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Hawaiian Islands Synoptic Discussion and Guidance

642
FXHW60 PHFO 250639
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
839 PM HST Sat Feb 24 2018

.SYNOPSIS...
An active upper level trough will sweep across Kauai County tomorrow
bringing heavy rain showers and thunderstorms to the islands of Kauai
and Niihau. Enhanced showers and thunderstorms from this upper
trough will likely reach Oahu by late afternoon and evening time
periods. Elsewhere moist and somewhat unstable conditions will
linger across the eastern half of the state on Sunday with rain
showers expected mainly over windward and mountain slopes from
Molokai to the Big Island. A drying trend begins on Monday
afternoon as a strong ridge builds in over the islands and we
transition to a windy east-southeast weather pattern with more
typical windward and mountain showers each day.

&&

.DISCUSSION...
An upper level low located roughly 1000 miles northwest of Kauai
this evening will slowly drift northward away from the state. An
upper trough moving around this low center will sweep through
Kauai on Sunday. Strong divergence aloft ahead of this trough will
produce widespread showers, heavy at times with isolated to scattered
thunderstorms across Kauai and Niihau tomorrow. A Flash Flood
Watch will likely be issued later this evening to cover the
elevated threat for Flash Flooding over Kauai County. Enhanced
showers and thunderstorms will probably reach Oahu by Sunday evening,
some of these developing showers will become locally heavy at
times. Elsewhere our latest forecast guidance continues to show
east-southeast winds producing windward and mountain showers over
each island with elevated chances along the east and southeast
slopes of the Big Island.

For next week, a strong ridge builds into the state from the
northeast starting on Monday with a surface trough just to the
west of the islands. Stronger winds are forecast to blow across
the region from Monday night onward. These strong winds may
create unusual downsloping effects due to the less typical east
to southeast direction and wind advisories will likely be needed
later this week. Downward vertical motions (subsidence) under this
ridge will start a drying trend across the island from Monday
afternoon onward with a much needed break in the weeks long wet
weather pattern.

&&

.AVIATION...
High pressure to the northeast of the state and a trough of low
pressure to the west will keep an east to southeast flow in place
through Sunday. Moist and unstable conditions will prevail through
the period, with widespread MVFR cigs in showers affecting mainly
windward areas. A strong upper level disturbance will rotate
across the western islands on Sunday, bringing the potential for
heavy rainfall and thunderstorms to Kauai and Oahu. As a result,
have mentioned VCTS in the PHLI, PHNL and PHJR TAFs to reflect
most probable time frame for seeing thunderstorms. Conditions
should begin to improve statewide Sunday evening as the winds
begin to shift around to more of an easterly trade wind direction
and the best forcing for ascent associated with the upper level
trough lifts northward and away from the island chain.

AIRMET Sierra is in effect for mountain obscuration across
windward sections of Kauai, Oahu, Molokai, Maui and the Big
Island. These conditions will likely prevail through the day on
Sunday.

&&

.MARINE...
A moist and somewhat unstable airmass will remain over the state
overnight, with isolated thunderstorms possible over the coastal
waters. Thunderstorm chances will focus over the western sections
on Sunday and Sunday night, as a trough aloft passes overhead.

A Small Craft Advisory (SCA) remains posted for most waters,
including all windward areas, due to strong east-southeast winds
and elevated seas. Strong high pressure far to the north-northeast,
along with a surface trough west of the state, will produce
breezy east to southeast winds through the remainder of the
weekend, with winds expected to strengthen further during the
first half of next week. The SCA may need to be expanded to
cover additional waters early next week as the winds increase,
with gales possible in some areas.

A long and rather broad fetch of easterly winds on the south side
of the distant subtropical high will continue to produce advisory
level surf along east facing shores into early next week. The
latest model guidance continues to show that surf could reach
warning levels on those shores as early as Monday night or
Tuesday. This combined with high astronomical tides could cause
some coastal inundation issues along some low lying areas exposed
to east.

Relatively small surf is expected along north and west facing
shores during the upcoming week. Small, mainly background south
swells can be expected through the week as well.

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

&&

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

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

&&

$$

DISCUSSION...Bohlin
AVIATION...Jelsema
MARINE...Jacobson

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



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