Flash Flooding in Hawaii, April 7 to 10, 2026

Area Forecast Discussion
National Weather Service Honolulu HI
329 AM HST Tue Apr 7 2026

.SYNOPSIS...
A notable shift in the weather pattern is underway today as an
upper-level trough and developing surface low organize northwest
of the islands. This evolving system is expected to pull deep
tropical moisture northward across the state, ushering in a
prolonged period of strong southerly winds, widespread showers
and thunderstorms, and elevated risk of flooding from midweek
through the weekend.

&&

.DISCUSSION...
Latest observations show an upper-level trough just northwest of
the Hawaiian Islands slowly meandering eastward over the next
several days while ridging at the surface lingers northeast of
the state. Finally, a cold front well west of the islands is
forecast to track eastward into the vicinity over the course of
the next day or so. Winds across the islands remains light to
moderate along the windward side of all islands, while even
lighter winds/land breezes hold place along on the leeward areas,
especially along the Kona coastline of the Big Island due to wind
blockage.

A broad surface trough is forecast to develop just ahead of the
aforementioned front prior to its approach, resulting in the
veering of southeast winds to become more southerly across the
Hawaiian Islands. This will cause tropical moisture to be pulled
in northward in the southerly flow. Additionally, a moisture band
associated with the trough will also develop along side of Kauai
and Niihau early this morning, bringing a chance of heavy shower
activity. Guidance also supports some heavier showers and
thunderstorms developing over the interior of most islands, given
the mixture of daytime heating, sea breezes, and the shortwave
trough ahead of the front.

As the upper-level trough exits to the east, tropical moisture
continues being advected poleward and may lead to periods of
showers tonight through Wednesday morning. Upper-level forcing is
forecast to strength again tomorrow as another vigorous shortwave
pivots the base of the previous trough. This may enhance lift over
the region as a new surface low consolidates just west of the
islands. The deep tropical moisture overspreading the state,
combined with the increasing large-scale lifting, supports another
widespread and organized rainfall event for the entire state.
Because of this, a Flood Watch has been issued for all islands
beginning Wednesday morning and lasting through Friday afternoon.

A Winter Storm Watch has also been issued beginning Wednesday
morning for a combination of freezing rain and snow on the
summits of the Big Island above 12,500 feet, lasting through
Friday afternoon as well.

There has been careful consideration for a Wind Advisory beginning
Wednesday night as southerly winds begin to ramp up, especially
in and around heavier showers and thunderstorms, but also in the
vicinity of mountain ranges where downsloping gusts over northern
and windward slopes can occur. Severe wind gusts (58+ mph) may be
possible, though most areas will be in the range of 40 to 50 mph.
Future shifts will monitor and refine the situation in the next 24
hours or so as the pattern evolves.

As the week comes to an end, model guidance seemingly appears to
have come to an agreement compared to the previous run. The
general consensus of the GFS and the ECWMF shows the latest
pattern is expected to continue through the weekend and into early
next week as the moisture band anchors over the state. Longer
range models of the GFS finally show much of the rainfall clearing
most of the islands by midweek next week.

Of important note, and as previous discussions have already
mentioned, one important consideration with this week's rainfall
is antecedent conditions. The islands experienced significant
flash flood events in recent weeks, and despite several days of
drier trade wind weather, soils remain vulnerable in many areas.
As a result, any time period of higher rainfall rates could
quickly lead to enhanced runoff, renewed rises in streams and
reservoirs, and localized flash flooding concerns.

&&

.AVIATION...
Southerly winds have spread across the region, and an area of rain
is spreading over Niihau from the south. This area of rain in
expected to move over Kauai shortly. Additional rain is expected
today, particularly during the afternoon hours. While VFR
conditions will start the day, MVFR and isolated IFR conditions
are possible, particularly this afternoon.

Anticipate AIRMET Sierra being issued soon to account for mountain
obscuration on Kauai. This will likely be expanded to additional
areas later today.

&&

.MARINE...
Southeasterly winds have spread across the islands with a front
stalling to the west of the islands, and a surface right far to
the north. Winds are expected to increase out of the south
tomorrow and Thursday, as a low develops across the stalled front.

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. Thunderstorms have developed in the
offshore waters to the northwest of Kauai, and thunderstorms are
possible over the offshore waters through at least Friday.

A moderate long-period northwest swell will fill in today, giving
more of a boost to north shore surf late today through Wednesday.
A small, short- period north-northeast swell will fill in today
before peaking Wednesday and subsiding Thursday. This overlap will
make for some rough choppy conditions along north facing shores.

A slightly larger, long- period south swell, will increase today
with a more noticeable increase tomorrow. Surf could reach
advisory levels by Wednesday before slowly easing Thursday into
the weekend. Strengthening southerly winds will make for rough and
choppy surf by late next week.

East shore surf will remain small and below the seasonal average due
to a lack of strong trade winds over and upstream of the islands,
except for some wrap from the northerly swells.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch from Wednesday morning through Friday afternoon for
all Hawaii islands-

Winter Storm Watch from Wednesday morning through Friday
afternoon for Big Island Summits.

&&

DISCUSSION...Pierce
AVIATION...M Ballard
MARINE...M Ballard