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Hawaiian Islands Synoptic Discussion and Guidance

645
FXHW60 PHFO 252002
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1002 AM HST Sat Sep 25 2021

.SYNOPSIS...
Clouds and showers will favor windward and mauka areas as a
locally breezy trade wind weather pattern continues through the
forecast period. A surface trough passing south of the islands
will increase shower coverage over windward Big Island tonight
into Sunday.

&&

.DISCUSSION...

Morning soundings indicate strong stability in place over the
islands this morning, particularly over the western end of the
state where the dewpoint depression above the trade inversion is
on the order of 50 degrees C. This has kept a firm lid on the
morning's shower activity while also supporting some morning
stratus over Windward Kauai and Big Island that has already burned
off. As evident regional visible imagery, mid-level flow over the
area has become increasingly veered in response to a trough
currently positioned southeast of the state. As a result, the band
of offshore stratocu will continue to track northwestward /roughly
parallel to the island chain/ as breezy easterly trades at the
surface focus limited clouds and showers over windward areas
through today.

Established southeasterly mid-level flow will then steer an
organized area of shower-bearing stratocu toward the Big Island
tonight. This activity, located about 400 miles southeast of Hilo
at press time, is demonstrating a reasonably healthy appearance
on satellite, its weak circulation serving as evidence of modest
organization. Cloud tops in this area of interest topped out
around 10-15kft during the favorable nocturnal period last night,
and a similar presentation is likely tonight as it approaches the
Big Island around midnight HST. Diminishing stability through the
lowest several thousand feet will lead to a high coverage of
moderate trade wind showers focused over Windward Big Island and,
to a lesser extent, Windward Maui. A few locally heavy showers
will be possible but their expected coverage and duration fall
well short of causing any flooding concerns. Trades will receive
a slight boost as the trough passes by to the south, but will
remain mostly in the breezy category for the remainder of the
forecast period. Drier trade wind weather will then resume as the
approaching moisture axis shifts west of the islands on Sunday
night.


&&

.AVIATION...

Latest satellite and radar imagery shows cloud bands and a few
showers favoring north through east sections of the islands and
adjacent waters this morning. Although isolated MVFR conditions
will be possible throughout the day for windward sections of the
islands, VFR ceilings and visibility will prevail for most
locations. Another round of enhanced moisture is expected to
approach the Big Island late this evening and tonight, which could
result in heavier and more widespread showers, reduced
visibilities, lower ceilings, and mountain obscuration for
windward portions of the island. Elsewhere, MVFR conditions will
remain isolated.

Strong high pressure north of the islands will continue to
support breezy easterly trade winds across the state through the
rest of the weekend. As a result, AIRMET Tango remains in effect
for moderate mechanical turbulence on the lee side of the
mountains below 9000 ft. This AIRMET is expected to remain in
effect through the rest of the weekend.

&&

.MARINE...

Moderate to locally strong trade winds will continue through the
week. A slight increase in the trades will occur tonight and hold
into Sunday, as a surface trough passing south of the islands
briefly tightens the local pressure gradient. A Small Craft
Advisory (SCA) remains in effect for the typically windy waters
around the Big Island and Maui. The SCA will likely be expanded to
cover additional zones tonight and Sunday as winds briefly
increase and will probably be trimmed back to the typically windy
waters Sunday night and Monday as the trough departs. High
pressure anchored north of the state will produce border line SCA
conditions for the typically windy waters Tuesday and Wednesday
with slightly stronger winds expected late in the week.

After spiking above guidance yesterday, the north-northwest swell
has declined to 4 to 6 ft at 13 seconds this morning. North shore
surf will remain above September average today, decline to around
average (about 6 ft) tonight and Sunday, then fade by Tuesday. No
other significant northwest swells are due this week. South
shore surf is holding around September average (around 5 ft). The
south swell will drop slightly on Sunday then fade, leading to
mainly summertime background surf heights through the rest of the
week. Trade wind swell is running at about 5 ft 7 seconds, which
is near September average. Little change is due through the
weekend, followed by a small decline early in the work week and
and increase back to near average Thursday or Friday.


&&

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


&&

$$


DISCUSSION...JVC
AVIATION...Vaughan
MARINE...Wroe

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



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