913
FXHW60 PHFO 171336
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
336 AM HST Mon Nov 17 2025

.SYNOPSIS...

Moderate trade winds will favor showers across windward and mauka
areas today, while an upper level low just southwest of the
islands pulls a swath of high clouds overhead. The upper level
low will strengthen Tuesday afternoon into Wednesday and bring an
increased chance of heavy showers, a few isolated thunderstorms
on increasing trade winds, with flood potential increasing over
the eastern end of the island chain. Chances of heavy rainfall
will diminish late Wednesday into Thursday with breezy and
somewhat wet trade wind weather expected. Another round of
unsettled weather is possible next weekend.

&&

.DISCUSSION...

Looking at radar and satellite imagery, there are scattered
showers ongoing across the whole island chain, with high clouds
streaming in from the southwest and low clouds riding in on the
trades. An upper level low centered just southwest of the island
chain will continue to pull the shield of high clouds over the
area from the south. With the low eroding the mid level ridge
overnight and the elimination of the inversion, enhanced showers
in the trades continues across the western half of the island
chain this morning. Expect trades to diminish somewhat into early
Tuesday as the atmosphere continues to destabilize and a cold
front sinks closer south, pushing the weaker subtropical ridge far
northeast of the state.

The upper level trough driving the front will begin to absorb the
upper level low just southwest of the state. The resulting
negatively titled upper level trough will produce difluence aloft
within a narrow jet stream over the islands. The greatest forcing
associated with this jet will likely be just south of the Big
Island, where an area of deep tropical moisture will be drawn
northward.

The GFS and ECMWF ensemble guidance suggests that the deepest
moisture and greatest threat of heavy rainfall will remain south
of the islands through at least midday Tuesday, with chances for
rainfall along southeast and windward Big Island increasing during
the afternoon. Have elected to remove scattered thunderstorms
from the previous forecast as uncertainty is to high. Closer to
the colder temperatures aloft, along the upper level trough axis,
an isolated thunderstorm or heavy shower cannot be completely
ruled out across the other islands. The highest chances for heavy
rainfall favors late Tuesday afternoon or early evening into
Wednesday, though differences in model guidance are producing some
uncertainty. The wetter GFS is maintaining a more persistent
upper level trough, while the ECMWF depicts a weaker upper level
trough being absorbed more quickly into the large upper trough to
the west. Both models suggest that the front will dissipate just
north of the islands and that trade winds will ramp up as strong
surface high pressure passes far to the north.

Under this breezy, moist, and unstable trade wind flow, windward
and mauka areas will be the focus for heavy rainfall, while
leeward areas will experience brief heavy and potentially
frequent passing showers. The greatest threat for flooding
continues to point toward the eastern end of the island chain,
mainly on Big Island, where PWATs are in the 2in range. The GFS
Ensemble 90th percentile solution shows 24 hour rainfall totals of
around 5 to 7 inches. Given the model differences regarding
potential rainfall totals, confidence is not high enough to issue
a Flood Watch at this time.

Along with the flood threat, the high summits of the Big Island
could experience periods of heavy snow and strong winds, however the
the million dollar question will be the snow level, which will be
hovering close to summit elevation. Holding off on any winter
weather headlines at this time also due to the uncertainty.

The heavy rainfall threat will decrease during the day on
Wednesday. The strong surface high passing to the north will
maintain breezy trades, and even though PWATs will be decreasing,
the GFS and ECMWF keep abundant moisture around the islands,
pointing to a rather wet pattern for Thursday.

A brief period of stable and somewhat drier trade wind conditions
is possible on Friday, followed by another round of potentially
active weather this weekend. Guidance is not in agreement on the
evolution, timing, location or intensity on the next system but it
bears watching.

&&

.AVIATION...

Moderate trades will veer slightly east southeast today while
weakening over the western half of the state. With weaker winds
local land/sea breezes look to set up locally. High clouds are
building in over the eastern end of the state and will eventually
produce widespread overcast conds across the entire state. Where
the trades are a little weaker some leeward and interior isol SHRA
are possible as well as where winds are a little stronger some
windward and mauka SHRA are possible. Low cigs along within any
SHRA could produce MVFR conds but for today VFR looks to prevail.
Wetter weather is coming Tuesday evening through at least Thursday
and may impact some sites with rain and sub-VFR conds.

AIRMET Sierra is in effect for mtn obsc for windward portions of
Kauai, Oahu and the Big Island. Conds expected to improve later
this morning. This AIRMET may be needed over the next few days.

AIRMET Tango may be needed later this afternoon into Tuesday for
turb in the mid to upper levels with this next weather system
getting closer.

AIRMET Zulu may be needed later today into Tuesday for some light
icing due to the influx of high clouds.

&&

.MARINE...

A high pressure ridge north of the islands will continue to drift
to the east as a front to the northwest moves to the east. This
will weaken the trade winds near the islands and cause the winds
to veer from a more east to southeast direction. The front is
expected to enter the northwest offshore waters Tuesday, before
weakening and lifting to the north. Near gale-force winds are
possible behind the front. The upper level trough associated with
the front will help to bring the possibility of thunderstorms to
the central and northern offshore waters into the middle of the
week. Additional thunderstorms are possible over the southern
offshore waters due increased moisture being drawn northward on
the east side of the upper level trough mentioned above. A
surface trough forming over the eastern end of the state on
Tuesday will bring the possibility of increased rain over the
eastern end of the state.

Behind the front, a new high building north of the islands will
help trades to strengthen. Combined with incoming northwesterly
swells, expect winds and seas to exceed Small Craft Advisory
thresholds for exposed coastal waters during the second half of
the week.

As alluded to above, there are overlapping northwest swell
expected this week that will keep surf heights boosted along
exposed N and W facing shores. The challenge with this pattern
will be in tracking the multiple swell energies from similar
directions and similar periods. Wave height model guidance has
trended towards combining these different swells for the Monday
through Friday time period. Continued to decrease the NW swell
heights from model output by a few feet with this forecast package
to account for this discrepancy.

A small medium period NW (310-320 degree) swell will continue to
slowly decline today, and the next moderate, medium to long
period NNW (320-340 degree) swell will arrive late Tuesday,
peaking Wednesday near High Surf Advisory thresholds along exposed
N and W facing shores before gradually declining through the end
of the week. Another overlapping small, medium period NNE (010-020
degree) swell arrives from Thursday into Friday. Then another
small, long period NW (320-330 degree) swell arrives in Hawaiian
waters by Friday, then holds through the weekend.

Surf along E facing shores will continue to decline today due to
the weakening of the local and upstream trade winds. As the high
pressure ridge moves east later this week, the combination of a
long easterly fetch of 15 to 20 kt winds upstream of the islands
from Tuesday through Thursday, along with strengthening local
winds along the cold front, will produce moderate and choppy surf
along E shores. For S shores, periods of tiny background south
swell energy will linger.

&&

.FIRE WEATHER...

Moderate trades and higher humidity will maintain conditions below
critical fire weather thresholds. Wet conditions are expected
Tuesday into Thursday, with heavy rainfall possible over recent
burn areas on the Big Island.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$


DISCUSSION...Walsh
AVIATION...Walsh
MARINE...M Ballard
FIRE WEATHER...Walsh