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Hawaiian Islands Synoptic Discussion and Guidance

856
FXHW60 PHFO 101957
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
957 AM HST Tue Dec 10 2019

.SYNOPSIS...
Mostly dry weather and light winds will continue through Wednesday
as a stable airmass lies over the area. A front will be near Kauai
on Thursday with increasing shower coverage expected especially
over the west end of the state. Showers will be most numerous over
windward and mauka areas as trade winds strengthen due to high
pressure building in to our north. A bit drier weather is expected
over the weekend and the front weakens into a trough and begins to
move away from the area.

&&

.DISCUSSION...

High pressure lies far to the east-northeast of the area while a
ridge extends west-southwest from the high to a position just
north of Kauai. This places the islands in light easterly flow.
Winds are light enough allowing for local land and sea breeze
development. The northern periphery of an area of enhanced
moisture sliding by just south of the state has been providing for
some passing showers over southeast sections of the Big Island
while most of the remainder of the state is dry as a mid level
ridge is in place across the area. Expect generally dry weather
for most areas through the day with things expected to dry out
eventually over the southeast Big Island slopes.

On Wednesday, a broad upper level trough along with its associated
frontal system will be approaching the area from the northwest.
Winds will remain light continuing to favor daytime sea breezes
and nighttime land breezes. A few showers may develop during the
afternoon hours mainly over interior areas.

By Thursday, the front will be located just northwest of Kauai
with the upper trough digging down just west of the state. Winds
will remain on the lighter side favoring an easterly direction.
However they will be light enough for some continuation of local
land and sea breeze circulations. Shower coverage is expected to
increase over and around Kauai with some increase possible over
interior areas of the other islands.

The front is forecast to reach Kauai on Friday with shower
activity increasing especially over the west end of the state.
Meanwhile, strong high pressure will be building in northwest of
the state. This will result in strengthening trade winds
especially over the west end of the island chain. This wind
direction along with the front's moisture field, will cause
windward and mauka areas to receive the majority of the showers
but some will likely spill over to leeward areas as well.

The high will be passing by north of the area Friday night and
Saturday with moderate to locally breezy trade winds expected
across the entire area. Meanwhile the front will have weakened
into a trough with its moisture field beginning to shift westward.
Shower coverage will likely taper off but there will still be
passing windward and mauka showers at times with some leeward
spillover still a possibility on into Sunday.

Sunday night and Monday, a new high will be building in north-
northwest of the area. Moisture will the old front may get caught
up in increasing northeast flow with increasing shower activity
possible especially over windward and mauka areas.

&&

.AVIATION...
With a weak background flow, daytime sea breezes and nighttime
land breezes will develop over the state through Wednesday. VFR
will predominate over most areas most of the time.

This morning, the one area where MVFR exists is over southeast
portions of the Big Island as a weak trough passes south of the
area, bringing lower ceilings and showers. Thus, AIRMET Sierra
for mountain obscuration is in effect for the Big Island from Hilo
to South Cape.

&&

.MARINE...
Trade winds will remain at gentle to locally moderate strength
through tomorrow as a surface ridge holds about 200 nm north of
Kauai and a weak front falls apart about 100 miles north of the
Garden Isle. Another front will approach then stall just northwest
of Kauai on Thursday as strong high pressure builds to the
distant northwest. Trade winds will increase locally late
Thursday, and as the front is pushed down to near Kauai on Friday,
winds will build further. The front will weaken and move off to
the west by Saturday, and local winds will decline to moderate to
fresh strength as the high moves off to the northeast. Expect
stronger trade winds starting Sunday.

Northwest swell will hold around similar heights through tonight
then rise. NOAA NDBC buoys northwest of Kauai have been
registering the swell at 8 to 10 ft at 13 to 14 seconds since last
night, while local PacIOOS buoys have been measuring the swell at
7 to 8 ft. The next pulse of northwest swell will reach the
islands on Wednesday, then build into early Thursday. A High Surf
Advisory (HSA) is currently in effect for most north and west
facing shores from Kauai to Maui, and an upgrade to a High Surf
Warning will likely be needed for late Wednesday and Thursday.
North and west shore surf will drop below advisory levels late
Friday through the weekend. Another pulse of northwest swell may
produce advisory level surf early next week.

Surf along east facing shores will be remain below seasonal
average for the next few days, then pick up. Strong northeast
winds along the front that will stall near Kauai will likely send
large, short period swells to Kauai and Oahu, with east shore
surf dropping down to near or below seasonal average during the
weekend.

A Small Craft Advisory (SCA) will likely be needed on Wednesday
and Thursday as the building northwest swell pushes seas in most
areas above 10 ft. The SCA may need to be expanded for locally
strong trade winds Thursday night of Friday. Border line SCA
conditions are due on Saturday for both seas and local winds,
though stronger trades will probably require an SCA Sunday
onward.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST Wednesday for Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

&&

$$

DISCUSSION...Burke
AVIATION...Kinel
MARINE...Wroe

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