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Hawaiian Islands Synoptic Discussion and Guidance

785
FXHW60 PHFO 240627
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
827 PM HST Tue Jan 23 2018

.SYNOPSIS...
Moderate to breezy trade winds will begin to trend down Wednesday
night through Thursday night as the strong ridge that has been in
place to the north weakens and shifts southward. Periods of heavy
rain along with a few storms moving ashore will remain a
possibility overnight into Wednesday for the the windward Big
Island, due to instability associated with an upper trough and a
sufficient amount of low-level moisture. Elsewhere, more of a
typical trade wind pattern is anticipated with showers focusing
over windward locales into Thursday. Friday through the weekend,
winds are forecast to become light and variable, which will result
in land and sea breeze conditions each day. The best chance for
clouds and showers through this period will remain over interior
areas through the afternoons and early evenings.

&&

.DISCUSSION...
Water vapor imagery showed a compact upper trough over the eastern
end of the island chain with a shield of middle- to upper-level
moisture/clouds racing northeastward within the subtropical jet over
and east of the Big Island. Plenty of middle- to upper-level dry air
associated with a weak upper ridge extending northward was shown
over the western end of the island chain. At the surface, the latest
analysis showed a ridge extending west and southwestward to the
north and northwest of the islands and a cold front extending
southward to the east of Midway from a storm-force low near the date
line.

Satellite-derived precipitable water (PW) imagery showed a pocket of
above average values (1.5-1.8") moving up the island chain from the
Big Island to Oahu. This increasing moisture combined with moderate
to breezy trades has translated to an uptick in windward showers
across Maui County and Oahu. For the Big Island, no significant
changes are anticipated overnight into Wednesday. Periods of heavy
rain with a few thunderstorms remain in the forecast, which will
keep the threat for flooding in place for the windward sections of
the Big Island. Additionally, the combination of the upper
trough, cooler temperatures and moisture support freezing
temperatures and periods of snow and freezing rain at the summits.

A pattern shift is anticipated through the second half of the week
and over the upcoming weekend as the storm-force low north of Midway
slowly tracks east-northeastward and drags a cold front to an area
north and northwest of the islands. This will result in a weakness
developing within the ridge north of the state that has been
responsible for moderate to breezy trades for the past several days.
Land and sea breeze conditions will result each beginning as early
as Thursday for the northwest islands, then all islands come Friday.
Over the weekend, some uncertainty still exists as the ECMWF
continues to reflect more of a transition back toward light
trades. Will continue to take a blend of the GFS and ECMWF through
this period, which shows land and sea breeze conditions
prevailing. A broad upper trough over the islands combined with a
sufficient amount of low-level moisture will support clouds and
showers focusing over interior areas through the afternoon and
evening hours each day.

Early next week, guidance remains in decent agreement and depicts
falling upper heights associated with an approaching upper trough.
The attendant cold front is forecast to approach from the west-
northwest on Monday, then move over the northwest islands by
midweek. This pattern suggests light and variable winds continuing
into Monday, before shifting out of the south-southwest ahead of the
approaching front. Although uncertainty remains high this far out in
time, an uptick in rainfall chances will be possible during this
time next week.

&&

.AVIATION...
An area of layered clouds covers much of the Big island, providing
a number of potential aviation hazards this evening. However,
except for Hilo, most terminals will see VFR conditions overnight.

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

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 the night.

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

&&

.MARINE...
The current Small Craft Advisory will be in effect through
Wednesday afternoon for a combination of winds and combined seas.
Both are expected to decrease over the next 24 hours. Once winds
and seas drop below advisory levels, they are expected to remain
below criteria through the weekend.

With the combination of locally driven trade wind swell and a
background east swell along east facing shores, advisory level
surf continues, 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 Wednesday. At the same
time; a new, long-period, background south-southwest swell is
forecast to fill in, then hold into Thursday, which should be
enough to keep southern shores from going flat.

Several northwest swells are expected Thursday through the
weekend, but surf is expected to remain below advisory levels.
See the latest Oahu Surf Discussion (SRDHFO) for more specifics on
these swells.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
High Surf Advisory until 6 AM HST Thursday for Kauai Windward-
Oahu Koolau-Olomana-Molokai Windward-Maui Windward West-Windward
Haleakala-South Big Island-Big Island North and East.

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.

&&

$$

AVIATION...POWELL
MARINE...KINEL
DISCUSSION...GIBBS

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