Flash Flood Warning 33, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
854 AM HST Wed Apr 8 2026

.SYNOPSIS...
Low pressure west of the islands will draw tropical moisture
northward leading to periods of heavy rainfall, thunderstorms, and
an increased flash flooding risk. Strong and gusty south to
southeasterly winds are also expected Thursday and Thursday night.
Unsettled conditions linger into next week.

&&

.SHORT TERM UPDATE...
Issued at 852 AM HST Wed Apr 8 2026

Adjusted forecast precip chances today into night based on current
observational trends and morning hi-res guidance. Expect the first
wave of showers to lift into the area from the south mid/late
afternoon today, heaviest over Kauai and Oahu where instability is
maximized. SE Big Island received several inches of rain last
night as winds aloft funneled moisture into that area where an
assist from local terrain allowed continued regenerative showers.
Backing and weakening wind profile tonight suggest diminished
potential, though showers may stream onshore at times through the
night. Suspect the mountain breeze eventually wins out.

The latest guidance is still trending stronger with the ambient
wind field Thursday and Thursday night as low pressure deepens
around 30N and the local gradient tightens considerably. Difficult
to diagnose downsloping potential with surface and 925mb winds
out of the south as opposed to southwest which generally targets
areas like Windward Oahu. The latest hi-res guidance does indicate
downsloping potential over northern portions of Kauai and Oahu.
Given 925mb winds easily in excess of 40 kts, it should not be
difficult to achieve High Wind Warning thresholds (gusts to 50
kts/58 mph) should downsloping materialize. A High Wind Watch has
therefore been issued for select zones of those islands. Potential
expansion to Molokai is not out of the question. Elsewhere on
Oahu and Kauai, the Wind Advisory threshold (gusts to 50+mph) but
confidence in mixing down sufficient wind gusts without any
inversion or subsidence in the column is quite low at this time.
Finally, a second area of concern for High Wind Warning potential
is immediately west of the ridge on SW Big Island in the vicinity
of Milolii in the South Kona District of the Big Island. Potential
here will be evaluated for future forecast packages.

Big Island Summits Winter Wx Advisory is a bit tenuous as
expected, but remains in good shape for now. Will likely issue a
Wind Advisory for the BI Summits this afternoon for tomorrow into
Friday.

&&

.PREV DISCUSSION...
Current satellite imagery this morning shows an area of low
pressure beginning to develop west of the islands. A potent upper
level trough to the northwest is currently moving southeast and
will enhance the surface trough to the west through the next few
days. Heavy showers and thunderstorms can be seen mostly around
east Maui and southeast Big Island, as well as the waters south of
Maui.

South to southeast winds will strengthen today, pulling deep
tropical moisture northward over the islands with precipitable
water values around 2". An upper level trough will also move
closer to the state and enhance large- scale lift. As a result,
widespread heavy rain is expected to gradually increase through
the day statewide. A Flood Watch is in place for all islands from
this morning through Friday afternoon. A Winter Weather Advisory
is also in effect this morning for a combination of freezing rain
and snow on the summits of the Big Island above 12,500 feet,
lasting through Friday morning.

There has been increasing focus on the need for a Wind Advisory
or a High Wind Watch beginning Wednesday night as southerly winds
begin to ramp up. These winds will be especially strong strong in
and around heavier showers and thunderstorms, and in the vicinity
of mountain ranges where downsloping gusts over northern and
windward slopes can occur. Severe wind gusts (60+ mph) look increasingly
likely with latest high resolution model runs. Late tonight
through Thursday morning could see advisory level winds over
Kauai and Oahu (30-39 mph) and then possibly high wind warning
levels (40+ mph) by Thursday afternoon through Friday morning.
Future shifts will monitor and refine the situation as the time
period approaches.

Thursday night through Friday night. Intense upper-level forcing
develops. The combination of right entrance dynamics and curved flow
will maximize upper-level divergence over the western end of the
state by Thursday evening. A mature jet-front system takes shape
west of Kauai as this forcing rapidly organizes and ramps up and
showers and thunderstorms along the frontal zone begin to
intensify. The heavy rain then surges eastward Thursday night
bringing the heaviest rainfall to Kauai late Thurs night into
early Friday, Oahu during the first half of Friday, and Maui
County and the Big Island during the second half of Friday.

High-end advisory winds are likely on the Big Island Summits and
possibly Haleakala Summit on Maui Thursday night through Friday.
Snow potential on the Big Island Summits will be limited by marginal
temperatures.

Saturday onward.
The resident band of tropical moisture will likely linger over
the islands for the foreseeable future, although a short break in
rain is anticipated by Saturday as drier air briefly moves in.
More periods of rain are possible through the remainder of the
weekend into next week as more upper level disturbances track over
the state. This may yield periods of localized heavy rain, but
that remains to be seen.

&&

.AVIATION...
Issued at 852 AM HST Wed Apr 8 2026

Latest radar imagery shows rain showers affecting mainly Maui and
the Big Island this morning. While dry conditions prevail across
the islands west of these two, areas of low clouds have been
observed thanks to recent rainfall and light winds. Expect a
mixture of VFR/MVFR/IFR conditions for most of today. Weather
models suggest another round of widespread showers and isolated
thunderstorms this afternoon and evening. Some will be heavy.

AIRMET Sierra is in effect for mountain obscuration on the
southeast portions of the Big Island. Other islands are expected
to be added today. AIRMET Zulu is in effect for icing on Big
Island, and this also may be expanded to other islands today.

&&

.MARINE...
Issued at 852 AM HST Wed Apr 8 2026

South to southeast winds are expected to steadily increase late
today into Thursday, as a low pressure system develops just west
of the state. A Small Craft Advisory (SCA) is now in effect for
the waters around the Big Island and the SCA will expand in
coverage tonight as the winds continue to strengthen. Showers,
some locally heavy, will be possible through Friday as a result of
the southerly winds bringing tropical moisture to the north over
the islands. As the low pressure system further deepens Thursday
into Friday, we will likely see a greater coverage of
thunderstorms and winds are expected to increase to gale force
across the coastal waters. A Gale Watch has been issued starting
Thursday for all Hawaiian waters. Depending on how the low
pressure system evolves, Gale force winds could be possible by
Thursday night. Very rough boating conditions are expected late
Thursday into Friday as seas quickly rise in response to the
strong to gale force southerly winds.

The current south-southwest swell continued to fill in overnight
and latest near-shore buoys are at around 4 feet 17 to 18 seconds
this morning. We should see this swell continue to fill in a bit
more throughout the day today and slowly drop tonight through the
rest of the week. A High Surf Advisory has is in effect for all
south facing shores of the Hawaiian Islands through tonight.

Along north facing shores, a new moderate long-period northwest
(310-320 degree) swell continues to fill in this morning and will
steadily rise throughout the day today. This swell should peak
just below advisory thresholds later today. A smaller short-period
northwest swell is expected over the weekend into early next week.

As the long-period south swell declines on Thursday, strong to
gale force southerly winds are expected to develop on Thursday
and strengthen even further Thursday night into Friday. This
increase of southerly winds will bring a rapid increase of short-
period energy Thursday into Friday and will once again bring surf
heights above the advisory thresholds for south facing shores.
Large disorganized surf is expected along many south facing shores
Thursday into Friday.

Surf along east facing shores could see an increase later this
week due to the strengthening southeasterly winds especially for
southeast exposures.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch through Friday afternoon for all Hawaiian Islands.

High Surf Advisory until 6 AM HST Thursday for south facing shores
of all Hawaiian Islands.

High Wind Watch from Thursday morning through late Thursday
night for Central Oahu-Kauai Mountains-Kauai North-Koolau
Leeward-Koolau Windward-Oahu North Shore-Olomana-Waianae
Mountains.

Winter Weather Advisory until 6 AM HST Friday for Big Island
Summits.

Small Craft Advisory from 6 PM this evening to 6 AM HST Thursday
for Kauai Channel-Kauai Leeward Waters-Kauai Northwest Waters-
Kauai Windward Waters-Oahu Leeward Waters.

Gale Watch from Thursday morning through late Thursday night for
all nearshore waters.

Small Craft Advisory until 6 AM HST Thursday for Big Island
Leeward Waters-Big Island Southeast Waters-Big Island Windward
Waters.

&&

SHORT TERM UPDATE...JVC
DISCUSSION...Tsamous
AVIATION...Parker
MARINE...Walsh