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Hawaiian Islands Synoptic Discussion and Guidance

072
FXHW60 PHFO 111344
AFDHFO

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

.SYNOPSIS...
A mostly dry and light wind pattern is expected today. A front
approaching from the northwest will bring increasing clouds and
showers to Kauai late Thursday. Moisture from the front will
become focused over the windward and mauka areas of all islands Friday
as trades begin to strengthen. A wet trade wind pattern will
persist through the weekend.

&&

.DISCUSSION...
Have made some modifications to the forecast through the period,
but the general flavor remains the same. The winds have been
lightened a bit today over land in keeping with the latest runs
and observations. Winds in the other periods have also been
weakened just a bit to better align with the current model runs,
but still trend upwards over the weekend. Moisture into Kauai has
be delayed just a bit, with arrival a bit later on Thursday, and
lingering into Friday. Other changes to the PoPs and related
fields generally trend the forecast towards the GFS solution.

The overnight surface analysis shows a weakening stationary front
to the north and a cold front behind that system. The front is
expected to move towards the islands, and will bring an increase
in clouds and showers to Kauai, with the bulk of the moisture
coming in Thursday night. Behind the front, a ridge of high
pressure will build in, so expect the front to die out and be
replaced with building high pressure. This will bring increasing
trade winds to the islands, while leftover frontal moisture to the
northeast of the islands gets carried in. This will focus clouds
and showers over windward and mauka areas starting Friday and on
through the weekend.

Winds will be light enough today to support some afternoon cloud
development over the interiors of the islands. Some clearing will
occur tonight, with a few more showers expected around the
islands tomorrow.

The upper level dynamics continue to be the question for Friday
through the weekend. The GFS continues to develop an upper level
low along the southern end of an upper level trough that moves
towards the islands. However the latest run keeps the low further
to the west of the islands. The ECMWF keeps a slightly more
defined upper trough passing over the islands. The end result for
the islands in either solution continues to be an enhancement of
the trade wind showers for Friday night through Saturday night.
Have boosted the QPF values for the western half of the state to
better reflect the higher rainfall totals this would bring.

Another front passing north of the islands next week will continue
to bring moisture in on the trade wind flow, but upper level
ridging should help to limit rainfall totals.

&&

.AVIATION...
With weak easterly background flow, nighttime land breezes and
daytime sea breezes will develop over the much of the state
through Wednesday night. This will lead to some cloud buildups
over island interiors in the afternoons and evenings, then
clearing out overnight. Little in the way of showers is expected
due to a stable airmass over the area. VFR will dominate over
most areas. Easterly winds and clouds will gradually increase
beginning Wednesday night as a front approaches from the
northwest.

&&

.MARINE...
The ridge of high pressure located 430 nm NE of Kauai, will be
given way to new surface high located far NW of the area in the
next 48 hours. Our winds will be more of a trade wind flavor
today, and slightly more moderate as well. An old frontal
boundary located 130 nm NW of Kauai is just about done, as a new
front presses southeastward. This new front will be entering the
far NW offshore waters later today, accompanied by NE winds 15 to
25 kt. This front will nudge slightly further east, stalling just
NW of Kauai Thursday night. The new surface high of 1033 mb will
be passing north of Kauai between Thursday night and Friday,
resulting in a boost in NE winds to gale force north of the
front. The waters around Kauai County will be grazed with small
craft level winds of 23 kt or more Thursday night and Friday.
Also on Friday, the front starts to weaken, allowing locally
strong trades to develop over the typically sensitive areas to the
trades, that is waters around Maui County and waters south of the
Big Island. As the surface high continues eastward, the trades
weakens over the western end of the island chain by Saturday, but
the locally strong trades over the eastern part of the island
chain continues through the weekend, and at least through the
early next week with a new high passing north of the islands.

The northwest swell continues to build across our local waters
this early Wednesday morning. The Waimea PacIOOS buoy has risen
to just under 10 feet with a 15 second period. Deep ocean buoy
51001, northwest of Kauai, has indicated a temporary break, with
the swell holding at 9 feet in the past hour. Another larger pulse
will be arriving in the next couple of hours at the buoy, resuming
the upward trend. A High Surf Warning is in effect for the north
and west facing shores of Niihau, Kauai, Oahu, and Molokai; and
the north facing shores of Maui. The lower half of the Kona coast
will see a rise in the surf but not reach advisory level. The surf
will peak early Thursday then lower to below the advisory level
late Friday. Another pulse of northwest swell may produce advisory
level surf early next week.

Due to the weak trades, surf along east facing shores will
continue below seasonal average for today. A boost in the trades
Thursday into Friday will likely lead to higher surf along the
east facing shores of all islands, but especially for Kauai where
it may necessitate a high surf advisory for the island's east
facing shores. Additionally, waters will turn particularly very
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, the SCA has been expanded to
cover all windward waters, Kauai to the Big Island, as well as the
leeward waters of Kauai and Oahu, including the Kaiwi Channel.
The SCA will also cover the strong NE winds slated for Kauai
County waters Thursday night into Friday. The SCA, due to locally
strong NE 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 SCA, due to
locally strong trades, to likely continue through the weekend and
into early next week.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Warning until 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 this morning to 6 PM HST Thursday
for Oahu Leeward Waters-Kaiwi Channel-Maui County Windward
Waters-Big Island Windward Waters.

Small Craft Advisory until 6 PM HST Thursday for Kauai Northwest
Waters-Kauai Windward Waters-Kauai Leeward Waters-Kauai Channel-
Oahu Windward Waters.


&&

$$


DISCUSSION...M Ballard
AVIATION...TS
MARINE...H Lau

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



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