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Hawaiian Islands Synoptic Discussion and Guidance

966
FXHW60 PHFO 251357
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
400 AM HST Sun Aug 29 2019

.SYNOPSIS...
Trade wind weather will continue through Monday, thanks to high
pressure far north of the area. Clouds and passing light showers
will tend to favor windward and mauka areas with most leeward
locations remaining dry. Winds will slowly slack beginning Tuesday
through the rest of the week as a low pressure develops far north of
the area, with a mixed trade wind and land and sea breezes weather
regime expected.

&&

.DISCUSSION...
A surface high pressure centered far north of the state will
continue to bring moderate to locally breezy trade wind to the area
through Monday. Meanwhile, a mid level ridge will remain near the
islands as well, and serve to keep airmass in the island vicinity
mainly stable. Both early morning Lihue and Hilo soundings pretty
much confirmed airmass stability, with trade wind inversion based at
around 6 to 7 thousand feet. Furthermore, Hilo sounding is slightly
drier, with precipitable water near one and three quarter of an
inch. Latest satellite imagery indicated a line of low clouds spread
across the windward areas from the Big Island to Oahu, and will
reach Kauai in the next few hours. Radar data indicated these
showers are passing the islands somewhat swiftly, and are affecting
mainly windward and mountain areas. Due to the quick movement of
these showers in a mostly stable airmass, precipitation amounts will
stay mostly on the light side.

Do expect current weather pattern to persist through Monday. Hence,
trade wind weather will prevail, with mainly light windward and
mauka showers expected. Afternoon and evening clouds and showers are
also expected over the leeward Big Island slopes.

Changes in the weather are expected beginning Tuesday, as the high
current far north of the area is forecast to weaken and move off to
the northeast. Moreover, a low pressure system is expected to
develop far north of the area. This will result in the trade winds
to slowly weaken quite a bit over the later half of the week.
Localized land and sea breezes may develop under this lighter trade
wind regime. Forecast charts only showed a slight increase in
airmass instability. Thus, shower activity will be highly dependent
on available moisture and still be primarily focused over windward
and mauka areas. Daytime heating may allow for afternoon clouds and
showers to develop over interior sections as well as over the
leeward Big Island slopes.

&&

.AVIATION...
High pressure north-northeast of Hawaii continues to bring breezy
trade winds to the state. AIRMET Tango continues for mechanical
turbulence downwind of all the mountains below 8,000 feet. This
AIRMET will likely continue through today and into tonight with
little change in winds expected.

VFR conditions are expected to prevail with passing MVFR conditions
in passing clouds and showers primarily over windward slopes.

&&

.MARINE...
Fresh to strong easterly trade winds will spread westward today
as high pressure holds far north of the state and a surface trough
northwest of Kauai moves away. The Small Craft Advisory (SCA) has
been expanded from the typically windy waters around the Big
Island and Maui to briefly cover Maui County windward waters and
the Kaiwi Channel today. Trades will gradually decrease during
the first half of the work week as the high weakens and drifts to
the northeast. The SCA will likely be trimmed back to the
typically windy waters on Monday and should be dropped entirely by
Tuesday.

South shore surf will be at summertime background levels for the
next couple of days, and a southwest swell out of the Tasman Sea
is due to arrive Tuesday and peak Wednesday and Thursday. Islands
in the southwest Pacific usually block a significant amount of
energy from swell sources out of the Tasman Sea, but we could see
inconsistent sets above August average during the peak. This swell
will decrease heading into next weekend.

Expect a slight and brief increase in east shore surf today as
trades strengthen, followed by a gradual decline to near average
conditions on Monday and well below August average for the
remainder of the week. Small north shore surf will fade today as
a small pulse of northwest swell diminishes.

&&

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

Small Craft Advisory until 6 PM HST this evening for Kaiwi
Channel-Maui County Windward Waters.

&&

$$

DISCUSSION...Hui
AVIATION...M Ballard
MARINE...Powell

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



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