347
FXHW60 PHFO 211956
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
956 AM HST Mon Sep 21 2026

.SYNOPSIS...
Breezy trades weaken slightly during midweek then restrengthen
by late in the week. Potential tropical development threatens to
bring heavy rainfall to eastern portions of the state during the
latter half of the week, though uncertainty remains higher than
usual at this time.

&&

.SHORT TERM UPDATE...
Skies were partly to mostly cloudy windward/mauka and mostly sunny
to sunny leeward. Radar detected isolated light showers south of
Hilo, with dry conditions elsewhere as of 9:50 AM HST. Trades
will continue at moderate to breezy speeds today through
tomorrow.

The chance of formation of the remnants of TD 15-E into Nolo (the
next name on the list) are up to 90% now. Model spread remains
especially large on the track of this system, and this is not
likely to improve until the system actually forms and becomes
something that the models can "sink their teeth into", so to
speak. The latest ensembles continue to display a large spread in
timing and location of the track. The general consensus is that
the storm will remain south of the islands. However, there is a
large spread concerning how far south...from just barely to
several hundred miles. The latest Google DeepMind ensemble shows
this spread well.

The system is being monitored closely by us and NHC, of course,
and we will update you as we learn anything new. This will be a
rapidly evolving situation. Impacts should start with increased
rainfall on Big Island (and perhaps Maui County) as early as
Wednesday if the storm tracks toward the northern side of the
ensemble envelope. A track more to the southern side would result
in impacts starting late Wednesday night or Thursday. Winds will
increase across the eastern end of the state regardless of which
track the system ends up taking. How strong the system gets,
combined with how far away it is, will determine wind speeds. At
this time, we are forecasting gusts over 70 mph on the summits of
Big Island (over 60 on Haleakala) late this week, but that is
subject to significant change.

Every 6 hours we will get new model runs that will help
us start to narrow things down, and as always we will look to NHC
for the official forecast track.

No forecast changes are needed at this time. See below for
additional details from the previous discussion.

&&

.PREV DISCUSSION...
Issued at 301 AM HST Mon Sep 21 2026

Attention going forward is on the time frame spanning Wednesday
into the weekend. Uncertainty is running significantly higher than
normal as Former TD-15-E lurks well southeast of the islands and
presents as a disorganized area of showers and thunderstorms.
General consensus exists that activity will organize during the
first half of this week and advance in the general direction of the
Big Island thereafter. Initially, uncertainty in system movement
likely stems from the current lack of organization and typical
uncertainty related to the cyclone`s development sequence.
However, by the end of the week, considerable differences arise
among the models regarding the evolution of governing mid-latitude
troughing as it approaches the state from the north. This will in
turn be key to determining placement of the convergent moisture
plume wrapping around the NE/E periphery of a potential tropical
cyclone.

Broadly speaking, there is consensus that a positively tilted
longwave trough will pivot toward the area and spur a closed low
NW of Kauai. The remaining portion of the trough then quickly
advances SE through the islands and, at some point, forces any
tropical system to the south and away from the forecast area. How
quickly this unfolds is dependent upon the amplitude and forward
motion of the trough. A less phased solution (GFS/UKMET), with a
well-defined closed low forming early, shunts the tropical to the
south much more quickly and suggests a rather brief/24-hr window
for flash flooding potential centered around Thursday. A more
phased solution (ECMWF) keeps the longwave trough in tact longer
and results in considerable delay forcing the system southward.
This makes all the difference as modeled rainfall totals run the
gamut from being measured in inches over a day or so to measured
in feet over several days. Accordingly, there is concern that the
heavy rainfall in the case of a closer brush with this system will
occur over the same areas impacted by Hurricane Lala, though this
remains far from certain. AI guidance, including the GDM, have
been equally unhelpful as they too demonstrate considerable spread
given the rather short lead time.

The shift of tails and extreme forecast index derived from the
00z EC ensemble sums up the current modeling situation well. This
information hints strongly at high end rain potential over
much/all of the Big Island Thursday, Friday, and Saturday with
consensus at this time tending to focus on the Hilo Districts and
portions of Puna. The official forecast is guided by this
information but additional revision to the magnitude and location
of the heaviest rainfall is likely during the next couple of days.
Maintained isolated thunder in the forecast during this period and
introduced mention of heavy rain over the Big Island for the
second half of the week.

A Flood Watch the Big Island and possibly Maui will be considered
for future forecast packages. Strong trades likely worthy of wind
headlines will also develop late this week into the weekend.

&&

.AVIATION...
Issued at 301 AM HST Mon Sep 21 2026

Trades will remain breezy through today, but are expected to
slightly weaken by midweek. Showers will primarily impact
windward and mountain areas, with possibly a higher frequency over
windward Big Island today. Kona slopes of the Big Island could
also see some afternoon showers. MVFR conditions are expected
within showers while VFR prevails elsewhere.

AIRMET Tango for moderate low level turbulence downwind of island
terrain will remain in effect through today due to the breezy
trade winds. An upper level trough positioned NW of Kauai, which
is currently advecting high clouds over the the state, could also
cause some moderate upper level turbulence over Kauai by midday.

AIRMET Sierra is not currently in effect, but could be issued
later today due to upstream clouds and showers.

&&

.MARINE...
Issued at 301 AM HST Mon Sep 21 2026

Fresh to locally strong trade winds expected through tonight
before decreasing slightly through midweek. A Small Craft
Advisory is in effect for the typical windy waters of Maui County
and the Big Island through Tuesday evening. Wind speeds will
restrengthen during the latter half of the week as a trough or
potential tropical disturbance moves near the islands. Shower
coverage will also increase, along with the potential for isolated
thunderstorms, particularly across the eastern waters.

Surf along south facing shores will remain above average over the
next couple of days as a long period SSW swell holds relatively
steady. This swell will then gradually fade heading into mid
week. A slightly smaller, long-period south-southwesterly swell
will fill in late this week into next weekend generated by a
series of lows near New Zealand. West facing shores will
experience a bit of the southerly wrap through most of the week.

Breezy trade winds will maintain moderate surf along east facing
shores for the next several days. Surf along north facing shores
will remain flat to tiny through Monday, but will see a slight
bump Tuesday into Wednesday as a small, medium-period northwest
swell fills in from an Aleutian low. Then a small, long-period
west-northwest swell generated by Tropical Storm Dujuan as it
curves northeast near Japan and potentially strengthens into a
hurricane will build locally Friday into next weekend, increasing
surf along north and west facing shores.

&&

.FIRE WEATHER...
Issued at 301 AM HST Mon Sep 21 2026
Breezy trades continue through the week, likely strengthening this
weekend. However, dewpoints and subsequent RH values will remain
elevated throughout this time. The inversion hovers around 6kft
this morning but will rise over the eastern end of the state as
deeper moisture entrained in the trades works into the area.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Tuesday for Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters-
Maalaea Bay-Pailolo Channel.


&&

$$

DISCUSSION...JVC
AVIATION...DT
MARINE...Walsh