Flash Flood Warning 20, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
859 PM HST Fri Mar 20 2026

.SYNOPSIS...
In the big picture, two surface low pressure systems will continue
move slowly northeastward through the northern Hawaii region into
the weekend, producing additional periods of moderate to heavy
rain, thunderstorms, and elevated threats for flash flooding. A
combination of low level forcing and passing upper level troughs
will drive these smaller scale heavy rain band and thunderstorm
formations. Additional weather threats include strengthening south
to southwest kona winds on Saturday that will produce down
sloping wind gusts in the 30 to 45 MPH range along steep north and
east slopes of island mountain ranges lasting through Saturday
night. Improving weather trends will develop from west to east on
Monday as we transition to a wet trade wind weather pattern
lasting through the end of next week.

&&

.SHORT TERM UPDATE...
Diving into the smaller scale (mesoscale) analysis this evening,
we see a long lived low level convergence band just east of Oahu,
this band is drifting slowly eastward. A very unstable atmosphere
coupled with increasing forcing aloft from an upper level
subtropical jet stream will cause heavy rain and thunderstorms to
fill in along this low level band through the overnight hours and
through Saturday. Some caution here additional heavy rain bands
may yet develop any other island. As this current convergence band
continues to drift eastward the primary heavy rain threats in the
short term will increase over the islands of Maui County into
Saturday. Saturday night through Sunday the large scale flooding
threats increase over the Big Island of Hawaii. Hourly rainfall
rates of 2 to 3 inches are likely, even higher hourly rainfall
rates are possible under developing thunderstorms. Confidence is
growing on this forecast solution as the latest satellite imagery
trends supports this conclusion.

This is a dangerous life threatening weather pattern evolving
across the state of Hawaii through Sunday. Flash flooding is the
primary threat with damaging wind gusts possible. We will closely
monitor these small scale heavy shower and thunderstorm bands as
they develop for flooding potential across the state. Island by
island forecast impacts of these rapidly developing small scale
features are typically more predictable on an hour by hour basis.
Longer range forecasts of these smaller scale bands are extremely
challenging. Overall additional widespread rainfall amounts in the
larger scale through Sunday statewide remains in the 3 to 8 inch
range with the understanding that these smaller scale convective
cloud bands can double or sometimes triple these ranges for some
localized areas of each island.

&&

.PREVIOUS DISCUSSION...
Issued at 342 PM HST Fri Mar 20 2026

This afternoon radar (as of 3:40 PM HST) shows a large area of
moderate to heavy showers over Maui County, and scattered showers
over Kauai and Oahu. Light showers are expected to move into
northwest Big Island this evening. Winds were generally out of the
south at speeds averaging 10 to 20 mph with gusts 20 to 30 mph.
Flood warnings are still in effect on Oahu, with flood advisories
on Maui County. Rainfall amounts on Oahu in the past 6 hours were
generally in the 2 to 6 inch range, with a maximum of 8.58 inches
at Wahiawa.

The forecast remains largely unchanged, with a flood watch in
effect for all areas except the peaks on the Big Island through
Sunday. Periods of showers, heavy at times, will continue to move
through the islands. Southwest flow aloft will remain in place
thanks to an upper level trough to our west. Smaller "pieces" of
energy will move through this trough and occasionally boost the
shower activity, possibly leading to isolated thunderstorms. The
exact location of these increased showers is impossible to know
ahead of time (beyond the state of modeling science out here in
the middle of the ocean). Therefore, folks should be aware that
flooding will continue to be possible in all areas where the flood
watch has been issued.

Forecast models are reasonably consistent in keeping the axis of
heaviest rainfall over the central islands tonight and Saturday,
then shifting it slowly eastward Saturday night into Sunday. This
will slowly take the risk of flooding eastward with it.
Some drying will occur from Monday into Wednesday as trade winds
become established over the state, but windward areas will see
numerous light to moderate showers. Winds are expected to be a
little weaker both Saturday and Sunday (in most areas) than they
have been today. The exception will be the peaks on the Big
Island, where winds will approach wind advisory criteria Saturday
evening through Sunday morning. Several inches of snow are likely
to fall on the summits Saturday night into early next week.

The long range forecast shows the possibility of another trough
moving into the area from the northwest late next week. At this
time, the setup is not a kona low system, but could push a cold
front through the state. Details will become available as we get
closer in time.

&&

.AVIATION...
Bands of rain with embedded heavy showers and storms are expected
to continue over Oahu and Maui County through the night. These
rainbands are producing periods of IFR to LIFR CIGS/VSBYS, which
will persist into the weekend as an upper level trough approaches
from the west. Kauai and Big Island are expected to be mostly VFR
through tonight, with MVFR conditions possible within an
occasional shower.

Low-level winds continue to be out of the S-SSW, and may veer more
SW over the western half of the state on Saturday. Winds are
strongest over Maui county, with gusts reported up to 39 kt at
PHOG. These winds may result in localized mechanical turbulence
leeward of terrain. Winds will remain gusty through the evening,
but gradually weaken through the weekend.

AIRMET Sierra for mountain obscuration is in effect across all
islands except Kauai and the Big Island, and will likely continue
into the weekend. AIRMET Tango is in effect for moderate
turbulence downwind of terrain for Oahu and Maui county below 090.
AIRMET Zulu for light icing in the 140-FL260 layer will continue
to be a concern within deeper cloud layers associated with the
upper level disturbance.

&&

.MARINE...
Issued at 342 PM HST Fri Mar 20 2026

Light to moderate S winds may strengthen tonight as surface low
pressure west of Kauai deepens. Moderate to locally strong S/SW
prevail through Saturday, and periods of heavy showers and
thunderstorms are expected. The low will lift north on Sunday
allowing moderate N winds to develop around Kauai which will then
veer to NE and spread south. Fresh to strong NE winds may require
a Small Craft Advisory over most waters.

N swell will increase into Saturday in maintenance of moderate
swell through the weekend along N facing shores. A small,
overlapping WNW swell then arrives late Saturday and holds through
Monday. Surf along E shores remain well below average through the
weekend. NE trades then develop early next bringing increase NE
fresh swell and an increase in short period choppy surf which may
approach the High Surf Advisory threshold.

A moderate S swell slowly diminishes through today followed by a
small reinforcing SSW swell this weekend into Monday. Smaller
surf is expected along south facing shores through the remainder
of the week.

&&

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

Winter Storm Watch from Saturday evening through Sunday
afternoon for Big Island Summits.

&&

SHORT TERM DISCUSSION...Bohlin
PREVIOUS DISCUSSION...Parker
AVIATION...Evans
MARINE...JVC