High Surf in Hawaii, February 2 to 5, 2018

Area Forecast Discussion
National Weather Service Honolulu HI
308 AM HST Fri Feb 2 2018

.SYNOPSIS...
Warm and muggy conditions with light to moderate southerly winds
will hold in place through the weekend as a broad upper trough and
front remain positioned west of the state. Periods of heavy showers
will remain possible each day, especially over the smaller northwest
islands. A strong upper trough and cold front will approach and
slowly move down the island chain Sunday night through midweek
bringing widespread showers and the potential for thunderstorms.
Drier air and northerly winds will fill in across the state behind
this front as a ridge of high pressure builds north of the state.

&&

.DISCUSSION...
Recently animated water vapor loop showed a broad upper trough
positioned northwest of the state over the central Pacific with
several shortwave troughs rotating through its base. The bulk of the
middle- to upper-level moisture, however, was shown north of the
islands within the deep southerly flow east of this upper trough and
mostly dry conditions aloft over Hawaii. Despite the drier air
aloft, satellite-derived precipitable water (PW) imagery depicted
well above average values (1.5-1.8") for this time of year with a
large plume of tropical moisture being drawn northward across the
region within deep southerly flow. The latest surface analysis
showed a storm-force low associated with the upper trough centered
well northeast of Midway and a cold front extending southwestward to
an area around 200 miles west of Kauai.

Today through the weekend, guidance has initialized well with the
current conditions and indicates no significant pattern changes. A
plume of high PWs will remain positioned over the smaller northwest
islands within the deep southerly flow ahead of the aforementioned
front and upper trough to the west. Warm and muggy conditions with
dewpoints remaining in the upper 60s to lower 70s will continue.
Bands of showers, some heavy at times, will develop and lift
northward over the smaller northwest islands periodically. Any band
that becomes focused and trains over any particular area could
translate to flash flooding. Guidance suggests the mostly likely
islands to be impacted will include: Niihau, Kauai and Oahu. As a
result, the flash flood watch remains in effect today and may need
to be extended. Additionally, a few thunderstorms can't be ruled
out over these areas this morning.

Sunday night through Wednesday, guidance remains in good agreement
and depicts a strong shortwave trough digging southeastward driving
a cold front down the island chain beginning Sunday night. Falling
upper heights associated with the shortwave trough, a sufficient
amount of deep layer shear (30-40 kt) and moisture will support
another active period with heavy showers and thunderstorms
developing and advancing down the island chain Monday through
Tuesday. In addition to the active weather associated with this
front, deep moisture, lift and cold temperatures aloft will likely
support a wintery mix at the Big Island Summits Monday night through
Tuesday. Drier air and northerly winds will fill in across the state
behind this front as a ridge of high pressure builds north of the
state.

&&

.AVIATION...
Moist southerly flow will continue across the state today and
tonight, ahead of a stalled front located to our northwest. MVFR
ceilings and mountain obscuration remain quite extensive across
the western half of the state early this morning, and AIRMET
Sierra for mountain obscuration is currently posted for Kauai,
Oahu, Molokai and Lanai. With moist low levels in place, mountain
obscuration will likely remain a concern for at least western
parts of the state through today. Isolated IFR conditions in
heavier showers and a few thunderstorms will also be possible
across Kauai, Oahu, Molokai and Lanai through today. VFR
conditions should prevail across Maui and the Big Island, with
just isolated showers and/or MVFR ceilings possible.

&&

.MARINE...
A High Surf Advisory (HSA) is in effect through tonight for most
north and west facing shores on the smaller islands. The current
northwest swell has been subsiding after peaking around noon
yesterday. Surf may drop below advisory levels today, but an new
large northwest swell will be building today. The new swell will
be larger, with a longer period, and a more westerly direction.
Surf may build above warning levels on north and west facing
shores of the smaller islands tonight, and the swell will likely
reach the Kona coast of the Big Island. Surf will likely remain
above the advisory threshold through Monday. The persistent north
and east swells are both slowly subsiding and will fade out
overnight.

A Small Craft Advisory (SCA) is in effect for most coastal
waters. A large northwest swell has been producing combined seas
above the 10 foot SCA threshold in many areas. The new northwest
swell will spread southeast over the coastal waters this evening.
That swell will boost the seas even higher. A trough to the west
and a ridge to the east will combine to produce southerly winds
over the Hawaiian coastal waters through the weekend, generally
from the southeast toward the Big Island and from the south or
southwest toward Kauai County. The southeast winds are reaching
the 25 knot SCA threshold in some areas around the Big Island.
The southeast winds blowing over the northwest swell may make for
steeper waves and rough seas.

A deep trough aloft is making the atmosphere quite unstable, so
thunderstorms are expected over the coastal and offshore waters
through the weekend. Some of these thunderstorms could produce
dangerous lightning, strong winds and small hail. Marine Weather
Statements or Special Marine Warnings may be needed at times.

A strong new front is expected to move into the offshore waters
from the northwest Sunday and to reach the coastal waters Sunday
night. Winds will rapidly shift from southwest ahead of the front
to northwest behind the front. The front is forecast to stall over
the central coastal waters and weaken to a trough early next week.

See the latest Collaborative Surf Discussion for more information
on swells and surf.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch through this afternoon for Niihau-Kauai-Oahu.

High Surf Advisory until 6 AM HST Saturday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala.

High Wind Warning until noon HST today for Big Island Summits.

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

&&

DISCUSSION...Gibbs
AVIATION...Jacobson
MARINE...Donaldson