High Surf in Hawaii, December 1 to 2, 2019

Area Forecast Discussion
National Weather Service Honolulu HI
321 AM HST Sun Dec 1 2019

.SYNOPSIS...
Windy conditions are expected across most of the state today. Wet
weather this morning, will give way to a decrease in showers
across the smaller islands this afternoon. Showers will then begin
to diminish this evening across the Big Island. The trades will
ease tonight and Monday, with more typical trade wind weather
returning, featuring mainly windward and mauka showers and the
occasional leeward spillover. Moderate to locally breezy
conditions are then expected through the remainder of the work
week, with a continuation of mainly windward and mauka showers and
the occasional leeward spillover.

&&

.DISCUSSION...
Currently at the surface, a 1032 mb high centered around 1100
miles north-northwest of Honolulu, is driving breezy to windy
trades across the island chain early this morning. Infrared
satellite imagery shows mostly cloudy skies across the state.
Meanwhile, radar imagery shows numerous showers across Kauai,
Oahu and Maui County, with showers beginning to approach the Big
Island, all in association with an old front. Main short term
concerns revolve around trade wind and rainfall trends through
tonight.

Today and tonight,
The old frontal remnants will continue to sink steadily southward
through the islands today, bringing windy and wet weather to the
entire island chain. Showers will be most frequent over windward
and mauka areas, but quite a bit of shower activity will affect
leeward locales as well. The brunt of the shower activity should
occur before noon today across the smaller islands, with drier
air and less shower activity working in for the afternoon. The
drier air will likely take until this evening to work into the
Big Island however. Fairly typical trade wind weather should then
prevail behind the frontal remnants, with mainly windward and
mauka showers and the occasional leeward spillover. Based on an
overnight ASCAT pass showing open ocean winds of 25 to 30+ knots
along and behind the old frontal remnants as well as observational
data reaching advisory levels, the Wind Advisory has been
extended through 6 PM this evening and expanded to include most of
the state. The trades are expected to ease by tonight, but breezy
conditions will likely continue.

Monday through next Saturday,
High pressure north of the state will get reinforced by a new
and stronger high building to the northwest of the islands Monday
through Wednesday. This new high will then gradually weaken as it
shifts southeastward, well to the north and northeast of the
islands through the rest of the week. Moderate to locally breezy
trades should prevail through the period, although some minor
fluctuations in speed can be expected. As for weather details, a
fairly typical trade wind pattern should prevail through the work
week and into next weekend, featuring windward and mauka showers
and the occasional leeward spillover. There could be a bit of an
increase in shower coverage and intensity Monday night and
Tuesday, and again Thursday night through Friday night as upper
level disturbances move overhead and give a boost to the inversion
heights.

&&

.AVIATION...
A band of clouds and showers associated with an old frontal
boundary will continue to move through from north to south
today. Windy and showery conditions will continue through this
time. Expect periodic MVFR conditions in low clouds/showers,
especially along the windward slopes. AIRMET SIERRA is now in
effect for north thru east sections of Kauai, Oahu, Molokai and
Maui above 1000 feet due to mountain obscuration from low clouds
and showers. This will be updated later this morning (around
daybreak) to include the Big Island as this band of clouds and
showers continues southward.

AIRMET TANGO remains in effect over and immediately south thru
west of mountains on all islands below 9000 feet due to tempo
moderate turbulence.

Although conditions should gradually improve through the day
today (especially over the western end of the state) as this band
of clouds and showers continues southward, strong trades will
hold as high pressure builds to the north in its wake.

&&

.MARINE...
Strong surface high pressure is centered far north-northwest of
the state, while a surface low pressure system is located far
northeast of the islands. The strong northerly flow between these
features continues to push a remnant frontal boundary down over
the coastal waters early this morning. As a result, strong
northeast to east trade winds, along with elevated, rough seas are
expected across all waters. The winds will likely reach gale
force over some of the windier waters. Therefore, a Gale Warning
has been issued through this afternoon for the Alenuihaha and
Pailolo Channels and Maalaea Bay. A Small Craft Advisory (SCA)
remains in effect through tonight for all remaining Hawaiian
waters. The latest guidance suggests winds will weaken somewhat
by early Monday. The SCA will likely need to be extended into the
daytime hours Monday. Winds should continue to gradually weaken
from Monday night through Tuesday as the high weakens north of
the area due to a passing front. Seas will remain elevated
through Monday, before slowly subsiding from Monday night through
mid-week.

The captured fetch associated with the low pressure system,
combined with a broad area of strong northerly flow between the
high and the low has sent a large, long-period north swell into
the state. In addition to this swell, wind waves generated
locally by the strong trade winds and a large, short-period
northeast swell will produce elevated, rough surf along north and
east facing shores into Monday. The significant wave heights
recently observed at the the PacIOOS buoys over the windward
waters are 13 to 14 feet with a peak wave period of around 16
seconds (the average wave period is 10-11 seconds). The NDBC buoy
51000 located north of the islands is showing a swell height of
12 feet with a wave period of 16 seconds (the significant wave
heights are 14 to 15 feet). Based on these observations, which
appear to be about 2 feet above the Wave Watch III Model
guidance, surf may approach the High Surf Warning criteria along
some north facing shores later today. In addition, the rough surf
will be elevated along east facing shores. As a result, a High
Surf Advisory (HSA) remains in effect through early Monday
morning for north and east facing shores of Kauai, Niihau, Oahu,
Molokai, Maui, and Big Island. The HSA will likely need to be
extended through the day for most of these same shores on Monday.

The large north and northeast swells will likely produce significant
harbor surges and very large breaking waves near the entrances at
north facing harbors, including Hilo and Kahului Harbors into Monday.
The harbor surges in Hilo Harbor may also cause water to cover
portions of Bayfront Highway. A Marine Weather Statement has been
issued to alert mariners using these north facing harbors about
these impacts. In addition, the large breaking waves and wave
runup may also result in overwash and beach erosion along some
vulnerable shore lines.

South facing shores will continue to receive small, mainly
background south swells today, with a slightly larger, long-period
south swell expected from tonight through Tuesday. Some exposed
south facing shores may receive wrap from the large trade wind
generated waves into early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 PM HST this evening for Niihau-Kauai-Oahu-
Molokai-Lanai-Kahoolawe-Maui Windward West-Maui Leeward West-
Maui Central Valley-Windward Haleakala-Leeward Haleakala-Big
Island North and East-Kohala.

High Surf Advisory until 6 AM HST Monday for Niihau-Kauai
Windward-Kauai Leeward-Oahu North Shore-Oahu Koolau-Olomana-
Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala-South Big Island-Big Island North and East.

Small Craft Advisory until 6 AM HST Monday 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-Big Island Leeward Waters-Big Island Southeast
Waters.

Gale Warning until 6 PM HST this evening for Maalaea Bay-Pailolo
Channel-Alenuihaha Channel.

&&

DISCUSSION...Jelsema
AVIATION...Gibbs
MARINE...Houston