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Hawaiian Islands Synoptic Discussion and Guidance

329
FXHW60 PHFO 111320
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
320 AM HST Mon Dec 11 2017

.SYNOPSIS...
A cold front located around 500 miles northwest of Kauai will
approach the state through the day today, then move down the
island chain Tuesday morning through Wednesday. Winds will remain
light today, then shift out of the southwest and strengthen
tonight ahead of the front. Breezy north winds and relatively
cooler conditions will fill in behind the front Tuesday through
Thursday, with unsettled weather possible from Friday into the
upcoming weekend.

&&

.DISCUSSION...
Satellite imagery and observations showed a band of clouds
associated with a dissipated front that had stalled over the state
Sunday slowly retreating northward. Showers within this band
remain light with the best coverage located over the adjacent
waters from Oahu to Kauai. This cloud band will continue to lift
northward as a shortwave trough and its attendant cold front
approach the region from the north. Ahead of this front, winds
will remain light with the background flow shifting from the
south-southeast this morning to the southwest later today into
tonight, likely giving way to sea breezes this afternoon.

The latest hi-res model guidance remains in good agreement tonight
through midweek and brings this next cold front over Kauai early
Tuesday, then east of the state by Wednesday night. A better
source of moisture associated with this next boundary combined
with upper height falls and isentropic ascent will likely
translate to decent rainfall coverage across the state. Model
reflectivities depict pre-frontal showers developing over the
northwest islands later today through tonight, followed by the
main band of showers along the front that should reach Kauai
Tuesday morning, Oahu through the early afternoon hours, then the
rest of the islands Tuesday night into Wednesday. Another shot of
cool and dry air will fill in (PWs down toward half of an inch;
dewpoints back into the mid/upper 50s) behind this front Tuesday
through Wednesday. Winds will shift out of the north-northeast and
become breezy Wednesday through Thursday.

For the extended period, the latest GFS/ECMWF/CMC model cycles
continue to indicate that the shortwave will develop into a
cutoff low just E of the islands on Thursday, and then drift W
over the islands Friday through the weekend. If this scenario
evolves, island weather would become very active, with the
potential for heavy rain and thunderstorms, as well as snowfall
and high winds at the Big Island summits. However, some of the
ensemble solutions depict the upper low remaining centered east of
the islands, which would correspond to a drier solution. Provided
these differences, no significant changes are introduced in this
morning package through this extended period.

&&

.AVIATION...
Cloud and shower bands are drifting northwestward over Oahu and
Kauai this morning. These low cloud and showers will bring brief
periods of MVFR conditions to Kauai and Oahu this morning. The
next cold front will move in from the Northwest reaching Kauai by
Tuesday morning.

VFR will prevail at the terminals today. No AIRMETs in effect
with none expected until the next cold front arrival on Tuesday
morning.

&&

.MARINE...
The current north-northwest swell peaked across the coastal
waters during the afternoon and early evening hours on Monday,
and has been slowly declining overnight. Upstream buoys and wave
guidance show that this decline will continue today. The high surf
warning has been dropped and replaced by a high surf advisory for
north and west facing shores of the smaller islands, while the
high surf advisory continues for the north facing shores of the
Big Island. We expect surf to drop below advisory levels by late
this afternoon.

This swell continues to produce seas at or above 10 feet across
some windward waters, and the Small Craft Advisory (SCA) has been
extended through today for the Maui County and Big Island windward
zones. Seas should drop below 10 feet in those areas during the
afternoon.

The next north-northwest swell is expected to arrive Tuesday,
then peak Tuesday night into Wednesday. This swell is forecast to
be even larger than the current swell, and is expected to produce
warning- level surf on north and west facing shores of the smaller
islands. Surf on the north facing shores of the Big Island will
probably reach advisory levels in some areas, but with a lot of
local variations due to the expected 330 degree direction of this
swell. This swell will also bring seas back above 10 feet across
most of the marine area, beginning as early as midday Tuesday for
the far northwest waters, and spreading across the rest of the
area Tuesday night into Wednesday morning.

The next cold front is forecast to move down the island chain
Tuesday through Wednesday morning, with northerly winds
increasing behind the front. Winds behind this front will likely
reach SCA levels across much of the area by Wednesday afternoon or
Wednesday night.

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 this evening for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Molokai-Maui Windward West-Maui Central Valley-Windward
Haleakala-Big Island North and East.

Small Craft Advisory until 6 PM HST this evening for Maui County
Windward Waters-Big Island Windward Waters.

&&

$$

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

Bulletins, Forecasts and Observations are courtesy of Honolulu National Weather Service Forecast Office