093
FXHW60 PHFO 220629
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
829 PM HST Fri Aug 21 2026

.SYNOPSIS...
Strong trades develop this weekend as Tropical Storm Moke moves
from east to west, south of the islands. This system will deliver
periods of rainfall to all islands, but heaviest along windward
and southeast slopes of the Big Island. A Flood Watch remains in
effect for the Big Island.

&&

.SHORT TERM UPDATE...
This evening`s IR satellite imagery loop showed an area of
convection several hundred miles southeast of the Big Island
associated with Tropical Storm Moke. According to the 5 PM HST NHC
advisory, the center of Moke around 595 miles southeast of Hilo
moving west northwest at 12 miles per hour. Closer to home, IR
satellite showed dissipating cloud cover over the interior, due to
the loss of diurnal heating, along with scattered low clouds
drifting toward the windward coasts of most islands. Radar has
been very quiet this evening with little in the way of shower
coverage of rainfall accumulation. Relatively dry air can be noted
below the 6000 ft subsidence inversion height on the 00Z Hilo
sounding.

Through Saturday, local pressure gradients will continue to tighten
across the state and allow trade winds to gradually increase. The
forecast focus now will be to determine which zones are to be
included in a Wind Advisory, or High Wind Warning if need be, for
Saturday night through Sunday as Tropical Storm Moke moves from
east to west, south of the islands. The short term forecast looked
on track, so no major adjustments to the grids were warranted in
this update.

&&

.PREV DISCUSSION...
Issued at 259 PM HST Fri Aug 21 2026

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...

Tonight into Saturday, trade winds will fill back in, gradually
strengthening across the island chain from east to west becoming
locally strong by the afternoon. AIRMET Tango will likely be
needed for low level turbulence downwind of island terrain.

While occasional pockets of MVFR conditions will be possible
within passing showers along windward and mauka locations, VFR
conditions are expected to prevail through Saturday.


&&

.MARINE...
A trough in the northwest offshore waters will move west 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 Big Island. The SCA may need to be extended into
western waters this weekend as trades strengthen in response to
Tropical Storm Moke passing south of the state. Seas greater than
10 feet will may occur over the eastern half of the coastal
waters.

Tropical storm conditions are expected Saturday night into Sunday
across the southern offshore waters. Thus, a Tropical Storm
Warning is in effect for these waters this weekend.

Surf along south-facing shores will remain elevated through the
weekend. Another reinforcing south swell is expected over the
weekend which should boost surf to near the summer averages this
weekend. A series of south swells next week, with above average
surf during the second half of next week, may potentially
increase surf to advisory levels.

Surf along east-facing shores remains below average tonight then
is expected to build this weekend per strengthening trades. Surf
along east and south-facing shores will be compounded by a
southeast swell generated by Tropical Storm Moke that will build
in through Sunday and then become south to southwesterly through
early next week. This short period captured fetch from TS Moke could
cause surf to approach advisory levels along the east-facing
shores of Big Island Saturday night into Sunday. Moke`s wind field
is forecast to be relatively small and, with potential shifts in
forecast tracks, forecast confidence for Big Island east-facing
shore advisory-level surf is low.

North-facing shores will remain tiny through tonight. Small, medium
to long period west to north swell originating from Hurricane Lala
will arrive this weekend.


&&

.FIRE WEATHER...
Issued at 259 PM HST Fri Aug 21 2026

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 East-Big Island Interior-Big Island North-Big
Island South-Big Island Southeast-Big Island Summits-Kohala-Kona.

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


&&

$$

SHORT TERM UPDATE...Thomas
DISCUSSION...JVC
AVIATION...Vaughan
MARINE...Blood