High Surf in Hawaii, November 18 to 19, 2016

Area Forecast Discussion
National Weather Service Honolulu HI
316 AM HST Fri Nov 18 2016

.SYNOPSIS...
High pressure systems passing to the north of the islands will
maintain breezy trade winds across the region through the
weekend and into next week. Clouds and showers being carried in
on the trades will be focused over windward and mauka areas. Some
of the showers may reach leeward areas of the smaller islands at
times with the stronger winds. Showers will increase during the
weekend as an old frontal boundary drifts southward through the
state. A rather wet trade wind pattern will persist into early
next week as moisture lingers across the islands.

&&

.DISCUSSION...
Dry and breezy trade wind weather prevails across the Aloha State
on this early Friday morning. Very little rainfall has occurred
over the islands during the night, and radar currently shows
widely scattered light showers over windward areas and adjacent
waters. Surface analysis depicts a 1033 mb high centered about
1200 miles NNW of Kauai, with a tight pressure gradient over the
state to the south of this high. Aloft, a mid/upper level ridge
lies over the islands. MIMIC-TPW satellite imagery shows a dry
airmass around the islands and extending far upstream, with only
some small pockets of slightly higher moisture embedded. A narrow
ribbon of higher moisture associated with a weak front is located
about 150 miles north of Kauai and is sinking slowly southward.
The 12Z soundings show a dry and stable atmosphere in place, with
strong inversions based between 5500 and 6000 feet, and below
normal PW values near one inch.

Rather dry and breezy trade wind weather will continue today,
followed by a somewhat wetter and still breezy pattern tonight
through the weekend, as the remnants of an old front drift
southward over the state. Showers will remain mostly concentrated
across windward areas, but some will be carried leeward due to the
strength of the trade winds.

Heading into next week, another strong surface high will build
far north of the state, maintaining or even increasing the breezy
trade winds especially by midweek. Models continue to show
increasing lower and even mid level moisture over the islands
during this time, so would expect a generally wetter trade wind
pattern, with highest rain chances remaining windward but some
scattered showers spilling over leeward.

By the end of the forecast period on Thanksgiving Day, models
continue to show indications of an upper level trough developing
over the state, with considerable lower/mid level moisture
remaining in place. An active and quite wet trade wind pattern
could be the result. Strong high pressure north of the state
would keep breezy/windy conditions in place as well. Will
continue to monitor later model cycles.

&&

.AVIATION...
Breezy trade winds will continue through Friday with clouds and
isolated showers favoring windward and mountain areas. VFR Conditions
will predominate, but passing showers will produce localized MVFR
CIGS/VIS at times.

AIRMET TANGO for low level turb over and immediately south through
west of the mountains will remain posted due to brisk winds at summit
levels. No other AIRMETs are expected.

&&

.MARINE...
A new long-period northwest swell has arrived this morning and
continues to rise at the nearshore CDIP buoys. Buoy 51101, a couple
of hundred nautical miles northwest of Kauai, peaked around midnight
(11 ft at 15 sec), which is in line with the ECMWF-Wave guidance and
a few feet higher than predicted by the GFS-WAVEWATCH III. The
Hanalei CDIP buoy has just climbed into the 10 to 11 ft range and
appears to be peaking. Swell heights at the Pauwela, Maui buoy have
recently begun to rise, which should continue to through the morning
hours before leveling off. As a result, the morning marine forecast
package will include a slight upward adjustment by one to two feet
based on what has already been observed at 51101 and the Hanalei
buoy. Outside of these differences, the latest model guidance
remains on track and depicts this swell beginning to ease tonight
through the weekend.

Impacts from this swell will include hazardous boating conditions,
especially around harbor entrances, and dangerous surf along the
exposed north and west facing shores. A Small Craft Advisory and
High Surf advisory remain in effect for the exposed waters and
coastal areas through tonight. If the timing of this event lines up
with the latest guidance, surf should drop below advisory-levels by
Saturday as this swell steadily eases. This downward trend is
expected to continue over the weekend.

In addition to the large swell and surf, advisory-level winds are
expected over portions of the marine area through the weekend due to
high pressure building far north of the state in the wake of a
dissipating front approaching from the north. The strongest winds
will remain over the typically windier channel waters each day.
Winds may near gale levels over these windier areas by the mid-week
period as strong high pressure builds north of the region in the
wake of another front passing well north of the state.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Saturday for north and west
facing shores of Niihau-Kauai-Molokai and north facing shores of
Oahu-Maui-Big Island.

Small Craft Advisory until 6 PM HST Sunday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

Small Craft Advisory until 6 PM HST Saturday 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-Big Island
Windward Waters.

&&

DISCUSSION...Jacobson
AVIATION...Bedal
MARINE...Gibbs