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Hawaiian Islands Synoptic Discussion and Guidance

874
FXHW60 PHFO 110150
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
350 PM 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 airmass over the area is dry and stable as
indicated by the 00Z soundings. Thus very little in the way of
rainfall is expected overnight.

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 associated with 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 from the east, nighttime land breezes
and daytime sea breezes will develop over the much of the state
through Wednesday. VFR will predominate over most areas most of
the time.

One area where MVFR still exists, though, is over southeast
portions of the Big Island, where lower ceilings and showers
persist, though they are on the decrease. Thus, AIRMET Sierra for
mountain obscuration remains in effect for the Big Island from
Apua Point to South Cape. However, the AIRMET will likely be
cancelled at the next issuance time of 0400Z/1800 HST.

&&

.MARINE...
Trade winds will remain at gentle to locally moderate strength
through tomorrow, with stronger winds expected to develop Thursday
night and Friday. A surface ridge currently sits about 225 nm
north of Kauai, while a weak front is falling apart about 125
miles north of the Garden Isle. Locally, little change is due
through tomorrow. 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
and the strong high passes to the north, winds should build
further, especially around Kauai. The front will weaken and
retreat 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 Sunday and Monday as another area of
high pressure passes far north of the state.

Northwest swell will hold around similar heights through tonight
then rise significantly on Wednesday. NDBC buoys northwest of
Kauai have been registering the swell at 9 to 11 ft at 13 to 14
seconds today, with some increase in energy within the 15 to 16
range noted recently, and local PacIOOS buoys have been measuring
the swell at 6 to 7 ft. Border line advisory surf is expected to
continue across most north and west facing shores from Kauai to
Maui tonight, followed by a significant rise in swell Wednesday
and Wednesday night. A High Surf Warning has been posted for this
rise, even though warning level surf is not expected until
afternoon on Kauai and Oahu and on Molokai and Maui until
evening. The large surf will peak early Thursday then decline
below the advisory level late Friday. 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 THursday night and
Friday, with east shore surf dropping down to near or below
seasonal average during the weekend.

A Small Craft Advisory (SCA) has been issued for hazardous seas
building tonight and Wednesday. The SCA covers waters around
Kauai and north of Oahu tonight, then expands to most waters
exposed to the northwest swell on Wednesday. The SCA may need to
be expanded for locally strong trade winds Thursday night or
Friday. Border line SCA conditions are due on Saturday for both
seas and local winds, though stronger trades will probably require
an SCA for the typically windy waters around the Big Island and
Maui Sunday onward.

&&

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

Small Craft Advisory from 6 AM Wednesday to 6 PM HST Thursday
for Oahu Leeward Waters-Kaiwi Channel-Maui County Windward
Waters-Big Island Windward Waters.

Small Craft Advisory from midnight tonight to 6 PM HST Thursday
for Kauai Northwest Waters-Kauai Windward Waters-Kauai Leeward
Waters-Kauai Channel-Oahu Windward Waters.

&&

$$

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

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



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