Area Forecast Discussion for Hawaii

Area Forecast Discussion   Issued: 10/08/2026 03:59:00 PM UTC

                        
480
FXHW60 PHFO 080559
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
759 PM HST Wed Oct 7 2026

.SYNOPSIS...
Strengthening low pressure roughly 750 miles north of Kauai will
produce increased south winds through Thursday and will draw deep
tropical moisture over at least the western half of the island
chain. The threat of heavy flooding rainfall and thunderstorms
will be greatest tonight and Thursday over Kauai and Oahu, where
a Flood Watch is in effect. Chances for heavy rain will diminish
on Friday as the low weakens several hundred miles northwest of
Kauai. The low will slowly fill Saturday and Sunday, though a few
heavy showers may linger through the weekend under southeast to
east winds.


&&

.SHORT TERM UPDATE...

Forecast in excellent shape so far this evening. Slug of deep
tropical moisture is making efficient inroads into the area as it is
drawn north by a digging upper low west of Kauai. So far, radar
indicates a wide coverage of poorly organized moderate to locally
heavy showers from Oahu to Kauai with the greatest coverage further
west. The most recent radar trends show an accelerating expansion of
shower coverage around Kauai and south of Oahu. Some signs of
increased organization are noted southwest of Kauai as well. These
are likely signs that the maturing right entrance region is
beginning to exert its influence as a mature jet-front system
develops over the western end of the state through tonight. The
primary limiting factor for further organization is a lack of
surface convergence as emerging deep frontal features will be washed
out in the boundary layer. With that said, deep layer forcing should
be sufficient to maintain a fairly organized band of heavy rain over
Kauai with continued widely scattered/locally heavy showers over
Oahu.

Going forward, guidance varies in their depictions of exactly how
progressive this band will be and whether it will impact Kauai,
Oahu, both, or simply remain in the channel. It is evident at this
juncture that it will impact Kauai, the primary uncertainty being
the time at which it will advance eastward. The high res guidance,
which can be prone to erroneous eastward propagation, is expectedly
more progressive. If organized heavy rain can remain tied to larger
scale forcing, then suspect it will be rather stationary over Kauai
through much of the night before advancing eastward coincident with
mid-level troughing during predawn hours potentially affecting Oahu
through Thursday morning. In advance of this feature, a wide
coverage of locally heavy showers will persist and still may
potentially affect Maui County during Thursday. Potential for a
spatial expansion of the Flood Watch will be considered for the
morning package, but is not warranted at this time.

&&

.PREV DISCUSSION...
Issued at 323 PM HST Wed Oct 7 2026
The atmosphere is destabilizing this afternoon as deepening low
pressure north of the state produces increased southerly winds.
The main surface low is forming along a front roughly 750 miles
north of Kauai, leading to light to moderate south to southeast
winds and increasing humidity with dew points surging into the
lower 70s over the islands this afternoon. Meanwhile, a sharp
upper level trough is digging toward Kauai from the west, and a
short wave rounding this upper level trough is rapidly
destabilizing the atmosphere, especially over Kauai and Oahu. The
southerly flow is also tapping moisture surrounding a tropical
disturbance a couple hundred miles south of Kauai and drawing it
northward. As of this writing, only scant rainfall has occurred
over land today, but the leading edge of the heavy showers is
pushing into the coastal waters south of Kauai and Oahu. Expect
these showers to reach Kauai, and possibly Oahu, before sunset.

Heavy showers will develop across the western half of the island
chain this evening and persist into Thursday, bringing a flood
threat focused from Kauai to Oahu where a Flood Watch is in
place. The aforementioned upper level shortwave will swing over
Kauai tonight through Thursday afternoon as a jet stream entrance
region along the eastern flank of the low provides moist ascent
over the western half of the island chain. This will coincide with
deep moisture, having precipitable water in excess of two inches,
moving over the islands from the south. As a result, expect heavy
showers and thunderstorms to move over Kauai and Oahu from the
south this evening and maintain a flood threat through at least
Thursday morning. The main concern in this flow would be heavy
training showers over leeward areas that have a lower threshold
for flood impacts, though the entirety of Kauai and Oahu are at
risk of flooding rain. Maui County will be on the outer edge of
the zone of concern for heavy showers, and even though it is not
included in the Flood Watch, there is some chance of impacts,
especially on Molokai and Lanai. The flood threat diminishes
significantly on Big Island. Southeast slopes of the Puna and Kau
Districts could experience a brief period of heavy rainfall, and
spotty showers will be possible elsewhere on the island, mainly in
the afternoon. While gusty downsloping winds to the north of
terrain are typically a concern in this setup, the flow does not
appear to be strong enough for gusts to reach the advisory level
of 50 mph.

The low will stall and begin to gradually weaken roughly 800
miles northwest of Kauai on Friday. The jet stream on its eastern
flank will retreat northwest of the islands. This will reduce
upper level support for widespread heavy rainfall. However, light
southeast winds will persist and rich but shallow moisture will
remain pooled over the islands. Under these humid conditions with
dew points hovering in the low to mid 70s, isolated pockets of
heavy showers could still flare up, especially along terrain. At
this time, an extension of the Flood Watch is not planned, though
it is possible.

Guidance suggests light east to east-southeast will gradually push
the high precipitable water air mass westward during the weekend
as the low fills northwest of Kauai. Chances of heavy rainfall
will diminish, though some isolated nuisance flooding will still
be possible near terrain. In addition, this should bring some
relief from the humidity as dew points slowly lower.

Guidance divergences somewhat early next week. The GFS clears out
the low and ushers in a moderate trade wind flow, but the ECMWF
maintains some lingering remnants of the low north of the state
and keeps the trade wind flow very light.

&&

.AVIATION...

Light southerly winds will increase in speed and become more
southerly through Thursday due to a surface trough approaching
from the west. Increasing showers and isolated thunderstorms are
forecast tonight and Thursday as an area of enhanced moisture
moves up from the south and instability increases. MVFR conditions
likely and IFR conditions possible within heavier showers and
thunderstorms. VFR prevails elsewhere.

AIRMET Sierra is in effect for tempo mountain obscuration across
all of Kauai and south through west facing portions of Oahu,
Molokai and Lanai. This AIRMET will likely be needed for portions
of the island chain through Thursday.


&&

.MARINE...
Issued at 323 PM HST Wed Oct 7 2026
Gentle to moderate south to southeast winds will continue over
the next couple of days as a low pressure system develops north
of Hawaii. Enhanced showers with the potential for a few
thunderstorms are forecast for the western half of the state
through Thursday. A surface trough will linger near the area
through the weekend and early next week keeping potential for
variable winds and unsettled weather.

South swell will maintain into next week with pulses of long
period reinforcement arriving Thursday and Saturday. Surf will
stay below advisory levels along south facing shores.

Small to moderate surf along north and west facing shores will
continue through the rest of the week as swell energy is
reinforced by the slowly moving surface low northwest of the
Hawaiian Islands. A larger swell will arrive around Tuesday,
bringing north and west shore surf to just below advisory
thresholds. Surf along east facing shores will remain small as
local winds remain fairly weak.

&&

.FIRE WEATHER...
Issued at 323 PM HST Wed Oct 7 2026
Moist and humid conditions under moderate south winds will develop
overnight and persist through Thursday. Chances for widespread
rainfall diminish on Friday while humid and light southeast winds
prevail. Light to moderate easterly trade winds are expected this
weekend. The inversion will be wiped out by evening and will
likely not redevelop until the weekend.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Thursday afternoon Niihau, Kauai, and Oahu.
Wind Advisory until 4 PM HST Thursday for Big Island Summits.


&&

$$
SHORT TERM DISCUSSION...JVC
DISCUSSION...Wroe
AVIATION...Farris
MARINE...Foster
FIRE WEATHER...Wroe