Area Forecast Discussion
Issued by NWS Honolulu, HI

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000
FXHW60 PHFO 221348
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
348 AM HST Fri Oct 22 2021

.SYNOPSIS...
A band of low clouds and showers will bring scattered showers to
Kauai and Oahu through tonight. An area of tropical moisture will
move onto the Big Island tonight through Saturday morning and
bring increased showers. Trade winds will gradually strengthen
through the weekend before tapering off Tuesday.

&&

.DISCUSSION...
Infrared satellite this morning shows a northeast to southwest
oriented band of low clouds draped over Kauai and Oahu associated
with a surface trough. Individual cloud elements within the band
are tracking to the west southwest by moderate to locally fresh
trade winds. Radar shows scattered showers mainly impacting the
windward slopes of both islands. A weak, nearly stationary front
lies about 300 miles north of Hawaii. A high pressure system
behind the front is driving the trade winds.

Another, stronger high will develop far to the north Saturday
night and dig closer to island chain resulting in a slight
strengthening of the winds this weekend. Trades will direct any
incoming clouds and showers over windward sides of the islands,
with some showers occasionally reaching leeward sides of the
smaller islands.

The band of clouds and showers currently over Kauai and Oahu is
expected to remain in place today and then slowly move west of
the state tonight. Kauai could still see a few lingering showers
into Saturday. Meanwhile, Maui County should remain mostly sunny
and dry. To the east, an area of tropical moisture will move in
late in the day today over the windward and southeast sections of
the Big Island and bring a boost in showers likely through
Saturday morning. This area of moisture will pass south of the
smaller islands over the weekend.

The front to the north is expected to slowly dissipate over the
next couple of days, but a slight backing of the trade wind
direction on Sunday could bring a little bit of the upstream
remnant clouds and showers over the state late Saturday into Monday.

Long range GFS and ECMWF show a cutoff upper low tracking in from
the northeast over the Big Island Monday and Tuesday before
lifting back to the northeast Wednesday and Thursday. A backdoor
cold front also approaches from the northeast and stalls around
midweek. Trade winds will weaken on Tuesday as this front
approaches.


&&

.AVIATION...
The shower band that brought increased low clouds and showers
into windward and mountain areas of Oahu overnight has dispersed
somewhat early this morning, but will gradually shift westward
today, allowing shower activity to increase over windward Kauai as
well. Elsewhere, isolated -SHRA and VFR conditions will prevail.

Trade winds are expected to strengthen through tonight, which
will bring an increase in shower activity along windward portions
the remaining islands, beginning with the Big Island late this
evening and spreading westward into early Saturday morning.

AIRMET SIERRA for tempo mountain obscuration on Oahu has been
dropped as shower activity has decreased. As the trades strengthen
today, AIRMET Tango may be needed for low-level turbulence in the
lee of the mountains.


&&

.MARINE...
No significant changes to the coastal waters forecast this
morning. Frontal remnants remain near the islands but will
continue to dissipate today. A new high building north of the area
will boost trade winds into early next week. A Small Craft
Advisory (SCA) is in effect for the typical windier waters near
Maui county and the Big Island through Saturday but will likely be
extended as SCA level winds will likely persist into early next
week.

A new north swell is expected to arrive later this morning, and
boost surf along exposed shores through Saturday. The swell looks
likely to peak tonight just below advisory levels. The potential
remains for this swell to produce moderate harbor surges in the
Kahului and Hilo harbors, however have held off on issuing a
statement until we see some evidence of the swell at the buoys.
East facing shores with northerly exposure will also see a boost
in surf with this swell. During the Sunday through Wednesday
time period, a series of small north and north northwest swells
can be expected.

Small, mainly background south and southeast swells are expected
through Monday with a slightly larger south swell expected to fill
in Tuesday and Wednesday. As the trade winds rebound, they will
produce short period choppy surf along east facing shores through
the weekend with a slight downward trend expected into the middle
of next week as trade wind speeds decrease.


&&

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


&&

$$


DISCUSSION...Foster
AVIATION...TS
MARINE...M Ballard


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