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Temp9.5 C
RH41 %
WindNE 14 mph
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Hawaiian Islands Synoptic Discussion and Guidance

391
FXHW60 PHFO 172000
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST Tue Sep 17 2019

.SYNOPSIS...
Increasingly stable easterly trade winds will build over much of
the state, while a slightly unstable east-southeast flow produces
enhanced clouds and showers over the western end of the island
chain into tonight. Typical trade wind weather is expected on
Wednesday and Thursday, with showers focused over windward
terrain and across leeward Big Island slopes each afternoon. A
return to unstable conditions is possible on Friday through the
weekend.

&&

.DISCUSSION...
A somewhat stable, easterly trade wind flow is settling over most
of the state, while a slightly unstable east-southeast flow is
lingering over the western end of the island chain, mainly near
Kauai. Surface high pressure far north of the state is driving
the trade winds, and a broad and complex area of low pressure west
and southwest of the islands is causing the easterly trade winds
to veer out of the east-southeast near Kauai and Oahu. A mid to
upper level ridge is building over the state from the south and
bringing increasingly stable conditions to most areas. As a
result, the overnight Hilo sounding revealed a strengthening
inversion near 7,000 ft, and scant rainfall has been observed
along windward slopes from the Big Island to Oahu since last
night. An upper level trough to the west is maintaining somewhat
moist and unstable conditions closer to Kauai, where the
overnight sounding detected no inversion and PW values of nearly 2
inches. This is maintaining more active showers over and around
Kauai along with thin high clouds.

For today, expect typical late summer trade wind weather from the
Big Island to Molokai. Modest showers will be focused along
windward terrain, and spotty afternoon showers, some heavy, will
flare along leeward slopes of the Big Island during the
afternoon. Conditions will remain slightly unstable on Kauai and,
to a lesser degree, on Oahu. Leeward and interior showers, some
briefly heavy, will develop during the afternoon hours as sea
breezes overpower the east-southeast background flow.

Stable trade wind conditions are due on Wednesday and Thursday.
Moderate to locally breezy east to east-southeast winds will
persist, and deep mid to upper level ridging will produce stable
conditions. This will focus showers over windward slopes and will
inhibit heavy showers over all areas except the leeward Big Island
slopes in the afternoon hours.

A return of unsettled conditions is looking increasingly likely
heading into the weekend. An upper level trough will drop from
the north on Thursday night and Friday, wiping out the inversion
and setting the stage for spotty heavy showers in the trade wind
flow. During the weekend, the upper trough will develop into a low
as it moves over the state, and a surface trough could drift over
the islands from the south. Expect lighter winds and humid
conditions along with the possibility of heavy rainfall.

&&

.AVIATION...
A 1025 mb surface high pressure cell well north of the state will
allow for light to moderate easterly trades to continue through
the remainder forecast period. Relatively deep low level moisture
remains in place over the western portion of the islands today.
Thus, expect scattered SHRA/+SHRA to mainly affect Kauai, which
may bring brief periods of MVFR conditions. Otherwise, isolated
SHRA elsewhere.

No AIRMETs are currently in effect.

&&

.MARINE...
Moderate to locally fresh, east to east-southeast winds will hold
through midweek as a tropical disturbance to the south-southwest
of the islands lifts northwestward. A shift back to a more typical
east to northeast direction is expected Thursday through Friday,
possibly strengthening to Small Craft Advisory speeds for a short
time around Maui and the Big Island. Guidance shows a return of
light to moderate east-southeast flow late Saturday through Sunday
due to an approaching trough from the east-southeast, which would
support localized land and sea breeze conditions near the coast.

A series of small swells from the southwest, south, and southeast
is expected this week. The largest of these swells looks to be
towards the end of the week as a long-period south-southwest swell
and potential short-period west-southwest swell combine across
the area. Surf heights are expected to be below advisory levels
with these swells.

Small surf is expected along north, east, and west facing shores
through the week. Expect a small swell from Tropical Cyclone Kiko
around Thursday along exposed east facing shores before dropping
over the weekend. Exposed west shores may see some of the
aforementioned west-southwest swell late Thursday into Friday. A
small northwest swell is possible late in the weekend into early
next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Wroe
AVIATION...Thomas
MARINE...TS

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