Area Forecast Discussion for Hawaii

Area Forecast Discussion   Issued: 09/18/2026 05:03:00 AM UTC

                        
367
FXHW60 PHFO 171903
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
903 AM HST Thu Sep 17 2026

.SYNOPSIS...

Moisture associated with the remnants of Norbert will move from
east to west across the state through Friday and bring an
increase in shower activity, especially for windward and mauka
locations. Some showers will be heavy at times with a slight
chance of thunderstorms. A Flood Watch remains in effect for all
the main Hawaiian islands through early Friday morning. Drier air
will move back in this weekend behind the departing trough.
Moisture from the remnants of Tropical Depression 15-E may bring
another round of enhanced trade showers by mid next week.


&&

.DISCUSSION...

Morning satellite imagery shows clusters of deeper convection and
thunderstorms along an upper level trough axis, very slowly
advancing towards the eastern part of the state. Two such clusters
are flanking the north and south sides of the Big Island, with a
third cluster further north, well west of Kauai. This trough will
continue to move west across the state today, bringing with it the
remnant moisture from former tropical cyclone Norbert. This
moisture has already moved into the Big Island and Maui based on
satellite imagery, confirmed by the 12Z sounding at Hilo which had
2.58" of precipitable water. Weak to moderate instability (858
J/kg of surface-based CAPE on the 12Z Hilo sounding) moving west
across the state today will continue the threat of heavier showers
and thunderstorms, especially on the windward and mauka locations
and the Kona slopes on the Big Island. A Flood Watch remains in
effect for all main Hawaiian Islands through 6 AM Friday.

The trough will slowly depart to the west of the state on Friday,
but some residual moisture may be enough for heavier showers to
stick around near Kauai. Weak instability developing in the
afternoon could lead to another round of enhanced showers and
isolated thunderstorms Friday afternoon over leeward slopes of the
Big Island.

Saturday through Monday, moderate trades will help filter drier
air back into the lower levels and reduce the shower coverage to
scattered during mainly the late night/early morning hours. The
aforementioned upper level trough, or possibly weak upper level
low, will sit stagnant near Kauai and help advect some mid and
upper level cloudiness over the state from the southwest.

Tuesday through Thursday, there is yet another potential push of
deeper moisture from the remnants of tropical depression 15-E.
Both the GFS and ECMWF show a low level wave associated with this
moisture passing near or over the islands. This event is still
well into the long range forecast, so confidence is rather low.
However, it will still need to monitored closely for the potential
of more heavy rain.


&&

.AVIATION...
An approaching easterly trough, the remnants of TC Norbert, will
be passing over the islands through Friday. This trough is pulling
in a more moist air mass. This will result in a greater frequency
of showers with brief periods of moderate intensity rain. Decks
will lower tempo into IFR/MVFR categories during times of more
intense rainfall. Typically breezy trades will persist through the
period with wind-sheltered area breezes remaining light and
variable.

AIRMET SIERRA remains in effect for island-wide, windward exposed
mountain obscuration near or above 2,000 ft.


&&

.MARINE...

A broad trough, associated with the remnants of Norbert, will
allow moderate to locally strong northeast trades to hold through
today. A Small Craft Advisory (SCA) for the typical windy waters
around Maui and the Big Island is in effect. This SCA may be
expanded to include other areas in the coming days. Above normal
moisture and upper level support will allow isolated thunderstorms
with localized heavy rain and gusty outflows to develop over the
offshore waters through Saturday. A surface high far northeast of
the state could allow moderate to locally strong trades to return
over the weekend.

A series of long to medium period south-southwest swells will
taper off through tonight. A reinforcing southerly swell will
fill in early Friday bumping surf near High Surf Advisory
thresholds.

Surf along east-facing shores will increase today as trade winds
strengthen and a small swell from Norbert continues. Flat to tiny
surf along north facing shores is expected through the forecast
period.



&&

.FIRE WEATHER...
Fire weather risk will remain low through Friday due to moderate
to breezy winds and increased relative humidity and moisture from
the remnants of Norbert. Showers will decrease by this weekend.
Inversion heights will be elevated or very weak.


&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through late tonight for Big Island East-Big Island
Interior-Big Island North-Big Island South-Big Island Southeast-
Big Island Summits-Central Oahu-East Honolulu-Ewa Plain-
Haleakala Summit-Honolulu Metro-Kahoolawe-Kauai East-Kauai
Mountains-Kauai North-Kauai South-Kauai Southwest-Kipahulu-
Kohala-Kona-Koolau Leeward-Koolau Windward-Lanai Leeward-Lanai
Mauka-Lanai South-Lanai Windward-Maui Central Valley North-Maui
Central Valley South-Maui Leeward West-Maui Windward West-
Molokai Leeward South-Molokai North-Molokai Southeast-Molokai
West-Molokai Windward-Niihau-Oahu North Shore-Olomana-South
Haleakala-South Maui/Upcountry-Waianae Coast-Waianae Mountains-
Windward Haleakala.

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


&&

$$

DISCUSSION...SS
AVIATION...Phillips
MARINE...Foster
FIRE WEATHER...SS