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Hawaiian Islands Synoptic Discussion and Guidance

018
FXHW60 PHFO 112015
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1015 AM HST Wed Dec 11 2019

.SYNOPSIS...
Light trade winds and mostly dry weather will prevail through
Thursday. A front approaching from the northwest will bring
increasing clouds and showers to Kauai late Thursday into the
weekend, while trade winds increase statewide. High pressure passing
far north of the islands will support locally breezy trade winds
from Friday through the weekend, and will deliver passing windward
showers.

&&

.DISCUSSION...
Light trade winds will favor an E-SE direction through Thursday,
while the island atmosphere remains mostly dry and stable, due to a
nearby deep-layer ridge extending over the area from the E. Skies
over most areas are currently sunny, except for some low clouds over
windward portions of Maui and the Big Island, and adjacent waters.
Expect some spotty afternoon clouds to bring a slight chance of
showers this afternoon as sea breezes overtake the light trades.
Land breezes in most areas are expected to clear skies overnight
once again, except for a slight chance of a windward shower, with
little significant change going into Thursday.

A strong and fast-moving high far NW of the islands will track
steadily E through the weekend, with another high following a
similar track Sunday and Monday as they both pass far N of the
islands. As the first high tracks E, a NE-SW oriented frontal
boundary is expected to produce active weather over waters just NW
of Kauai, with strong to near gale force NE winds developing N of
the boundary, and moderate to fresh E trade winds developing S of
the boundary, and over the islands, Thursday night and Friday.

At this time, the stronger winds and active weather along and N of
the front are not expected to directly affect the islands or the
coastal waters, but it is expected to be close to Kauai, and thus
bears watching. This is especially the case because a closed low
aloft is expected to develop just W of Kauai around the same time,
likely prompting heavy showers along the boundary. Latest ECMWF
guidance brings the low closer to Kauai than GFS guidance, so some
uncertainty remains as to the potential for impacts on Kauai. As the
high continues moving E into the weekend, the low aloft and the
surface frontal boundary will move W, with locally breezy trade
winds over the islands. Moderate trade winds Sunday look to
strengthen Monday and Tuesday as the second high passes N of the
area.

Best chances for showers in the short term will be on the Kauai end
of the chain Thursday night into Saturday due to the nearby frontal
boundary and developing low aloft, and along windward Big Island,
where trade winds will likely deliver some showery low clouds from
time to time. Over the weekend, the breezy trade winds will deliver
passing clouds and showers that will focus over windward areas over
all islands, but no organized shower areas are expected, and weak
ridging aloft should cap incoming moisture.

A mid-level ridge will build over the area early next week, acting
to further strengthen the cap, so trades should deliver limited
showers at that time. Moisture associated with dissipated frontal
boundaries may arrive on the trade flow next week, which may help to
fuel modest increases in windward showers.

&&

.AVIATION...
With weak easterly background flow, daytime sea breezes and
nighttime land breezes will develop over the much of the state
through Thursday. This will lead to some cloud buildups over island
interiors and leeward sections in the afternoon and early evening,
then clearing out overnight. Little in the way of precipitation is
expected due to a stable air mass over the area. VFR will
predominate in most areas most of the time.

A front approaching from the northwest will begin to affect Kauai by
as early as late afternoon Thursday, and trade winds will begin to
increase. Trades will ramp up for most of the state by Friday as the
front stalls near the Garden Isle.

&&

.MARINE...
A front will enter the far northwest offshore waters later today,
accompanied by northeast winds in the 15 to 25 kt range. This front
will move eastward and stall Thursday night. A new high will pass
north of Kauai Thursday night and Friday, driving a boost in
northeast winds to gale force north of the front. Waters around
Kauai will see stronger winds Thursday night and Friday. Strong
trades will also develop over the typically windy local waters on
Friday. As the high continues eastward, trades will weaken over the
western end of the island chain by Saturday, but the locally strong
trades over the eastern waters will continue through the weekend and
into next week.

A northwest swell continues to build across local waters this
morning and a High Surf Warning remains in effect for north and west
facing shores of Niihau, Kauai, Oahu, and Molokai as well as for
north facing shores of Maui. The lower half of the Kona coast will
see a rise in surf heights but may not reach advisory level.
The surf will peak early Thursday, then lower to below advisory
level late Friday. Another northwest swell may produce advisory
level surf early next week.

Surf along east facing shores will continue below seasonal average
today. A boost in the trades Thursday into Friday will likely lead
to higher surf along east facing shores of all islands, but
especially for Kauai where a High Surf Advisory may be needed for
the island's east facing shores. Additionally, waters will turn
rough and choppy around Kauai. The trade swell will gradually back
down over the weekend as the trade wind eases.

Due to the large northwest swell, a Small Craft Advisory remains in
effect for all windward waters, Kauai to the Big Island, as well as
the leeward waters of Kauai and Oahu, including the Kaiwi Channel.
This advisory will also cover the strong northeast winds slated for
Kauai County waters Thursday night into Friday. The Small Craft
Advisory, due to locally strong northeast winds, will likely be
issued for waters around Maui County and waters south of the Big
Island Thursday night or Friday. Given the outlook described above,
expect this advisory to continue through the weekend and into early
next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 6 PM HST Thursday for North and West facing
shores of Niihau Kauai Oahu Molokai and North facing shores of Maui.

Small Craft Advisory until 6 PM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-Oahu
Windward Waters-Oahu Leeward Waters-Kaiwi Channel-Maui County
Windward Waters-Big Island Windward Waters.

&&

$$

DISCUSSION...Birchard
AVIATION...Kinel
MARINE...Powell

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