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Hawaiian Islands Synoptic Discussion and Guidance

FXHW60 PHFO 232000

Area Forecast Discussion
National Weather Service Honolulu HI
1000 AM HST TUE AUG 23 2016

Deep tropical moisture moving over the state will maintain very
humid conditions with localized heavy downpours and
thunderstorms. The greatest chance for heavy rainfall over the
islands will be during the afternoon, though moist and somewhat
unstable conditions will linger into Wednesday. Drier trade wind
weather will return to the islands by Thursday and prevail
through the weekend.


Conditions over the state can best described as deep tropical. A
weak low level trough originating from the deep tropics has
drifted to within 75 miles south of the Big Island, spreading
abundant moisture with exceptionally high precipitable water
values of more than two inches over the islands. The main focus
for thunderstorms and heavy rainfall was over windward Big Island
overnight, where totals of two to seven inches were common.
Thunderstorms within the trough have pushed offshore this morning
and are currently anchored along the trough and within the Big
Island plume to the south and southwest of the islands, confining
generally moderate showers to windward slopes this morning. A
cluster of thunderstorms in the trough is sending a thick shield
of high clouds over Oahu this morning.

Deep moisture will produce very humid conditions and localized
heavy showers with thunderstorms through at least the evening.
A deep trough located about 800 miles northwest of the state will
inch closer today, causing easterly trade winds to gradually veer
out of the southeast. While easterly trade winds will hang on
across most windward areas today, day time sea breezes will
dominate elsewhere, leading to another very humid day with dew
point temps hovering in the mid 70s. Even though a weak ridge will
remain overhead above 15000 ft or so, the abundant available
moisture will be tapped by sea breezes to produce spotty heavy
showers and thunderstorms. Due to the already impressive amount of
rainfall that has occurred on the Big Island, a Flash Flood Watch
remains in effect for that island through the evening. The Watch
does not include the other islands at this time, but localized
flooding is a concern, mainly during the afternoon.

Moisture will slowly diminish over the western half of the state
late tonight and early Wednesday while remaining deep over the
eastern end of the island chain. A southeasterly flow should
generate land breezes over the smaller islands, pushing higher
shower chances offshore, but windward and southeast slopes of the
Big Island, as well as east Maui, could stay active. Another round
of convection, featuring spotty heavy showers, is expected during
the afternoon.

Models remain consistent in bringing a drier trade wind weather
back to the state beginning late Wednesday night into Thursday
and continuing into the weekend, as high pressure builds to our
north and a drier airmass moves in from the east. This should
suppress seabreeze activity, with passing showers becoming focused
over windward areas.


A band of thick layered clouds has started to push its way up
into the waters south of Oahu. As of 1900Z, embedded thunderstorms
were approximately 35 NM south of PHNL. This band of showers and
thunderstorms has been slowly tracking north this morning and
PHNL and PHJR terminals should be aware of thunderstorms possibly
entering their vicinity.

Thunderstorm activity over eastern Big Island and the waters
north of Maui and the Big Island have settled down since early
this morning. Thunderstorm activity, however, remains in the
waters far south of South Point. Daytime heating and the ample
moisture over the state will help contribute to the chance of
thunderstorms this afternoon. This will hold true for all islands.

Showers will likely continue to move over the Big Island and PHTO
should expect to experience prevailing MVFR to TEMPO IFR
conditions. The islands of Maui, Lanai, and Molokai remain quiet
this morning, with VFR conditions at the terminals. An increase in
showers is also expected this afternoon. Predominately MVFR
conditions are likely to stick around at PHLI through the day.

AIRMET SIERRA for mountain obscuration is posted for the Big
Island, Oahu, and Kauai. This will be expanded as needed.


No marine advisories are in effect, but thunderstorms have been
added for all waters today. Currently, a large mass of
thunderstorms is noted about 35 nm south and southwest of Oahu.
This area could drift into additional waters around Oahu and Kauai
as the day progresses, and an isolated thunderstorm could flare up
anywhere, though the highest chance will be over land and south
through west of the Big Island.

Moderate to locally fresh east to southeast winds will prevail
through tonight, with winds strongest across windward waters and
the Alenuihaha Channel. Fresh to strong trade winds will
gradually fill in late Wednesday and Thursday then continue into
the weekend as high pressure rebuilds north of the state. Small
Craft Advisory winds are expected across the typically windier
waters during this time. No significant surf is expected through
the week.


Flash Flood Watch through this evening for Big Island.



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