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Hawaiian Islands Synoptic Discussion and Guidance

493
FXHW60 PHFO 250641
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
830 PM HST Sat Aug 24 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 become lighter during
the middle and latter half of next week as a low pressure develops
far north of the area, with a mixed trade wind with land and sea
breezes weather developing.

&&

.DISCUSSION...
A surface high pressure centered far north of the state is bringing
moderate to locally breezy trade wind to the area tonight. Aloft,
there is a mid level ridge in place near the islands. This weather
scenario is keeping a rather stable airmass across the area. Latest
satellite imagery as well as radar data do indicate there are some
clouds and showers in the island vicinity this evening. But these
showers will remain mostly light due to the stable airmass.
Furthermore, satellite imagery revealed more low clouds are present
just upstream, or just east of the islands. These clouds, carried by
the trades, will bring some more showers to the area, mainly
windward and mountains. In all, trade wind weather will prevail
across the area through the rest of the evening into early Sunday
morning, with clouds and showers affecting mainly windward and
mountain areas. Some clouds or even light showers may reach the lee
areas of the small islands under the somewhat robust trades.

The current weather scenario is expected to persist through Monday
as the surface high will stay while the upper ridge holds.
Therefore, trade wind weather will continue with mainly light
windward and mauka showers expected. Afternoon and evening clouds
and showers are also expected over the leeward Big Island slopes.

For Tuesday through the rest of next week, the high 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 cause
the trade winds to weaken quite a bit. 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 area. Daytime
heating may allow for afternoon clouds and showers to develop over
interior areas 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. Mountain waves evident on satellite
imagery downwind of the Koolau range on Oahu, along with VAD wind
profile readings of 25+ knots, are indicators of low level
turbulence. AIRMET Tango continues for mechanical turbulence
downwind of all the mountains below 8,000 feet. This AIRMET will
likely continue through Sunday 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 on Sunday
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 for Sunday. 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 on Sunday 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 tomorrow as the
current 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 from 6 AM to 6 PM HST Sunday for Kaiwi
Channel-Maui County Windward Waters.

&&

$$

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

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