699
FXHW60 PHFO 211853
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
853 AM HST Fri Aug 21 2026

.SYNOPSIS...
Strong trades develop this weekend as a tropical system delivers
heavy rainfall to windward and southeast slopes of the Big Island.
A Flood Watch remains in effect for the Big Island.


&&

.SHORT TERM UPDATE...

No changes for today and the near term forecast remains on track.
12z guidance has arrived and the GFS remains rather far south while
the EC closely matches the official NHC forecast along the northern
edge of the guidance envelope. Forecast rainfall in the 8-10" range
still looks reasonable over the Big Island, though the location of
that rainfall will be highly track-dependent. A southerly GFS-style
solution will essentially result in straight trades and heavy
rainfall piling up in the Hilo and Puna Districts, especially
upslope areas. A more northerly EC type of solution allows cyclonic
flow around TD 2C to envelop the Big Island and for veering mid-
level winds to turn SE and focus the rainfall maximum in the Ka`u
and Puna Districts. Given the NHC forecast, the continuity of the
ECMWF, as well more accurate initialization of the location of
analyzed center of the system, the official forecast leans heavily
in the direction of the EC which focuses most rainfall over the SE
slopes of the Big Island. Strong trades ensure fast-moving showers
elsewhere and a minimal flood threat with the exception of Windward
Maui where a few inches of rainfall is not out of the question. The
Flood Watch remains in good shape and it is not anticipated that it
will be changed in space or time. A Wind Advisory will likely be
needed for some or all of the area during the latter half of the
weekend. A High Wind Watch for the windiest portions of Maui County
and the Big Island is plausible pending further observations and
modeling.



&&

.AVIATION...
A surface trough continues to move west and should clear Kauai
this morning. The trough is maintaining light winds over the
western islands, allowing for localized land and sea breezes.
Behind the trough, trades are starting to fill back in, and will
begin to fill back in and strengthen gradually across the island
chain from east to west. Trade winds will continue to strengthen
on Saturday and AIRMET TANGO will likely need to be issued for low
level turbulence.

Due to the lighter wind flow pattern, clouds and showers will
likely form over the interior regions and elevated terrain this
afternoon and then transition to mainly windward locations this evening
into tonight. Background southeast flow this morning has pushed
some clouds and showers up against the mountains producing some
MVFR conditions on Kauai, Oahu, Maui and the Big Island. As such,
AIRMET SIERRA is in effect for mountain obscuration. The smaller
islands are expected to clear later this morning, and the Big
Island will persist into the afternoon hours. Otherwise, VFR
conditions are expected to prevail.


&&

.MARINE...
Issued at 355 AM HST Fri Aug 21 2026

A trough passing north of Kauai this morning will continue to
track northwestward and away from the state. As the trough moves
away, moderate to locally fresh trades will return later this
afternoon and could become locally strong tonight. A Small Craft
Advisory (SCA) will likely be needed for the windier waters
around Maui County and the Big Island by late tonight. Trade
winds will continue to strengthen over the weekend with gale force
winds possible across the windier waters around the Big Island as
Tropical Depression Two-C passes south of the state. A Gale Watch
will be possible for some of the windier waters around Maui County
and the Big Island late Saturday into Sunday. Across the offshore
waters, Tropical Storm conditions will be possible as early as
late Saturday. Tropical Storm conditions are expected Saturday
night into Sunday across the southern offshore waters.

Surf along south facing shores will see a modest increase today
from a new long-period south swell that began trickling in
yesterday. Another reinforcing swell is expected over the weekend,
which should boost surf heights near the summer average over the
weekend. A series of south swells are expected next week with
above average surf likely during the second half of next week.
Surf along east- facing shores remains below average with weak
trades, then is expected to build this weekend as the trades
restrengthen. Surf along east and south- facing shores will be
compounded by a southeast swell generated by Tropical Depression
Two-C that will build through Sunday and then become south, then
southwest through early next week. This short- period tropical
swell could cause surf to approach advisory levels along east
facing shores of the Big Island Saturday night into Sunday, though
forecast confidence is on the lower side as the wind field with
this tropical cyclone is expected to be relatively small and the
forecast track may shift slightly. North facing shores will remain
tiny through today, with a slight bump from Hurricane Lala
possible this weekend out of the west-northwest direction.

&&

.FIRE WEATHER...
Issued at 355 AM HST Fri Aug 21 2026
Trades remain weak today then strengthen considerably over
the weekend as a tropical system passes south of the state. KDBI
values remain low in the wake of Hurricane Lala and relative
humidity is forecast to remain elevated during this period of strong
winds. The inversion remains in the 5-7kft range during this time.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from late Saturday night through Monday afternoon
for Big Island East-Big Island Interior-Big Island North-Big
Island South-Big Island Southeast-Big Island Summits-Kohala-Kona.


&&

$$

SHORT TERM UPDATE...JVC
DISCUSSION...CO
AVIATION...M Ballard
MARINE...Kino
FIRE WEATHER...CO