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Hawaiian Islands Synoptic Discussion and Guidance

398
FXHW60 PHFO 120634
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
834 PM HST Wed Dec 11 2019

.SYNOPSIS...
Light trade winds and mostly dry weather will prevail through
Thursday. Strong high pressure passing north of the islands will
bring increasing trade winds Thursday night and Friday, with locally
breezy trades persisting into the weekend. The strengthening trade
winds will bring passing clouds and showers to windward portions of
the islands, but a front stalling just northwest of the area will
bring increased rainfall to Kauai and adjacent waters the next
couple of days. High clouds are expected to periodically pass over
the islands from Thursday night through Saturday.

&&

.DISCUSSION...
No changes to the forecast this evening. Satellite and radar
data show the clearing trend we were expecting over the central
islands with some clouds and showers lingering northwest of Kauai
and over/near the Big Island. Expect that trend to continue
overnight.

Light trade winds are expected through Thursday with a weakening
front to the north west of the islands. A mostly dry and stable
airmass over the islands will limit clouds and rain. Expect
similar conditions on Thursday however the clouds and showers that
form may be focused a bit more on the windward slopes.

High pressure far northwest of the islands will move to the east
through the weekend, followed by another high early next week.
Both highs will remain north of the islands. The front to the
northwest of the islands will be pushed over the western end of
the coastal waters beginning Thursday. Winds north of the front
are expected to be strong and out of the northeast, while out
ahead of the front moderate easterly trades are expected Thursday
night and Friday.

Kauai will be the most likely to experience stronger winds and
increased rainfall from the front. In the upper atmosphere, an
upper level trough is expected to drop down and form an upper
level low to the southwest of the islands, however there remain
some differences in the model solutions. At this time, the
forecast reflects the greatest chance for rainfall on Kauai to be
Thursday night into Saturday, with the most enhanced Friday night.

As the high mentioned above moves to the east this weekend, the
upper level low will move to the west, and locally breezy trade
winds will spread across the islands. As we transition to the
second high, trade winds could slacken a bit on Sunday, before
rebounding with the second high Monday and Tuesday. With the trade
wind flow over the islands. showers will be focused over windward
areas for the remaining islands.

Mid and upper level ridging moving over the region early next week
will help to stabilize the atmosphere limiting trade winds
showers, however leftover frontal moisture will be riding in on
the trades at times will bring some moisture bands in on the
trades from time to time.

&&

.AVIATION...
With a light east to southeast background flow, daytime sea breezes
and nighttime land breezes will develop over parts of the state
through Thursday. This will lead to some cloud buildups over island
interiors, and leeward and mauka sections in the afternoon and early
evening, with clearing overnight. Little in the way of rainfall is
expected due to a stable air mass over the area. VFR will dominate
in most areas.

A front approaching from the northwest will begin to affect Kauai
as early as Thursday afternoon, with increasing trade winds, clouds
and showers. Clouds and winds will increase for rest of the state
by Friday as the front stalls near the Garden Isle.

&&

.MARINE...
A large northwest swell will enter Hawaiian waters tonight and
build surf to above warning levels Thursday morning. A High Surf
Warning is in effect for north and west facing shores of Niihau,
Kauai, Oahu, and Molokai as well as for north facing shores of
Maui. A high surf advisory is in effect for the leeward Big Island
coast. 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.

A full moon high tide early Thursday morning (between 2-6am
depending on location) and the large NW swell will bring the
potential of overwash along coastal areas. As of 8pm, the swell is
running 1-3 feet larger than the WW3 predicted values at buoys
51001 and 51101. Added to the large swell and full moon tide,
is the fact that tide gauges across the state have been showing
water levels near 6/10 of a foot above predicted values. All of
these factors combined will allow waves to reach higher along
exposed coastlines and over low lying coastal roadways.

A front will enter the far northwest offshore waters tonight,
accompanied by northeast winds in the 15 to 25 kt range. This
front will move eastward and stall Thursday night NW of Kauai. A
new high will pass north of Kauai Thursday night and Friday,
driving a boost in northeast winds to near 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 across the coastal 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.

Surf along east facing shores is currently below the seasonal
average. A boost in the trades Thursday into Friday will lead to
higher surf along east facing shores of all islands, but
especially for Kauai where a High Surf Advisory may be needed for
east facing shores. The trade swell will gradually back down over
the weekend as winds ease.

Due to the large northwest swell, a Small Craft Advisory remains
in effect for all windward waters 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. A Small Craft Advisory for
winds will likely be issued for waters around Maui County and
south of the Big Island Thursday night. 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 Niihau-Kauai
Windward-Kauai Leeward-Waianae Coast-Oahu North Shore-Oahu
Koolau-Olomana-Molokai-Maui Windward West-Maui Central Valley-
Windward Haleakala.

High Surf Advisory until 6 PM HST Thursday for Kona.

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...M Ballard
AVIATION...TS
MARINE...Morrison

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



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