High Surf in Hawaii, September 22, 2022

Area Forecast Discussion...CORRECTED
National Weather Service Honolulu HI
342 AM HST Thu Sep 22 2022

.SYNOPSIS...
A large low pressure system far north of the Hawaiian Islands
will continue to weaken the trade winds through the first half of
next week. Local scale land and sea breezes will develop over each
island in response to these light trades. This means the weather
pattern will return to mountain and island interior sections with
clouds and showers from afternoon to early evening due to
converging sea breeze winds. More stable down sloping land breezes
will lead to drier trends each night lasting through the early
morning hours.

&&

.DISCUSSION...
A large low pressure system far north of the Hawaiian Islands will
continue to weaken the high pressure ridge north of the state this
week. Another trough associated with a dissipating cold front will
move in north of the state this weekend keeping the ridge in a
weakened state through next week Wednesday. Light to locally moderate
trade winds remain in the forecast through the middle of next
week. Local scale land and sea breezes will develop over each
island in the absence of stronger large scale trade wind flow.

Weather wise, expect clouds and brief periods of showers over
island mountain and interior sections each day due to converging
sea breeze winds. Typically drier leeward areas will also see
some measurable rainfall in this sea breeze pattern. An upper
level trough shown on both water vapor satellite imagery and in
the forecast guidance will likely expand rainfall coverage of
these sea breeze showers through Friday afternoon. Down sloping
land breezes will develop after sunset, produce more stable
conditions and decrease shower activity over the islands in the
overnight to early morning hours. Drier weather trends are
forecast from late Friday into Saturday for most areas.

Weather trends from Sunday onward become more uncertain as large
scale wind flow shifts to a more southeasterly direction with
clouds passing more parallel to the island mountain ranges and
keeping the smaller islands in a rain shadow just down stream of
the Big Island. A low level trough, on a westward track, passes
through the Big Island late Sunday into Monday. This low level
trough may enhance shower activity over the slopes of the Big
Island and Maui early next week.

&&

.AVIATION...
Light to locally moderate trade wind flow will continue to steer
ragged bands stratocumulus off the Pacific towards the Islands. A
weak eddy just west of the Big Island will keep the Kona coast
shrouded in clouds through mid-morning. In the afternoon,
localized sea breezes will focus daytime cumulus development over
island interiors. Brief heavy showers are possible over the
higher elevations of Maui and the Big Island later today.

No AIRMETs are in effect or anticipated at this time.

&&

.MARINE...
No Small Craft Advisory conditions are in the current forecast
through Monday. The western end of a weak surface ridge is about
300 nm north of Lihue early this morning. Elsewhere, a 1007 mb
surface low is near 35N 159W, or about 800 nm north of Lihue. A
cold front trailing from this feature extends down to near 30N
163W. As this cold front advances toward the east-southeast at
about 10 kt during the next 12 hours or so, it will cause the
surface ridge to shift down closer to the islands. This will
result in a continued easing of the background winds, which are
forecast to veer out of the east to east-southeast through Friday.
The front will eventually weaken to a surface trough, and shift
westward from late Friday through this weekend. At the same, there
will be a surface high far northeast of the islands. This will
likely produce gentle to moderate easterly trade winds from this
weekend into early next week.

The National Data Buoy Center buoys south of Hawaii, as well as
the PacIOOS buoys in the nearshore waters, are showing increasing
swell energy of 18 to 20 seconds from a south-southwest swell
that is spreading up across the coastal waters early this
morning. Since the swell height has been 1 to 2 feet above the
wave model guidance, we have decided to issue a High Surf Advisory
(HSA) for the south facing shores of all Hawaiian Islands. The
HSA is currently in effect through this afternoon, but this may
need to be extended into tonight if the observations show the
swell does not begin to subside later today. This swell will
slowly decline Friday, before the arrival of a reinforcing pulse
of south swell energy on Saturday, which will likely maintain
surf near the seasonal average along south facing shores this
weekend. Surf will gradually lower along south facing shores
early next week.

Surf will remain below the seasonal average along north facing
shores through early next week. The current small northwest swell
will fade late Friday. A small, short period north-northwest
swell is possible during the weekend, followed by another small
northwest swell next Tuesday. The weak trade winds over and
upstream of the islands will maintain small surf along east
facing shores through early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for all south
facing shores.

&&

DISCUSSION...Bohlin
AVIATION...Bedal
MARINE...Houston