High Surf in Hawaii, December 2 to 6, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
922 AM HST Sat Dec 2 2017

.SYNOPSIS...
High pressure north of the state will keep breezy to locally
windy trades blowing through the weekend. Showers will continue
today and tonight, with gradual improvement down the island chain.
A cold front with a band of showers is expected to move down the
island chain Monday night and Tuesday followed by strong north to
northeast winds. A cooler and drier airmass is expected for the
middle and latter part of next week.

&&

.DISCUSSION...
Several changes to the forecast this morning for the short term,
between now and tomorrow morning.

The Wind Advisory has been expanded to include portions of Kauai,
Oahu and Maui. A few gages on Oahu have already experienced
advisory level gusts, and we expect winds to increase today. The
greatest impact will be along mountain ridges and through gaps.

A Flash Flood Watch has been posted for the Big Island through
tonight. Although rains have tapered off on the Big Island at
present time, more showers are expected today. An upper level
trough sinking southward will bring additional instability to the
Big Island late today and tonight. This will enhance showers,
particularly on the windward side of the island which has already
received substantial rainfall over the last several days. With the
ground already saturated in many places, additional rain could
quickly lead to flash flooding. The trough is expected to move
south of the Big Island by tomorrow morning, with drier air moving
in.

Together with the Flash Flood Watch, a Winter Weather Advisory has
been posted for the Big Island summits above 12,000 feet through
tonight. The above mentioned upper level trough will usher in
colder air at the summits, with 600 mb temperatures expected to
remain near freezing today. As the layered clouds over the smaller
islands today sink southward, and combine with instability of the
upper level trough, any precipitation that falls at the summits
will be in the form of snow.

The overnight surface analysis shows a dissipating high pressure
system about 1600 miles northeast of Oahu, and a high about 1000
miles to north northeast, moving to the northeast. In between
lies a surface low about 1000 miles to the northeast. This low
remains embedded in an upper level trough that is sinking
southward. The trough axis is near Kauai this morning, and the
models show the trough sinking southward over the next 24 hours,
reaching the Big Island today. As the trough sinks towards the Big
Island it is expected to deepen, which will bring additional
instability to the Big Island.

Moisture associated with these systems is currently over the
smaller islands. Showers have been on the decrease over Kauai as
the band has shifted to the central islands. The bulk of the
moisture should reach the Big Island tonight, as the upper level
trough deepens.

By tomorrow, the upper level trough is south of the Big Island,
and drier air will continue to spread southward down the island
chain. The satellite derived precipitable water (PW) shows the
drier air just north of Kauai this morning. The lower than normal PW
values are expected to persist through Monday.

The next front is expected to reach Kauai Monday night, bringing
PW values close to normal in the band of moisture along the front.
High pressure behind the front will usher in breezy, drier air.
The strongest winds will be felt on Tuesday behind the front for
the smaller islands. Dewpoints behind the front will drop into
the upper 50s, making it feel cooler, during the second half of
the week.

&&

.AVIATION...
AIRMET Sierra continues for tempo mountain obscurations along the
north through east sides of Kauai, Oahu, Molokai, Maui and the Big
Island as low clouds and showers continue to blow in with the
trade winds. The low clouds and showers are expected to diminish
tonight into Sunday as drier air moves in the from the east.

AIRMET Zulu for light to moderate rime icing is posted for all of
the smaller islands in layers 140-220 with mid level clouds
creating the icing hazard. This AIRMET may extend to the Big
Island later today as the cloud band progresses southeastward.

Strong trade winds continue to produce turbulence to the lee of
the mountains on all islands, and AIRMET Tango remains in effect
below 8000 feet.

&&

.MARINE...
Rough boating conditions will continue through the weekend due to
a combination of fresh to strong trades and a slowly fading
north-northwest swell. The small craft advisory has been extended
through Sunday. Model guidance suggests the local winds will
temporarily relax and shift out of a more northerly direction
Monday as a front approaches and moves into the area from the
northwest. Strong northerly winds, however, will quickly fill in
across the waters in the wake of this boundary Monday night
through Tuesday, then relax through midweek as a ridge builds
over the islands.

The north-northwest (330 deg) swell that peaked over the past
couple of days will continue to ease through the weekend. Surf
associated with this source along exposed north and west facing
shores will continue to trend down.

A new northwest swell (320 deg) is forecast to fill in Sunday and
Sunday night, then build into the upcoming week. This swell will
become reinforced from the north-northwest (340 deg) Tuesday
through midweek as a decent sized area of gales develop out of
the north trailing a cold front projected to move down the island
chain.

Surf along exposed north and west facing shores associated with
this active pattern could near advisory levels again Monday, then
potentially reach warning levels Tuesday through midweek as the
reinforcing northerly swell and strong northerly winds fill in
behind the front.

Large surf associated with this northerly swell coinciding with
strong northerly winds, peak monthly tides, and beach erosion
from recent large surf events could translate to some overwash
onto vulnerable roadways Monday night through Wednesday. The best
chance for overwash will be centered around the pre-dawn high
tide cycles Tuesday through Wednesday.

Surf along east facing shores will remain rough and elevated near
or slightly above advisory-level through the weekend due to the
strong trades expected. Additionally, a reinforcing northeast (050
deg) swell associated with low pressure developing well northeast
of the state will likely keep the surf elevated along east facing
shores through early next week as the winds shift out of the
north.

Surf along south facing shores will come up slightly through the
weekend due to low pressure near New Zealand last weekend. A
similar source will be possible by midweek.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Wind Advisory until 6 AM HST Sunday for Kauai Mountains-Waianae
Coast-Oahu Koolau-Lanai-Maui Windward West-Maui Leeward West-
Maui Central Valley-Big Island North and East-Kohala.

High Surf Advisory until 6 PM HST Sunday for Niihau-Kauai
Windward-Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-
Windward Haleakala-South Big Island-Big Island North and East.

Flash Flood Watch through late tonight for Big Island.

Winter Weather Advisory until 6 AM HST Sunday for Big Island
Summits.

Small Craft Advisory until 6 PM HST Sunday for all Hawaiian
waters-

&&

DISCUSSION...M Ballard
AVIATION...Foster
MARINE...TS