High Surf in Hawaii, March 13, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
810 PM HST Sun Mar 12 2017

.SYNOPSIS...
A weak surface ridge will persist in the vicinity of the islands
through Thursday, maintaining light background winds along with
muggy and hazy conditions. Afternoon sea breezes will allow for
cloud build ups and a chance for showers over parts of the islands
each day, while land breezes will clear out most of the cloud
cover at night. Trade wind weather is expected to return by the
end of the work week as a front moves down the island chain.

&&

.DISCUSSION...
Currently at the surface, a 1026 mb high is centered around 1750
miles east-northeast of Hilo, with the ridge axis extending west-
southwestward and through the central islands. This has resulted
in a light wind regime across the islands, with sea breezes
now beginning to give way to land breezes in most areas. Infrared
satellite imagery shows partly cloudy to mostly clear skies in
most areas. Meanwhile, radar imagery shows some isolated showers
drifting into southern sections of Kauai, with rain free
conditions across the rest of the state. Main short term concern
for tonight revolves around the building west-northwest swell.

Tonight through Thursday,
The high pressure ridge axis will remain over or just north of
the island chain through the period. This will keep a light wind
regime in place, featuring overnight land breezes and daytime
seabreezes in most areas. Mid-level ridging will also build over
the island chain tonight and remain in place through Tuesday,
before some weak upper troughing builds over the area Wednesday
into Thursday. Overall, a much more stable airmass is expected to
build over the state tonight and remain in place through Thursday,
and this should keep the showers more isolated and generally on
the light side. Showers will tend to favor the coastal waters
during the overnight and morning hours, and the interior of the
island during the afternoon and evening hours as a result of
daytime heating. The light winds will keep the airmass rather
stagnant through the period as well, with muggy and hazy
conditions expected to continue.

Thursday night through next Sunday,
Model guidance is in fairly good agreement through the extended
portions of the forecast, with only some minor timing differences
noted. Overall, high pressure building north of the state will
shift a weakening cold front southward into Kauai as early as
Thursday night, with the front then progressing slowly and
steadily southward across the central and eastern islands Friday
through Saturday, before pushing south of the Big Island Saturday
night and Sunday. A band of showers is expected to accompany the
front as it moves through the islands, with showers most prevalent
over windward and mauka areas. Rainfall amounts should remain on
the light side as the front will be fairly progressive and the
moisture depth along the front will remain rather shallow. The
front will also bring a return of light to moderate trade winds
to the Aloha state by the end of the work week, with the trades
continuing through next weekend. This will bring a return of
cooler and less humid conditions, and also shift any lingering
volcanic haze south of the smaller islands.

&&

.AVIATION...
Expect prevailing VFR conditions tonight as land breezes will
help clear the islands. A high pressure ridge over the state will
continue to allow for a light southeast to south flow with
afternoon sea breezes and interior cloud build ups tomorrow.
Isolated to scattered showers tomorrow afternoon may introduce
isol to tempo MVFR CIGS/VIS to mainly interior areas, otherwise
VFR is expected. Hazy conditions will continue.

There are no AIRMETs in effect.

&&

.MARINE...
A moderate west-northwest (310 deg) swell associated with a gale-to-
storm force low that developed a few hundred miles northwest of
Midway on Friday is forecast to fill in and build across the waters
tonight, peak late tonight through Monday, then gradually ease
Tuesday through Wednesday. An earlier altimeter pass just east of
Midway showed seas up to 10 to 13 ft, which was a couple of feet
higher than depicted by the ECMWF-Wave and WAVEWATCH III. If this
higher trend unfolds across the local waters as the swell builds
down the island chain, surf heights will reach advisory levels
from late tonight through Monday. Provided this potential, an
advisory has been issued for exposed north and west facing shores
of the smaller islands beginning at midnight tonight and
continuing through Monday. Seas are forecast to remain just below
the small craft advisory level of 10 ft over the exposed waters.

A small reinforcing swell out of the west-northwest (300-320 deg)
associated with a gale that has developed east of Japan across the
northwest Pacific will fill in Thursday night and hold through
Friday.

A moderate northeast swell (020 deg) associated with a gale that is
forecast to develop and pass around 800 nm north of the state
Thursday through Friday will fill in across the local waters over
the weekend. Surf along east facing shores may near or reach
advisory levels if this feature materializes.

For the long-term, the latest ensemble and deterministic guidance
depicts a broad gale-to-storm force low developing in response to a
strong shortwave trough east of Japan through the mid-week period,
then lifting northeastward toward the Aleutian islands near the date
line by Saturday. If this feature evolves as projected, another
west-northwest swell (300 deg) will become a possibility over the
local waters by the end of next weekend.

Local winds will remain in the light to moderate range through much
of the week as a weak ridge of high pressure remains in place across
the islands. Land and sea breezes will remain likely each day near
the coasts, especially for Kauai and Oahu (offshore winds early
mornings/onshore winds afternoons). Models indicate trades
returning next weekend as high pressure builds north of the waters
in the wake of a weak cold front moving through the region later
in the week.

&&

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

&&

DISCUSSION...Jelsema
AVIATION...Eaton
MARINE...Kinel