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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 160156

Area Forecast Discussion
National Weather Service Honolulu HI
356 PM HST Mon Aug 15 2022

Trade winds have weakened across the region as a trough well to
the northwest breaks down the ridge north of the state. This
pattern will maintain a hybrid wind flow through Wednesday,
promoting leeward land and sea breezes while light easterly
trades continue over windward areas. Limited light showers over
the islands tonight will increase across the eastern portions of
the state Tuesday and Wednesday, as a weak trough passes to the
south. High pressure will then restrengthen north of the islands
Wednesday night, re-establishing moderate to locally breezy trade
winds while refocusing showers to more favored windward areas
Wednesday night into the weekend.


Current radar trends show mainly isolated showers across the
region this afternoon, with the overall intensity of the showers
remaining light through this evening as mainly stable conditions
linger. Otherwise, the latest satellite loop shows limited clouds
across the state this afternoon, mainly focused along windward
areas and leeward locations were sea breeze development enhanced
cloud cover. Additionally, a broken band of mid level moisture
continues to drift westward around the Big Island.

Showers are expected to remain limited in areal coverage across
the state through this evening with mainly stable conditions in
place. The weakened easterly trades will however continue to
focus a few showers over windward locations this evening, before
increasing in areal coverage some overnight. Also, the sea breeze
clouds and showers which formed this afternoon over interior and
leeward areas, are expected to diminish this evening with the
diurnal transition toward land breeze development.

A band of deeper layer moisture and slightly less stable air will
drift westward across the islands Tuesday and Wednesday, as a
weakly organized surface trough passes south of the region. This
deeper layer moisture could increase the areal coverage of showers
and enhance rainfall rates, especially across the Big Island and
Maui. Some thunderstorm probabilities continue to hint at
conditions being unstable enough to potentially generate a low
end chance of thunderstorms Tuesday afternoon and again Wednesday
afternoon, across the higher terrain of the Big Island as surface
heating may enhance deeper convection or thunderstorms in the
unstable layer above the trade wind inversion.

The trough northwest of the state will eventually weaken as it
exits northeastward Wednesday, allowing the ridge to build back
in north of the island chain and strengthen the trade winds.
Trades will increase to moderate to locally breezy by Thursday
and continue through the weekend. This will allow a transition
back toward a more typical trade wind weather pattern, with
showers focused along favored windward locations.


This afternoon, satellite imagery reveals a recent increase in
cloud coverage across windward sections of the Big Island which
prompted an AIRMET Sierra for tempo mountain obscuration from
Upolu Point to Hilo to Apua Point. This AIRMET is expected to
persist this evening before conditions improve Tuesday morning.

Meanwhile, an increase in clouds has also been noted across
interior sections of the smaller islands and along the Kona Coast
as a result of localized afternoon sea breezes within a light to
moderate trade wind regime. Mainly stable conditions have kept
shower activity somewhat limited across the state today. However,
as a trough passes south of the region Tuesday and Wednesday, an
increase in areal coverage of showers will be possible. As the
air becomes more unstable during the afternoon on Tuesday, an
isolated thunderstorm over the higher terrain of the Big Island
cannot be ruled out. Otherwise, VFR flight conditions are
expected to prevail at most locations with only brief MVFR
ceilings and visibilities in passing showers.


Moderate to fresh easterly trades continue to ease and will hold
in the gentle to moderate category tonight into Wednesday, as the
ridge north of the state weakens. As overnight land breezes and
daytime sea breezes take over near the coasts beginning this
afternoon, pockets of fresh breezes near some coasts can lead to
choppy conditions. A return of fresh to strong easterly trades is
expected during the second half of the week as the ridge builds to
the north. This may lead to Small Craft Advisory conditions for
the windier waters and channels over the eastern end of the state
by Thursday.

After a quiet spell through the first half of August, a more active
pattern is unfolding near New Zealand within Hawaii's swell window.
This will lead to an upward trend in surf along south facing shores
beginning next weekend. Until then, expect small conditions to persist
with mainly a combination of background south and southeasterly
energy moving through.

Surf along east facing shores will remain small through the first
half of the week as the trades ease. A return of choppy/short-
period surf is expected by the end of the week as the trades

A small, short-period north swell will hold tonight, then subside
tomorrow, providing a small bump in surf along north facing shores.
Another small, short period north swell may fill in Wednesday night
through Thursday, providing another small boost. Otherwise, flat
summertime conditions will prevail.






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

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It was last modified on: Wed, Dec 17 2014 - 1841 UTC
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