High Surf in Hawaii, December 14 to 16, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
347 AM HST Wed Dec 14 2016

.SYNOPSIS...
A fairly dry and stable airmass overhead today will limit shower
activity. A front approaching from the northwest will move across
the islands Thursday through Friday night, bringing rain and
possibility for a few thunderstorms. An upper low associated with
the front is expected to dig over the state through the weekend.

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.DISCUSSION...
As expected overnight, the winds on the Big Island summits dropped
below advisory levels, so the wind advisory was dropped at
midnight. Winds on the summits will likely climb again tomorrow,
with more of an increase Thursday night and into the weekend.

A band of showery low clouds is approaching the smaller islands,
and will bring a few more showers to the windward sides over the
next couple of hours. Beyond that, today should be a fairly dry
day with an upper level ridge over the islands.

There was little change in the latest model runs overnight with
regards to the next weather system that will bring unsettled
weather to the islands as early as tomorrow morning. The surface
and upper level low is currently forecast to be about 600 miles
north of Honolulu Thursday morning as a cold front begins to move
over Kauai. The front will move down the island chain through
Friday night, bringing rain to the islands as it does.

Colder temperatures aloft will accompany the front, with
temperatures near -9 celsius at 500 mb over the western islands
Thursday morning, on the cold side of normal for December. Colder
temperatures are expected by Friday morning, when 500 mb
temperatures are forecast to reach -13 or so.

The bulk of the low level moistures sits to the east of the
islands through the weekend, only being pulled in over the islands
early next week as the upper level low which is expected to move
over the islands late in the weekend, pulls away from the
islands. The cold air aloft will be plenty unstable to enhance
showers and spark a few thunderstorms, but the full extent of the
situation remains to be seen. The mention of thunderstorms
remains in the forecast over the smaller islands for Friday and
Friday night, but may be expanded in time and/or area as the
situation evolves.

The cold upper level temperatures will also bring the possibility
for additional winter weather on the Big Island summits. With the
front dissipating over the Big Island Friday night, there is still
some question as to how much winter weather the Big Island will
receive. Daytime highs are expected to reach near 40, but
overnight lows will be around freezing. Model wet bulb zero
heights get to around 10500 feet, which if timing of everything
works out, could bring winter weather a little lower on the
summits than during the recent events.

As the system moves away from the islands Tuesday, we could see
trades build back in, bringing the above mentioned low level
moisture into the islands which could mean some additional trade
wind showers mid week.

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.AVIATION...
High pressure north of the islands will keep a light to moderate
trade wind flow in place today, with winds diminishing tonight as
a cold front approaches from the northwest. Clouds and showers
will favor windward and mauka areas through the period, with some
brief MVFR cigs/vsbys possible in passing SHRA. Otherwise,
predominantly VFR conditions are expected through 15/12Z.

AIRMET Sierra is in effect for mountain obscuration across N thru
E sections of Kauai, Oahu, Molokai and Maui. Conditions are
expected to improve later this morning.

AIRMET Tango is in effect for low level turbulence over and
downwind of the mountains across all islands. Conditions are
expected to improve later this morning.

&&

.MARINE...
The current north-northeast swell has shown some signs of
reaching the Waimea Bay buoy overnight. The swell is expected to
peak later today. The swell is not expected to reach advisory
levels for north facing shores, however it has enough of an east
component that at least some east facing shores will see advisory
level surf. The current High Surf Advisory is posted through
tonight. It may need to be extended depending on how the swell
interacts with the east facing shores today. Combined seas are
expected to remain below the 10 ft Small Craft Advisory (SCA)
criteria.

The above mentioned storm system will start to affect the coastal
waters during the second half of the week. Winds behind the front
could bring SCA winds to the waters and the close proximity of
the low will ramp up seas. A large northwest swell is anticipated
to arrive Thursday night, and peak Friday night. This will likely
bring advisory, and even possibly warning level, surf to north
and west facing shores into Saturday. The combined seas could
reach SCA levels as early as late Thursday night. If southerly
winds develop as forecast around the low, the surf along south
facing shores may also see a slight rise and choppier waters.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Thursday for Niihau-Kauai
Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-
Windward Haleakala-Big Island North and East.

&&

DISCUSSION/MARINE...M Ballard
AVIATION...Jelsema