High Surf in Hawaii, November 25 to 28, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
845 PM HST Sat Nov 24 2018

.SYNOPSIS...
A weak ridge of high pressure north of the islands will keep
daytime sea breezes and nighttime land breezes blowing through the
rest of the weekend. The local airmass is dry and stable, so
mainly just afternoon interior clouds are expected, along with a
few windward clouds and showers over the Big Island. The trades
will start to return on Monday the high to the north strengthens,
and could briefly reach locally windy levels by midweek. A front
will approach the islands late in the week and our winds will turn
konas by Thursday or Friday.

&&

.DISCUSSION...
Overall forecast philosophy remains the same. Have made some
modifications to the forecast, but mostly to bring things in line
for PoPs and associated fields Tuesday and Wednesday afternoon
when trades are expected to be in place. Also made small
modification to PoPs for Big Island Hamakua slopes since satellite
and radar show a little activity there this evening.

High pressure about 1500 miles east-northeast of Oahu has a ridge
extending towards the islands with the western end just north of
Kauai. This will maintain the light winds over the islands through
Sunday, with local land and sea breezes. The few clouds over the
islands this evening will likely clear out with the overnight land
breeze, with increasing clouds over the islands tomorrow with the
sea breezes. Satellite derived precipitable water continues to
show a dry airmass upstream of the islands, so rainfall will be
limited.

A new high with a surface ridge building north of the islands will
help trades to start filtering across the islands on Monday, and
strengthen as we head into the middle of the week. Expecting
more typical trade wind clouds and showers over mainly windward
areas, beginning later on Monday and persisting through midweek.

A complex low is forecast to move in rapidly behind this high,
which will weaken the trades during the second half of the week,
and turn the winds to a more southeast direction. The low will
track to the east, with its associated cold front being dragged
towards the islands from the northwest. The latest run of the GFS
would bring the front to Kauai Thursday night into Friday. Will
continue to monitor the development of the front, but safe to say
we could see more showers and southerly winds during the end of
the new week.

&&

.AVIATION...
Expect a VFR picture overnight, dominated by a land breeze for
especially the smaller islands. There is a trade wind breeze
blowing through windward Big Island, and that is where most, if
not all, the weather is this hour. Radar and satellite indicate
the presence of some trade wind clouds and showers along the
Hamakua coast and on down through the Puna district. Tops of these
showers are around 10k feet. Satellite also reveal a large patch
of low clouds and embedded showers south of Cape Kumukahi,
embedded in the trade flow. Expect these clouds and showers to
cross the Kau coastal waters overnight and into Sunday morning. A
land breeze should keep these clouds and showers off the land
mass. A few light minor showers may affect windward Maui later
tonight. No AIRMETs are expected through Sunday morning.

&&

.MARINE...
Light to moderate east-to-southeast (090-110 deg) winds will prevail
across the Hawaiian waters through Monday in response to a weakness
that has developed within the ridge north of the state. Pockets
of fresh breezes are expected over the typical areas where wind
accelerations occur with this type of flow in place, such as: the
Hamakua coast, north and northeast Maui from Pauwela to Hana, the
channels between Maui and the Big island and the waters south of
the Big Island near South Point. Elsewhere near the coasts,
overnight land breezes (offshore winds) and afternoon sea breezes
(onshore winds) will be possible each day through Monday. A
return of fresh to locally strong trades appears likely Monday
night through midweek as high pressure builds north of the state.
A front is expected to approach the state from the northwest late
next week and will cause the winds to shift towards the south and
may bring some inclement weather.

The beginning of a long-duration high surf event with overlapping
large northwest swells in response to an active northern Pacific
pattern is expected tonight through midweek along exposed north
and west facing shores. The early evening observations at the
offshore northwest buoy (51101) are quickly rising and currently
running around 1-2 ft higher than predicted by guidance.
Advisory-level surf will be the result through the day Sunday
along north and west facing shores, then likely to warning levels
by or come Sunday night as a larger reinforcement fills in.
Warning-level surf could hold as long as early Tuesday before
lowering to advisory levels later Tuesday into Wednesday. The
current advisory will be upgraded to a warning overnight to
account for this potential. Seas will exceed small craft advisory
levels, likely by Sunday night over most waters.

In addition to the life-threatening surf expected for a long
period, impacts could also include significant beach erosion and
overwash onto the typically vulnerable low-lying coastal areas,
especially around the peak of the event Monday at/around the
monthly peak high tide cycles (around daybreak Monday/Tuesday for
highest tides).

Small surf will continue along south facing shores with mainly
background southern Pacific pulses moving through.

Short period choppy surf will return along east facing shores by
midweek as the trades return.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory from 6 AM to 6 PM HST Sunday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

&&

DISCUSSION...M Ballard
AVIATION...Lau
MARINE...Gibbs