High Surf in Hawaii, August 16 to 19, 2021

Area Forecast Discussion
National Weather Service Honolulu HI
321 AM HST Mon Aug 16 2021

.SYNOPSIS...
A light wind pattern will hold in place today through Wednesday,
with showers developing over the island interiors each afternoon,
and mainly dry conditions expected each night. More typical trade
wind weather will return Thursday through Saturday, with showers
favoring windward and mauka areas. The trades could become
disrupted again by Sunday, with a sea breezes favoring shower
development over the island interiors during the afternoon hours.

&&

.DISCUSSION...
Currently at the surface, a southwest to northeast oriented
trough of low pressure is located around 200 miles north-
northeast of Hilo, while a 1029 mb high is centered around 1300
miles north of Honolulu. The trough is moving slowly westward
early this morning and has weakened the trade winds to gentle
levels. Infrared satellite imagery shows clear to partly cloudy
conditions across much of the state, with a few pockets of more
extensive cloud cover near the Big Island and over windward
sections of Oahu and Kauai. Radar imagery shows scattered showers
moving into windward Oahu and Kauai, with very shower activity
reaching land over Maui County and the Big Island. Main short term
concerns revolve around trade wind trends and rain chances during
the next few days.

The trough of low pressure north-northeast of the state will track
slowly westward during the next couple days, disrupting the trade
wind flow across the island chain. The trades are expected to
become disrupted by late morning, with light and variable winds
featuring daytime sea breezes and overnight land breezes expected
this afternoon through Tuesday. Trade winds should begin to slowly
return from east to west across the state on Wednesday as the
trough shifts west of the islands and high pressure builds to the
distant northeast. Moderate to locally breezy trade winds should
then return Thursday through Saturday, with the trades possibly
decreasing again by Sunday as the remnants of tropical cyclone
Linda disrupt the gradient across the island chain.

As for the remaining weather details, a few trade wind showers
should continue to affect Oahu and Kauai this morning. A
convective shower regime should take hold this afternoon through
Wednesday, featuring the development of scattered showers over
interior and mauka locations each afternoon. These showers will
then fizzle out in the evening, followed by mainly dry conditions
overnight. A more typical trade wind pattern should become re-
established Wednesday night through Saturday, with showers
favoring windward and mauka areas. A sea breeze driven convective
shower pattern will be possible again by Sunday, favoring shower
development over the interior of the islands during the afternoon
hours.

&&

.AVIATION...
Weakening trade winds will allow local scale land and sea breezes
to become more dominant over the islands today. An upper level
low drifting into the region from the northeast direction will
decrease atmospheric stability allowing cloud heights to deepen.
Converging sea breezes will cause clouds to build and showers to
develop over each island during the afternoon to early evening
hours. Diverging land breezes forming after sunset will cause
clouds over island interiors to diminish through the early morning
hours.

No AIRMETs in effect and none are expected.

&&

.MARINE...
A surface trough passing north of the state the next few days
will disrupt trade flow and ultimately weaken winds around the
islands. The western movement of this trough should subtly veer
light to gentle breezes east southeast in time through early
Wednesday. Reestablishing east trade winds will strengthen by
Thursday as the trough moves west of the chain.

South shore surf will be on the rise today with the arrival of an
anticipated large, long period south swell. Offshore southern buoys
are showing the high period frontrunners while the nearshore PacIOOS
Barber's Point buoy is registering rising swell within the higher
energy bands. Several pulses within this swell will have this south
shore surf event lasting through mid week. A High Surf Advisory will
likely be in effect for south facing shores as early as this afternoon
but certainly for peaking surf at or slightly above 10 feet Tuesday
and Wednesday.

The ongoing fairly low, moderate period northwest-west swell will be
slowly declining through Tuesday. This will have north and west facing
shore surf gradually falling through late Tuesday. Surf along east
facing shores will remain relatively small the next several days.
Swell within the captured fetch from Hurricane Linda in the East
Pacific might make it to the east facing shores of the eastern
most islands as small, long period east swell by Wednesday.

&&

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

&&

DISCUSSION...Jelsema
AVIATION...Bohlin
MARINE...Blood