussion and Guidance

238
FXHW60 PHFO 160135
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
335 PM HST Mon Jul 15 2019

.SYNOPSIS...
A trade wind weather pattern can be expected through the
remainder of the week and on through the weekend, 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. Moisture embedded in the trades is providing for
passing light showers for some windward and mauka areas, primarily
over Kauai and Oahu. Daytime heating has also caused for clouds
and a few showers to form over the leeward slopes on the Big
Island this afternoon. These clouds will dissipate during the
overnight hours. A continuation of passing light showers for
windward and mauka areas can be expected through Tuesday with a
repeat performance of cloud buildups and a few showers over the
leeward Big Island slopes Tuesday afternoon.

Water vapor imagery along with model guidance shows a broad mid
and upper level trough centered a few hundred miles to the east
northeast of the Big Island. This trough is forecast to drift
westward over the next few days which will likely cause inversion
heights to rise a bit. A small area of enhanced moisture can be
seen on satellite imagery a few hundred miles east of the area.
Guidance indicates that this area of moisture will reach the Big
Island Tuesday afternoon or evening, then pass westward across the
remainder of the state Tuesday night into Wednesday morning. An
uptick in windward and mauka shower activity is expected with this
feature passing through especially for windward and mauka areas.
The trades will be sufficiently strong enough for some brief
passing showers over some leeward locations.

The surface high to the north is forecast to strengthen during the
middle and latter half of the week. The result will be for an
uptick in trade wind speeds across the area. Also by the middle of
the week, the upper trough will have weakened and moved to a
position just west of the state. Thus a typical windward and mauka
distribution of shower activity is expected and will be highly
dependent on incoming moisture availability. The high will weaken
a bit over the weekend with a slight drop off in trade wind
speeds.

&&

.AVIATION...
High pressure north of the state will keep a breezy to locally
windy, easterly trade wind flow in place through over the next
several days. Relatively dry conditions will prevail with a few
light showers affecting mainly windward and mountain areas. Brief
MVFR cigs/vis are possible as showers move through, but VFR
conditions are expected to prevail statewide over the next 24
hours.

AIRMET Tango remains in effect for moderate, low-level turbulence
over and downwind of the higher terrain of all islands. These
conditions are expected to continue for the next couple of days.

&&

.MARINE...
A high far north of Kauai will move east slowly over the next
several days. The pressure gradient south of the high will remain
tight enough to maintain locally strong easterly trade winds over
the Hawaii coastal waters through the week. A Small Craft
Advisory is in effect for the windy areas around the Big Island
and Maui. The advisory may need to be expanded around mid-week.

The large south swell affecting the Hawaii coastal waters is now
subsiding. However, a High Surf Advisory remains in effect for
all south facing shores through tonight. The swell is aimed just
southeast of the coastal waters, so surf will be highest on the
Big Island and lowest on Kauai. This swell will continue to slowly
subside through mid-week. The 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
relatively small. Surf along east facing shores will increase by
mid-week as the fetch becomes longer. Only very small surf is
expected along north facing shores through the week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Tuesday for Niihau-Kauai
Windward-Kauai Leeward-Oahu South Shore-Waianae Coast-Molokai
Leeward-Lanai Makai-Kahoolawe-Maui Leeward West-Maui Central
Valley-Windward Haleakala-Leeward Haleakala-Kona-South Big
Island-Big Island North and East.

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

&&

$$

DISCUSSION...Burke
AVIATION...TS
MARINE...Kinel

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