Area Forecast Discussion
Issued by NWS Honolulu, HI

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FXHW60 PHFO 112026
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1026 AM HST Mon Dec 11 2017

.SYNOPSIS...
A cold front will move down the island chain Tuesday morning
through Wednesday. Behind the front, breezy north winds bringing
cooler and drier conditions are expected through Thursday. A
moist airmass may move back across the state from the east over
the weekend.

&&

.DISCUSSION...
A broad trough positioned far to the north of the state is keeping
the forecast area under a weak surface ridge today. Upper air
soundings from Lihue and Hilo confirm that the state is under
drier than normal airmass with a stable layer limiting any clouds
and showers. The winds will remain light, however increasing
southwest flow will start to show up this afternoon for especially
the western half of the state as a new cold front approaches from
the northwest. Only minor updates to the temperature, pop, and
wind grid forecast for the daytime period to account for current
observation trends. Have pushed the arrival of areas of vog in
the smaller islands to later this afternoon or evening.

The short term outlook calls for the cold front to move down
across the islands chain. The front reaches Kauai Tuesday morning,
Oahu Tuesday afternoon, and Maui Tuesday evening. The front
eventually pushes across the Big Island on Wednesday. Along the
front there will be clouds, with the threat of rain showers,
although the moisture and dynamics do not look sufficient for a
widespread rain threat associated directly with the front. As the
winds shift to the north and northeast behind the front, there may
be a brief period for upslope orographic showers on the windward
sides, but the airmass will dry quickly behind the front, so any
rainfall should be moderate to light.

For Wednesday and Thursday, a progressive surface high will pass
by to the north of the state giving us a period gusty
northeasterly trades. It stays quite dry and stable, so windward
and mountain showers will be quite limited.

For the extended period, the latest GFS/ECMWF model guidance
continues to generally indicate that the upper level trough will
sharpen and develop into a cutoff low just E of the islands on
Friday, and then drift W over the islands through the weekend.
Depending on how the situation evolves, there is a chance of very
wet and unsettled weather this weekend. However, the latest model
runs suggest that the main moist and unstable air will remain east
and north of the state as the low moves west. Will continue to
monitor and trend the forecast as guidance evolves.

&&

.AVIATION...
AIRMET TANGO for moderate turbulence aloft is currently in
effect. We expect to cancel the AIRMET by this afternoon. More
turbulence aloft is possible Tuesday as a trough aloft digs SE
toward the main Hawaiian islands.

An old frontal cloud band over the area has dissipated, a surface
ridge has built over the area, and VFR conditions now prevail
across the islands.

Low level flow over the area will rapidly veer out of the south
ahead of a front approaching from the northwest. As the front
approaches, remnants of the old front will become organized into
a NE-SW convergence band over the Kauai channel. Clouds in this
band could produce enough MVFR ceilings over Kauai or Oahu to
require an AIRMET for MTN OBSC by this evening.

The front is expected to reach Kauai Tuesday morning and move
down to the Big Island by Wednesday afternoon. There will be
widespread MVFR and localized IFR ceilings and visibility along
the front.

&&

.MARINE...
The current north-northwest swell continues to decline. Buoy 51101
has shown a gradual trend over the last 24 hours, while some of
the nearshore buoys have shown a more stepped down trend. The Hilo
Bay buoy has been holding steady for the last couple of hours, but
we do anticipate it to trend downward through the day. A high surf
advisory continues today for north and west facing shores of the
smaller islands, and the north facing shores of the Big Island.

This swell continues to produce seas at or above 10 feet across
some windward waters near Maui and the Big Island, and so the
Small Craft Advisory (SCA) continues for those areas through this
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 will 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.


&&

$$

Discussion...Brenchley
Aviation...Donaldson
Marine...M Ballard



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