High Surf in Hawaii, December 9 to 14, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
831 PM HST Sat Dec 9 2017

.SYNOPSIS...
A weak cold front will continue to advance eastward down the island
chain through the overnight hours, then stall and dissipate over the
central islands Sunday. Lighter winds will start to turn southerly
on Monday ahead of a stronger front, which is expected to cross the
state from Tuesday into Wednesday. Breezy north winds will fill in
behind the front through Thursday, with unsettled weather possible
from Friday into the following weekend.

&&

.DISCUSSION...
Satellite imagery and observations showed a weak cold front moving
down the island chain with the bulk of its moisture remaining
confined from the surface to 8 KFT. The afternoon sounding at Lihue
reflected this low-level moisture below a strong inversion, topped
with an abundance of middle- to upper-level dry air (precipitable
water (PW) of 1.15 inches) prior to its arrival late this afternoon.
Radar imagery showed a narrow line of scattered showers along and
ahead of this boundary that had pushed south and east of Lihue early
this evening. The latest rainfall summary showed peak accumulations
associated with this activity up to a quarter of an inch over the
past three hours (3-6 pm HST) on Kauai. Expect similar accumulations
over Oahu this evening as it moves through, then possibly some
slightly higher amounts over Maui County and the northern/windward
slopes of the Big Island as the boundary stalls and slowly
dissipates later tonight through Sunday.

Although there is a significant amount of dry air behind the front
(MIMIC TPW values of 0.5-0.7 inches), it won't make much southward
progress across the islands Sunday into Monday. The trailing high
will quickly pass north of the state Sunday, with its surface ridge
expected to shift southward over us Sunday night/Monday. The weak
gradient will lead to another period of locally-driven land and sea
breezes, as the low-level background flow quickly veers from the
east Sunday night to the south and southwest Monday.

For the extended (Monday night-Friday) period, decent agreement is
now shown between the ECMWF and GFS solutions through midweek. They
depict the second, stronger, cold front reaching Kauai late Monday
night into Tuesday, then clearing the Big Island Wednesday through
Wednesday night. A deeper source of moisture along and ahead of this
boundary is shown with model PWs climbing above normal (1.5-1.7").
This moisture combined with upper height falls and isentropic lift
will translate to better rainfall coverage across the state,
especially along and ahead of the boundary as is advances down the
island chain. Guidance suggests this boundary pushing east of the
state with another shot cool and dry air filling in (PWs down
toward half of an inch; dewpoints back into the mid 50s) beginning
Tuesday night. Winds will shift out of the north-northeast and
become breezy Wednesday through Thursday.

Although forecast uncertainty remains high through the second half
of the upcoming week and next weekend, the GFS and ECMWF both
continue to depict a cutoff low developing just east-northeast of
the islands by Thursday as the upper pattern amplifies over the
region. While these deterministic solutions depict this upper low
retrograding or drifting westward over the islands Friday into the
weekend (more unsettled weather for the state), the ensemble fields
continue to suggest a large spread with its position and motion
through this time (some farther east giving way to a drier
forecast). Provided the run-to-run variability between model
solutions over the past couple of days, we will continue to monitor
later model cycles and refine the forecast over the coming days as
confidence rises.

&&

.AVIATION...
A weak and shallow cold front currently moving through Kauai will
continue to push low cloud ceilings and scattered showers down the
island chain overnight before stalling over Maui on Sunday
morning. Mountain obscuration AIRMETs may be needed as the frontal
band brings MVFR ceilings to each island. An AIRMET for mountain
obscuration remains in effect for Kauai this evening and will
likely be expanded to Oahu and possibly Molokai around 10Z
tonight.

On Sunday, MVFR ceilings and visibility in showers will be focused
over windward terrain of Maui County, and possibly Oahu. A stable
and moderate trade wind flow will produce VFR conditions
elsewhere.

&&

.MARINE...
The cold front appears to be south of Kauai and moving into Oahu
as of 8pm. This weak front should cross Oahu and Molokai overnight,
then stall near Maui on Sunday. NNE winds have increased behind
the front, but based on available land-based observations during
the last few hours it appears likely that these winds will remain
below 25 knots, even around Kauai and Oahu. Winds will quickly
veer around to the east and weaken on Sunday, then to the
southeast Sunday night as the dissipating front lifts back to the
north. Gentle to moderate S/SW winds are forecast Monday into
Tuesday as another, stronger front approaches. This front is
currently forecast to move down the island chain Tuesday through
Wednesday morning, with winds shifting to north and increasing
behind the front. N/NE winds will likely reach SCA levels across
most areas from Wednesday night or Thursday through Friday.

Seas will build rapidly tonight into Sunday morning as the next
NNW swell fills in and pushes seas back above 10 feet for many
areas. Therefore, a Small Craft Advisory is in effect for most
coastal waters zones through Sunday night. Seas should subside
below 10 feet again on Monday, but build rapidly once again late
Tuesday into Wednesday, as another even larger NNW swell arrives.

The NNW swell at Buoy 51101 is running slightly behind schedule at
the moment, but is still forecast to build quickly and peak
overnight. This swell will peak on Sunday around the islands,
producing warning-level surf for north and some west facing
shores of most smaller islands, before subsiding late Sunday night
and Monday. A High Surf Warning has been issued through Sunday
night for those areas. North facing shores of the Big Island will
be under a High Surf Advisory for Sunday and Sunday night. Warning-
level impacts are not expected on the Big Island due to the
330-340 swell direction, and the peak of the event occurring
closer to low tide. Tidal impacts are not expected to be an issue
as the predicted astronomical tides are lower than with the last
event.

Another even larger NNW swell is forecast to arrive Tuesday
afternoon, peaking Tuesday night into Wednesday. This swell is
expected to produce warning-level surf on north and west facing
shores.

See the latest Oahu Surf Discussion (SRDHFO) for more details on
the swell sources.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning from 10 PM this evening to 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 6 AM Sunday to 6 AM HST Monday for Big
Island North and East.

Small Craft Advisory from 10 PM this evening to 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-Alenuihaha Channel-Big Island Windward Waters.

&&

AVIATION...Bohlin
MARINE...Jacobson
REST OF DISCUSSION...Gibbs