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Temp-1.2 C
RH95 %
WindNE 2 mph
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Hawaiian Islands Synoptic Discussion and Guidance

033
FXHW60 PHFO 231959
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
959 AM HST Tue Jan 23 2018

.SYNOPSIS...
Moderate to breezy easterly trade winds will gradually decline
Wednesday night through Friday as the surface ridge currently far
north of the state is weakened and pushed southward. An area of
enhanced low level moisture will remain focused over windward
portions of the Big Island and Maui County today, and an upper
level trough overhead will trigger a few heavy showers and
thunderstorms across the eastern end of the state. The low level
moisture will slowly spread up the island chain tonight and
Wednesday, increasing rainfall chances, especially windward. By
Thursday or Friday, trade winds will likely weaken enough to allow
day time sea breezes.

&&

.DISCUSSION...
Moderate to breezy trade winds persist, and an upper level trough
is tapping an area of enhanced low level moisture, leading to wet
and unstable conditions across the eastern end of the state. A
surface high is passing about 800 miles north of Kauai, and its
trailing surface ridge is located about 400 miles to the northwest
of the Garden Isle. This is causing the locally breezy trades to
be more easterly than normal. A lingering, compact upper level
trough bisects the state, generating high clouds and producing
instability over the eastern end of the island chain. This
instability is tapping an area of enhanced low level moisture
moving along the trade wind flow, leading to a few heavy showers
and thunderstorms. The moist and unstable conditions have been
focused over windward Big Island and east Maui, where 2 to 4
inches of rainfall have been measured during the past 24 hours.
Conditions are, for now, somewhat stable over the rest of the
state, and modest shower activity has been occurring over windward
areas of Molokai and Oahu, while a small patch of enhanced
showers has been affecting Kauai this morning.

Easterly trade winds will hold in the locally breezy range into
tomorrow. The greatest amount of low level moisture and
instability is expected to remain focused across windward portions
of the Big Island and Maui during this time. This will support
chances for a few heavy showers and thunderstorms today. Daytime
sea breezes will likely trigger interior showers and thunderstorms
over the Big Island this afternoon, bringing chances for snow
and/or freezing rain showers over the high elevations of Mauna Kea
and Mauna Loa. We will monitor closely, but probabilities for
significant winter weather do not support a Winter Weather
Advisory at this time. Tonight and Wednesday, the low level
moisture should slowly spread up the island chain, bringing an
increase in mainly windward shower activity. The nearly stationary
upper level trough will weaken, raising the possibility of some
enhancement to the showers as well.

Trade winds will slowly decline starting Wednesday night, while
another upper level trough moving in from the west maintains some
instability. Showers should remain focused over windward slopes on
Thursday. On Friday, the surface ridge will be pushed to within a
couple hundred miles of Kauai, causing local winds to decrease
further and veer out of the east to east-southeast, and by
Saturday, winds will likely become light and variable across the
region. Afternoon sea breezes should be widespread, leading to
interior clouds and showers on all islands. The big unknown at
this point is the amount of available low level moisture to fuel
any heavy showers.

&&

.AVIATION...
A large band of MVFR clouds and showers continues to stream into
the eastern slopes of the Big Island amd Maui. An upper level
disturbance aloft may produce enough lift for isolated
thunderstorms near the Big Island and Maui this morning. Elsewhere
mostly VFR conditions will prevail with showers focused along
windward slopes of all islands.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of the Big Island. These conditions are
expected to continue persist throughout the day.

AIRMET Tango remains in effect for moderate low level turbulence
over and downwind of the terrain of all islands. The AIRMET is
expected to remain in place through this afternoon.

AIRMET ZULU is in effect for TEMPO light to moderate icing in
clouds around the Big Island from 12000 to 20000 feet.

&&

.MARINE...
The current Small Craft Advisory (SCA) goes through 6 pm tomorrow
for a combination of fresh to strong trade winds, and combined
seas generated primarily from the northwest swell. As the surface
ridge northeast of the islands breaks down, we are expecting winds
to drop below advisory levels. During the same time period, the
current northwest swell will be on the decline, so expect seas to
drop below 10 feet by tomorrow night. Winds and seas are then
expected to remain below criteria through the weekend.

The High Surf Advisory continues today for east, north and west
facing shores. Along east facing shores, the combination of
locally driven trade wind swell and a background east swell could
maintain advisory level surf through tomorrow, so will be
monitoring trends today to see if we need to extend the advisory.
The easterly swell will hold during the second half of the week,
and gradually trend down Friday through the weekend. The current
west-northwest swell is expected to hold during the first part of
day, and then slowly trend down into midweek. The current advisory
for north and west facing shores remains in place through 6 PM
today.

The south shores will continue to see a bump from an out-of-
season, long-period, south swell through tomorrow. A new, long-
period, background south-southwest swell, is forecast to fill in
by Wednesday, then hold into Thursday, which should be enough to
keep southern shores from going flat.

A small west-northwest swell is possible Thursday through Friday
from a gale to storm-force low currently north-northwest of
Midway. A small northerly swell could impact the area Thursday
through the weekend, and we will be monitoring future guidance. A
small, long-period, west-northwest swell will also become a
possibility late Friday through the weekend. Small surf will
result over the weekend for north and west facing shores, with
Kauai seeing the bulk of the energy.

See the latest Oahu Surf Discussion (SRDHFO) for additional
details on surf and swell.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 PM HST this evening for north facing
shores of Niihau, Kauai, Oahu, Molokai, and Maui; for west facing
shores of Niihau, Kauai, Oahu, and Molokai; and for east facing
shores of Niihau, Kauai, Oahu, Molokai, Maui, and the Big Island.

Small Craft Advisory until 6 PM HST Wednesday for Kauai
Northwest Waters-Kauai Windward Waters-Kauai Leeward Waters-
Kauai Channel-Oahu Windward Waters-Kaiwi Channel-Maui County
Windward Waters-Maalaea Bay-Pailolo Channel-Alenuihaha Channel-
Big Island Windward Waters-Big Island Leeward Waters-Big Island
Southeast Waters.

&&

$$

Discussion...Wroe
Aviation...Bedal
Marine...M Ballard

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