High Surf in Hawaii, December 23 to 25, 2017

Area Forecast Discussion
National Weather Service Honolulu HI
855 PM HST Sat Dec 23 2017

.SYNOPSIS...
Light to moderate northeasterly trade winds will veer to the east on
Christmas Day, delivering just a few showers to windward slopes and
coasts. A developing low will pass close to the islands by midweek,
with increasing southerly winds bringing an increased chance of
showers Tuesday and Wednesday. A cold front associated with the low
is expected to bring showers through Thursday. North to northeast
winds are expected Friday and Saturday, delivering a few windward
showers.

&&

.DISCUSSION...
A cool, dry and stable air mass prevails over the islands this
Christmas Eve Eve, with PWAT below an inch, a subsidence inversion
based between 4000' and 6000', and low-level dew points near 60. In
this regime, the broken to overcast stable low clouds streaming
toward the islands from the NE are barely able to produce measurable
precipitation. Although radar shows some light returns over windward
waters this evening, very little rainfall is being reported by the
rain gage network, and a recently-issued forecast update reduced
overnight shower chances.

Little significant change in island weather is expected the next 2
days, with light to moderate NE winds on Sunday veering to the E by
Christmas Day as surface high pressure passes quickly N of the area.
Moisture associated with a cold front about 425 miles NW of Kauai is
expected to be pushed S toward Kauai, with latest GFS guidance
stalling this moisture just N of Kauai early Monday. Otherwise mid-
level ridging and a relatively dry air mass will keep windward
showers at a minimum.

Forecast guidance continues to indicate a potent shortwave
sharpening as it moves toward the islands from the NW Monday night
and Tuesday, leading to the development of a surface low that is
forecast to pass several hundred miles N of Kauai around Wednesday.
Increasing S to SW winds are expected Monday night and Tuesday as
the low approaches, with this convergent low-level flow bringing
showers over the islands from the SW. The low center will then track
N of the islands on Wednesday, dragging a cold front with enhanced
showers across the state through Thursday. If current model runs
were to verify, locally gusty winds, heavy showers and a few
thunderstorms would occur from late Tuesday through Wednesday, but
forecast confidence is not sufficiently high to include them in the
forecast at this time.

A new high building far NW of the area may bring a drying trend, and
increasing N to NE winds Thursday and Friday. The very low
confidence forecast for New Year's Eve/Day continues to feature a
fairly typical trade wind flow.

&&

.AVIATION...
We are looking at a predominately VFR situation for the overnight
hours, and through mid Sunday morning. This is due to a relatively
dry and stable air mass, as noted by the afternoon soundings from
LIH and HILO. A ridge of high pressure located north of the islands
is providing mainly light northeast winds across the area.

Infrared satellite imagery shows a south to north oriented band of a
mix of stratocumulus (SC) and cumulus clouds just about to make
landfall along Maui's windward coast. Expect some brief periods of
MVFR ceilings, and perhaps a few light showers from these clouds.
The band is a couple of hundred miles long, so a high based ceiling
can be expected across Maui through most of the night. More than
likely, these clouds will spread across Lanai, and at least the
eastern half of Molokai. Satellite imagery is showing an expanding
patch of SC clouds along Oahu's windward shoreline. Some of these
clouds will spill downwind into the lee areas of Oahu in the coming
hours along with some light drizzle. The surface observation from
Kaneohe MCBH last hour indicated a cloud base at 4k feet. Elsewhere,
Kauai and its adjacent waters are mainly fair this hour. A land
breeze has developed there, which will keep the land relatively
clear of the low clouds and limiting them to along the coast. The
same goes to the windward shores of the Big Island where some low
clouds are present right off the coast, but a land breeze will keep
them where it is. Most of these clouds will burn off after sunrise.

No AIRMETs in effect, and none are expected overnight.

&&

.MARINE...
A Small Craft Advisory (SCA) remains in effect primarily as a
result of the incoming north-northwest swell which is producing
combined seas to 10 feet. The SCA is in effect for the waters
exposed to the north-northwest swell. The swell appears to be
peaking at buoy 51101 to the northwest of the islands, and we are
expecting it to begin to trend down tonight at the buoy, which
should be reflected in the local waters tomorrow. Currently the
SCA goes through the day Sunday. Seas may linger into the evening
hours of Sunday in the far eastern waters near the Big Island.

A High Surf Advisory remains in effect for north and west facing
shores of Niihau and Kauai and the north facing shores of Oahu,
Molokai, Maui and the Big Island through Sunday afternoon. With
the swell peaking at buoy 51101, anticipating the peak near Kauai
late tonight, and during the morning hours over the remaining
islands. Expect the swell to then gradually subsiding through
Monday.

A reinforcing north-northwest swell, from a low tracking north of
the islands, should build Sunday and peak Monday. Surf may near
the advisory level again Monday, before the swell turns more out
of the north Tuesday.

The longer term swell forecast, beyond Wednesday, is more
uncertain, but model runs tend to place a near gale force low
just north of the state. This would produce a large short period
north swell building late Wednesday.

South shores should see a welcomed bump in surf the beginning of
next week as small long period south-southwest swells are
expected.

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

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Sunday for North and West
facing shores of Niihau and Kauai, and for North facing
shores of Oahu, Molokai, Maui and the Big Island.

Small Craft Advisory until 6 PM HST Sunday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters-Kaiwi Channel-Maui County Windward Waters-
Alenuihaha Channel-Big Island Windward Waters.

&&

DISCUSSION...Birchard
AVIATION...Lau
Marine...M Ballard