High Surf in Hawaii, December 16 to 19, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
815 PM HST Sat Dec 15 2018

.SYNOPSIS...
Dry and pleasant conditions associated with high pressure will hold
into the start of the upcoming week. Trade winds will gradually
trend down Sunday through Monday, potentially becoming light enough
for land and sea breeze conditions for a brief period late Monday
through early Tuesday for some portions of the state as a weak cold
front approaches and moves into the area. The front is forecast to
quickly advance down the island chain late Tuesday through
Wednesday, bringing increasing rain chances. Gusty northerly winds
along with drier and cooler conditions will follow Wednesday through
Thursday. A return of a more typical trade wind pattern will be
possible Friday into next weekend.

&&

.DISCUSSION...
Short-term (through Monday night) guidance remains in good agreement
and reflects the dry pattern holding through Monday due to deep
layer ridging and a strong subsidence inversion. Precipitable water
values are forecast to range from three quarters of an inch to
around an inch, which is below average (drier) for this time of
year. After a long stretch of breezy to windy trades, the pressure
gradient is forecast to relax over the state as a weak cold front
and upper trough approach from the northwest. Trades will respond
and gradually trend down, potentially becoming weak enough for
localized land/sea breezes late Monday through Monday night.

For the extended (Tuesday through Friday), guidance remains in
decent agreement through midweek and shows the aforementioned cold
front advancing down the island chain beginning late Tuesday for
Kauai, then clearing the Big Island by the end of the day Wednesday.
A slight increase in moisture (albeit shallow) combined with this
boundary and upper trough will support increasing rain chances,
especially late Tuesday through Wednesday, as the boundary moves
down the island chain. Breezy northerly winds accompany this
boundary along with much drier conditions following its passage
Tuesday night through Wednesday. Dewpoints are forecast to dip into
the upper 50s for the smaller northwest islands. High temperatures
may struggle to reach the 80 deg mark Wednesday afternoon. The dry
and pleasant conditions will likely hold into the second half of the
week with a return of a more typical trade wind pattern.

Winds will remain elevated and around the advisory criteria through
the night for the summits on the Big Island. A gradual down trend is
anticipated through the day Sunday.

&&

.AVIATION...
A strong ridge of high pressure ridge extending north of the
Hawaiian islands will continue to generate moderate, mostly dry
trade winds across the state. Shower activity should be limited to
windward slopes and coasts. Isolted MVFR conditions possible in
passing showers, otherwise VFR conditions are expected to prevail.

AIRMET Tango remains in effect for TEMPO moderate turbulence
southwest of all island mountain chains.

&&

.MARINE...
Moderate to locally strong trade winds will continue through
Sunday night with high pressure to the north of the state. A front
approaching from the northwest will shift the high further to the
east, which will decrease the trades some for Monday. The front
is expected to begin impacting the northwest waters around Kauai
on Tuesday, and pushing down the island chain reaching the waters
around Big Island Tuesday night. Strong north winds are expected
with and behind the frontal passage. High pressure is expected to
return northeast of the state for the end of the week.

The new long-period northwest swell has reached 51101 to the
northwest of the islands, but several hours delayed from the model
depictions. It should take about 8 hours for that energy to reach
the main Hawaiian Islands, so have adjusted the Swell, Period and
WaveHeight grids to account for the delayed arrival. Surf is
expected to rise overnight and spread southeast across the
Hawaiian Islands. A High Surf Warning for north and west facing
shores of Niihau, Kauai, Oahu, Molokai, and north facing shores of
Maui and a High Surf Advisory for north facing shores of the Big
Island is in effect through Sunday night.

A Small Craft Advisory (SCA) is in effect for all Hawaiian waters
mostly due to the seas exceeding 10 feet, however the typically
windier areas near Maui County and the Big Island continue to see
winds reaching 25 kts.

This northwest swell expected to peak Sunday or Sunday night, and
then subside some on Monday. An even larger northwest swell is
expected to build late Monday, peaking well above warning levels
Tuesday night. Exposed coastal waters will once again rise to over
10 feet as these swells come rolling through. Expect the current
SCA to once again be expanded to most, if not all the coastal
waters for the first half of the new week as the new swell
arrives.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 AM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Surf Advisory from midnight tonight to 6 AM HST Monday for
Big Island North and East.

Wind Advisory until noon HST Sunday for Big Island Summits.

Small Craft Advisory until 6 AM HST Monday for all Hawaiian
waters-

&&

DISCUSSION...Gibbs
AVIATION...Bedal
MARINE...Ballard