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Temp-2.5 C
RH94 %
WindE 3 mph
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Hawaiian Islands Synoptic Discussion and Guidance

374
FXHW60 PHFO 240159
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
359 PM 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 into tonight, 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 early
Wednesday, increasing rainfall chances, especially windward. By
Friday, trade winds will likely weaken enough to allow day time
sea breezes,and an unstable land and sea breeze pattern is
possible this weekend.

&&

.DISCUSSION...
Moderate to breezy trade winds persist, and an upper level trough
is tapping an area of enhanced low level moisture, leading to
continued wet and unstable conditions across the eastern end of
the state. A surface high is passing about 900 miles north of
Oahu, and its trailing surface ridge is located about 425 miles
to the northwest of Kauai. The proximity of the ridge 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 heavy rainfall across windward Maui and Big Island,
with thunderstorms flaring in and around Hilo. The greatest amount
of rainfall has been on windward Big Island, where widespread
totals of 3 to 6 inches have been recorded in the past 24 hours.
Conditions are, for now, somewhat stable over the rest of the
state, where modest windward rainfall has been occurring.

For tonight, continued heavy rainfall is expected along windward
Big Island, while increasing low level moisture creeps up the
island chain. A weakening jet stream running along the flank of
the upper level trough will remain over the Big Island. This
should provide enough instability to maintain heavy showers and
thunderstorms through much of tonight. The moisture should also be
deep enough to reach the high summits of the Big Island, where
temperatures will be below freezing. As a result, a Winter Weather
Advisory has been posted of the high summits tonight. Across the
rest of the island chain, mainly windward rainfall will increase
as the above mentioned low level moisture spreads westward on the
locally breezy trades.

Easterly trade winds begin a gradual decline Wednesday and
Thursday, as showers remain largely focused over windward slopes.
The weakening upper level trough will linger over the region, but
we still expect low level moisture, as shower activity, to
decrease over windward Big Island. While we cannot rule out a
brief leeward shower Wednesday afternoon, the potential for
afternoon sea breezes and a few showers will increase Thursday
afternoon.

An unstable land and sea breeze regime may develop on Friday and
will likely be in place this weekend. 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
southeast, and by Saturday, winds will likely become light and
variable across the region. On all days, afternoon sea breezes
should be widespread, leading to interior clouds and showers on
all islands. There is still considerable uncertainty regarding
available low level moisture to fuel showers, but the GFS and
ECMWF are showing a deepening upper level trough overhead on
Saturday and Sunday, increasing the probability of a few heavy
showers.

&&

.AVIATION...
Light to locally brisk trade winds will continue. Clouds and
showers will favor windward slopes and coasts of all islands.
An upper level disturbance aloft has enhanced instability over
the eastern half of the state. Thunderstorms and locally heavy
downpours expected across Windward and Interior Big Island
through the evening hours and possibly beyond.

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

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 evening.

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 winds and combined seas. Both are expected to be
on the way down over the next 24 hours. Once winds and seas drop
below advisory levels tomorrow, they are expected to remain below
criteria through the weekend.

Surf along north and west facing shores has dropped below advisory
criteria, so that advisory is cancelled with this package. Along
east facing shores, the combination of locally driven trade wind
swell and a background east swell is maintaining advisory level
surf. So, the High Surf Advisory for those areas has been
extended. Even though the trade winds are expected to begin to
taper off Wednesday night, advisory level surf could continue
during the second half of the week.

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.

Several northwest swells are expected to Thursday through the
weekend, but all area expected to be below advisory levels. See the
latest Oahu Surf Discussion (SRDHFO) issued today for more
specifics on these swells.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Thursday for east facing shores
of Kauai, Oahu, Molokai, Maui, and the Big Island.

Winter Weather Advisory until 6 AM HST Wednesday for Big Island
Summits.

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