872
FXHW60 PHFO 220059
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
259 PM 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.

&&

.DISCUSSION...

Trade winds remain light over the western end of the state as
surface troughing continues lifting off to the northwest. This
trough, the remnant of TC Hernan, has draped a modest band of
moisture over Oahu this afternoon where sea breeze convergence has
capitalized on the resident weakly stable environment and led to
numerous showers over the interior. Prospects for afternoon showers
on the other islands remain limited due to diminished moisture
and/or stronger trade winds.

Attention then turns to Tropical Storm Moke which is currently
positioned about 650 miles southeast of Hilo and is lifting slowly
NW. Satellite presentation is characteristic of a storm affected by
southerly shear with convection concentrated north of the center of
circulation. Moke is also drawing north a band of rich moisture
originating from the deep tropics. PWAT values within this moisture
plume average about 2.5" which is what it will be when it reaches
the Big Island late Saturday night into Sunday. The forecast quandary
for today is seemingly answered by a northward jump in the 18z GFS
to much more closely match the 12z EC and the official NHC forecast.
This in turn adds confidence to the official forecast which was
based on the latter 2 sources and is considerably wetter for the
Ka`u and Puna Districts of the Big Island. Instead of straight
trades through the lowest 10kft or so as suggested by this morning`s
GFS, the northern periphery of the envelope of cyclonic flow will
envelop the Big Island allowing winds to veer to SE and steer
greater moisture content to the SE facing slopes of the Big Island.
The forecast for around 10" in this area, locally higher, remains on
track. Rain will begin over SE and Windward Big Island late Saturday
night or early Sunday morning, peak in intensity Sunday
afternoon/evening and then linger through Monday, particularly over
upslope areas where afternoon convection may be fairly robust on
Monday afternoon. Strong trades ensure fast-moving showers elsewhere
except perhaps Windward Maui where a few inches of rainfall appears
increasingly likely. Though trades may be on the wetter side on
Monday, prospects for flooding for Molokai, Oahu, and Kauai appear
quite limited. The Flood Watch for the Big Island remains in good
shape with no adjustments needed.

Secondary forecast concern is wind magnitude. As with the rainfall
forecast, the northward jump by the 18z GFS increases confidence in
the existing wind forecast. The pressure gradient north of TS Moke
will begin strengthening tonight with peak winds expected Sunday and
Sunday evening. Low stability and plenty of moisture advection will
make for inefficient and limited downsloping. Although the usual
suspects will be windy, will shy away from a High Wind Watch for any
zones at this time. Instead, this appears to be a solid Wind
Advisory (gusts to 50 mph) for at least Maui County and the Big
Island and possibly Oahu as well.

Low amplitude high pressure remains anchored along 30N supporting
breezy trades continuing through most or all of next week.

&&

.AVIATION...

A surface trough has moved north of Kauai, however light winds
remain over the western islands, leading to local land and sea
breezes. Clouds and showers formed over the interior of Oahu this
afternoon prompting AIRMET SIERRA for mountain obscuration.
Anticipate these clouds and showers to dissipate shortly after
sunset.

Behind the trough, trades are starting to fill back in, and will
gradually strengthen 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.

Addtional pockets of MVFR conditions are possible with passing
clouds and showers outside of those on Oahu this afternoon.
Otherwise, VFR conditions are expected to prevail.

&&

.MARINE...

A trough in the northwest offshore waters will continue to move
northwest as trade winds have begun to fill in across the coastal
waters from east to west. A Small Craft Advisory (SCA) has been
issued through Saturday afternoon for the windier waters and
channels around Maui County and the Big Island. As trade winds
continue to strengthen over the weekend as Tropical Storm Moke
passes south of the state, the SCA may need to be expanded
starting Saturday night through Sunday. Although seas greater than
10 feet will most likely be over the eastern half of the coastal
waters. Across the offshore waters, a Tropical Storm warning as
been issued as Tropical Storm conditions are expected Saturday
night into Sunday across the southern offshore waters.

Surf along south facing shores will remain elevated through the
weekend. 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
potentially up to High Surf Advisory levels.

Surf along east-facing shores remains below average tonight 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 Storm Moke 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 tonight
with a small medium to long period west north swell from
Hurricane Lala this weekend.

&&

.FIRE WEATHER...

Trade winds increase over the weekend and the inversion,
currently around 6-7kft, will steadily weaken and eventually
erode altogether as deep moisture associated with TS Moke moves
into the area late Saturday night into Sunday. Heavy rain is
expected over windward and SE facing slopes of the Big Island on
Sunday.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from late Saturday night through Monday afternoon
for Big Island-

Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.


&&

$$


DISCUSSION...JVC
AVIATION...M Ballard
MARINE...Shigesato
FIRE WEATHER...JVC