High Surf in Hawaii, December 29 to 31, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
330 AM HST Thu Dec 29 2016

.SYNOPSIS...
Trade winds are expected to increase later today and Friday
behind a front that will pass over the islands. Look for winds to
decrease again this weekend as the high behind the front moves to
the north. Trade winds will keep clouds and showers focused over
windward and mauka areas. An uptick in showers is expected today
and Friday in association with the front, with showers then
expected to decrease through the weekend as the front washes out.

&&

.DISCUSSION...
An area of showery low clouds embedded within trade flow is
passing over Oahu and the islands of Maui County. A second cloud
area has just reached the Big Island. However, PW of an inch or
less combined with weak ridging aloft will limit rainfall to
light amounts. Clouds and showers will favor windward and mauka
areas, with these portions expected to see steadier rainfall
until the low cloud areas pass west of the islands later today.

Satellite loops show a ragged 200 mile wide band of broken low
clouds about 200 miles north of the main Hawaiian Islands this
morning, moving southward at about 15 mph. Marking a weak frontal
boundary, this cloud band will push southward across the main
island chain later today and Friday, then stall and wash out
through the weekend. Since trade flow will persist during this
time, the increased clouds and showers associated with the weak
frontal band will affect mainly windward and mauka areas. The
strong high behind and far northwest of the front will boost trade
wind speeds after frontal passage. Trade winds will decrease
after Friday as the front washes out and the high far to the
northwest lifts out to the north. Trade flow will dry out as well
through the weekend, with fewer showers expected to moisten
windward and mauka areas.

Models keep a surface low to the northeast of the islands during
the start of next week, which will help limit trade wind speeds.
At the same time, models indicate a drier airmass over the
islands which would limit shower activity. By midweek, winds are
forecast to be light, possibly becoming light enough to support
localized land and sea breezes.

&&

.AVIATION...
AIRMET TANGO for low level turbulence over and immediately south
through west of the mountains continues, as do the breezy trade
winds. The enhanced trade wind showers are bringing periods of
MVFR CIGS/VIS to most windward areas. AIRMET SIERRA for mountain
obscuration is posted over all north and east facing slopes except
for Lanai and Maui, as they sit in a relatively dry slot with less
shower activity. Expect the more widespread shower activity to
start to ease up by mid morning. Clouds and showers will continue
to favor the windward and mountain areas. A front will move
closer to Kauai later this evening, where an uptick in showers
can be expected, especially for windward areas as it moves down
Kauai to Oahu. The already breezy trade winds will surge as the
front passes.

An upper level jet over Hawaii may introduce light to moderate
turbulence near and over the islands today. Pirep reports from the
last few hours suggest pockets of light turbulence around FL350
near the jet exit, approximately 175 to 250 NM east-northeast of
the island chain. AIRMET TANGO for upper level turbulence may need
to be issued later, if signs of tempo moderate turbulence form
closer to the islands.

&&

.MARINE...
An ill-defined front has entered our northern most offshore waters
accompanied by gale force winds of 35 kt. The front is moving
south around 12 kt, and is still on track to reach Kauai either
late this afternoon or early this evening. the front will weaken
as it oozes southward Friday across the central islands. The winds
behind the front will reach waters around Kauai at 30 kt then
weaken to 25 kt while crossing waters around the central islands.

Models show the front continuing slowly southward, dissipating
along the way while crossing the Big Island between Friday night
and Saturday. By this time the associated northeast winds will
weaken to below Small Craft Advisory limit of 25 kt, although
there may be a pocket or two of 25 kt around the surrounding
waters of Maui County including the Alenuihaha Channel. By Sunday,
the trades will be down at 20 kt or less area wide. This weakening
trend comes about as the driver of the strong trades, a strong
1040 mb high pressure cell, takes a northerly course away from the
islands.

Currently, the wind situation is rather messy and complex. The
ASCAT pass last evening caught the eastern half of the island
chain from the waters of eastern Oahu eastward to the Big island.
That pass showed 25 kt winds in the Kaiwi Channel and waters
north and south of Molokai and Oahu. The model winds are underdone
for the Alenuihaha Channel and waters south of the Big Island with
30 knotters. Some grid adjustments were made to account for this.
Although models show a trend for the winds to ease off across the
central waters today, there is the chance of pockets of SCA level
winds leaking ahead of the front. To cover this potential, the SCA
that was supposed to go into effect for waters around the central
islands at 6 pm HST this evening will be moved up becoming
effective this morning.

The SCA covering waters south of the Big Island is supposed to
end at 6 pm HST today. However, there is the possibility the SCA
level winds will continue into tonight. We will monitor the
situation closely.

The front is very near buoy 51001 located 185 nm nw of Kauai at 2
am HST with a predominant short 10 second period swell. This swell will
reach Kauai later this morning, peaking tonight at around 11 feet
10 seconds. A High Surf Advisory (HSA) will likely be needed for
tonight and Friday for exposed east facing shores from Kauai to
Maui. It appears the Big Island will be spared from this swell
but we will closely monitor the situation. The HSA will likely be
issued later today.

Additional pulses of NNE swell are expected into early next week,
possibly leading to advisory level surf along some east facing
shores and some harbor surge on Sunday night or Monday. It is
uncertain if there is a need for an SCA due to elevated seas with
these reinforcing swells.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST this evening for Big Island
Leeward Waters-Big Island Southeast Waters.

Small Craft Advisory until 6 PM HST Friday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui
County Windward Waters-Maui County Leeward Waters-Maalaea Bay-
Pailolo Channel-Alenuihaha Channel.

&&

DISCUSSION...Powell
AVIATION...Eaton
MARINE...Lau