High Surf in Hawaii, September 24 to 25, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
332 AM HST Fri Sep 24 2021

.SYNOPSIS...
A trade wind pattern will bring passing clouds and showers to
mainly windward and mountain areas through the weekend and into
next week, thanks to persistent ridging to our north. A weak
surface trough passing westward to the south of the islands will
boost rainfall across windward Big Island late Saturday and
Sunday.

&&

.DISCUSSION...
A high far northeast of the state keeps a trade wind weather
pattern across the forecast area this morning. A timely ASCAT pass
showed that winds increased in strength slightly overnight, enough
to issue a Small Craft Advisory for typically windy waters around
Maui and the Big Island. Overnight soundings show an inversion
base near 5500 feet, significantly lower than the previous
afternoon, which keeps convective buildups and rainfall limited.
Satellite and radar show low clouds and showers favoring windward
areas. A few low clouds cling to lower Kona slopes on the Big
Island as well.

Expect a trade wind weather pattern across the area through the
forecast period. Ridging aloft will maintain a stable airmass as
clouds and showers favor windward areas. A trough far southeast
of the state is expected to swing northwestward over the next few
days and skirt the Big Island, likely increasing clouds and
showers there as early as late Saturday and into early Sunday.
Trade winds will receive a boost in strength as the trough passes
by to our south.

&&

.AVIATION...
A high is now centered far northeast of the islands and is
remaining nearly stationary. Thus, trade winds will continue to be
in the moderate to locally breezy range through today and beyond.
Showers moving along on the trades will bring MVFR in lower
ceilings and visibilities to mainly windward and mauka sections
from time to time. VFR conditions will prevail otherwise.

No AIRMETs are currently in effect.

&&

.MARINE...
Surface high pressure northwest to north of the islands that is
building in behind a weak Central Pacific cold front will produce
a tight enough pressure gradient down across the state in
continued support of generally fresh to locally strong trade
winds. Early morning ASCAT satellite passes have warranted the
issuance of a Small Craft Advisory (SCA) for the typically windier
channels and bays around Maui County and waters south of Big
Island through Saturday afternoon. An extension in time will
likely be required for this SCA. A surface trough passing south of
the area this weekend, along with this tighter north pressure
gradient across the islands, will provide a subtle boost to
weekend trades. An incoming northwest swell has elevated sea
heights to near 11 feet in the northwest offshore waters. These
elevated seas will move into more western nearshore waters
through Saturday.

Long period energy from an incoming north northwest swell has
reached the northwest offshore buoys, as well as the nearshore
buoy off of Hanalei, Kauai. This new swell will build surf to as
high as double overhead over the more northwest-exposed reefs and
coastlines through Saturday. North and west facing shore surf
should peak below High Surf Advisory thresholds later today and
Saturday. This first northwest swell of the young autumn season
will begin to diminish late Saturday into Sunday. The next
mentionable smaller west northwest swell will come from Tropical
Cyclone Mindulle in the West Pacific and arrive next weekend.
South facing shore surf will hold at recent levels the next
several days as small, background southwest south swell energy
hangs on through the majority of next week. Short period wind
waves generated from fresh trade winds will support mainly small,
choppy surf along many east facing shores.

&&

.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...Powell
AVIATION...Kinel
MARINE...Blood