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Hawaiian Islands Synoptic Discussion and Guidance

865
FXHW60 PHFO 091346
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
346 AM HST Mon Dec 9 2019

.SYNOPSIS...
Light easterly winds will continue through the first half of the
week as a front stalls and diminishes just north of Kauai.
Outside of a few clouds and showers setting up over interior areas
each afternoon due to sea breezes, mostly dry and stable
conditions are anticipated. Increasing moisture associated with
another front that is forecast to move into the area will bring
increasing rainfall chances and the potential for gusty northeast
winds by the end of the week.

&&

.DISCUSSION...
Water vapor imagery showed a ridge extending eastward across the
region and a potent shortwave trough far northeast of the state
driving a cold front toward the west coast. The surface analysis
showed the tail-end of this frontal boundary beginning to stretch
out from east to west north of Kauai and a weak ridge extending west-
southwestward over the islands. Although mostly dry conditions
remain in place, some moisture is beginning to bleed out ahead of
this boundary near Kauai, which could be enough to trigger a few
showers over the adjacent coastal waters of Kauai through the
early morning hours today. Elsewhere, mostly dry and clear
conditions will prevail due to the drainage flow associated with
the early morning land breezes. The exception is over the
southeast Big island waters, where isolated to scattered showers
have begun to develop due to a passing trough to the south.

The latest short-term (through Tuesday) guidance depicts the
aforementioned frontal boundary stalling just north of Kauai today,
before retreating northward and diminishing tonight through
Tuesday. As this scenario materializes, a return of light trades
is expected over the eastern end of the state by Tuesday as the
ridge axis shifts back to the north. The land and sea breeze
regime, however, remains likely over the western end due to the
proximity of this ridge axis. Although dry conditions will
prevail, a few clouds and showers setting up over interior areas
can't be ruled out each afternoon due to the sea breezes,
especially over Kauai where the better source of moisture remains.

For the extended period (Wednesday through the weekend), guidance
depicts lowering upper heights associated with a strong short
wave trough approaching and moving into the area Wednesday through
Friday. Although it remains too far out to determine any
specifics due to persistent model differences, increasing moisture
and rainfall chances along with gusty northeast winds remain a
possibility Wednesday night through Friday as the attendant cold
front approaches and moves into the area. Just how far down the
island chain and how strong this boundary will be upon arrival
will continue to be the main forecast challenge this week.

&&

.AVIATION...
The light trade wind pattern continues. Expect sea breezes today
and land breezes tonight. This forecast period will be relatively
dry again through this afternoon. Any showers we have will be
isolated. There may be brief periods of MVFR conditions against
the slopes but otherwise expect VFR conditions across most of the
state.

There are no AIRMETs currently in effect or expected.

&&

.MARINE...
There are no warnings or advisories in effect for the Hawaii
coastal or offshore waters today. A ridge north of the area will
maintain light to moderate winds over the area through tomorrow night.
Winds will strengthen starting Wednesday as a front moves over
the area from the northwest and a strong high builds behind the
front.

A new northwest swell now spreading into the area is forecast to
product surf above the advisory threshold along north and west
facing shores starting this evening and continuing through
Wednesday morning. As this swell subsides on Wednesday, a new
very large northwest swell is expected. Surf from that swell will
produce surf well above warning levels along north and west facing
shores of the smaller islands. This swell may be large enough to
also produce advisory level surf along west facing shores of the
Big Island of Hawaii. The swell is expected to gradually subside
through Friday night


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

DISCUSSION...Gibbs
AVIATION...Chevalier
MARINE...Donaldson

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