Flash Flood Warning 40, NWS Honolulu, HI

Area Forecast Discussion
National Weather Service Honolulu HI
950 AM HST Fri May 15 2026

.SYNOPSIS...
Moderate to locally breezy trade winds will continue tonight,
with periods of showers favoring windward mountain areas. An upper
level low will briefly move over the region today and tonight,
enhancing threats for locally heavy rainfall and isolated
thunderstorms. Expect periods of heavy showers along with isolated
to scattered thunderstorms today across the islands in Kauai
County, Oahu, and Maui County. A wet trade wind weather pattern
however will likely continue through Sunday morning for most
areas. More typical trade wind weather will return from late
Sunday into the end of next week with brief passing showers in the
forecast as moderate to locally breezy trade winds continue to
blow across the region.

&&

.SHORT TERM UPDATE...
Strong showers and a few thunderstorms continue to develop ahead
of and under the approaching upper low. Heavy rain is occurring,
but so far the precipitation is moving NNE along at around 15 mph,
so there is a relatively low chance for flooding at this time.
However, the latest guidance is consistent with a continued
threat of flooding through today, and the flood watch will remain
in effect unchanged. There is a good chance that it will be
extended through tonight in some areas, and Big Island may need to
be added. These decisions will be made in the afternoon forecast,
giving us time to fully include the latest data over the next few
hours.

&&

.PREV DISCUSSION...
Issued at 301 AM HST Fri May 15 2026

An unstable upper level low continues to drop into the islands
from the north this morning with thunderstorms forming less than
100 miles north of Niihau, Kauai, and Oahu. Expect periods of
heavy showers along with isolated to scattered thunderstorms today
across the islands in Kauai County, Oahu, and Maui County. A
Flood Watch was issued through this afternoon for these islands
due to the heavy rain and flooding threats across most of the
state. This watch may need to be extended into the overnight hours
for some areas as this upper low pivots over Kauai, then weakens
and drifts eastward across the state.

Unstable cold air aloft within this upper level low will enhance
shower activity statewide, showers will become heavy at times
with isolated to scattered thunderstorms. Small scale low level
wind convergence bands and thunderstorm outflows will likely drive
the heaviest shower activity today. Higher rainfall rates will
likely lead to flash flooding in some areas through at least the
afternoon hours and possibly into the overnight time period. This
low will slowly weaken as it drifts eastward on Saturday,
decreasing the threat for heavy rainfall, especially as the low
departs the Hawaii region on Sunday. A wet trade wind weather
pattern however will likely continue through Sunday morning for
most areas.

A broad high pressure ridge remains in place north of the islands
through much of next week. Expect a more typical moderate to
locally breezy trade wind weather blowing across the Hawaiian
Islands from late Sunday into the end of next week. Brief passing
showers will occur favoring the windward mountain areas in the
overnight to early morning hours.

&&

.AVIATION...
Surface trough near Kauai this morning will result in gentle to
moderate trade to ESE winds (90 to 120 degrees) across the state.
Upper low tracking over the western half of Hawaii today will
destabilize the atmosphere producing scattered heavy showers and
thunderstorms, particularly across Kauai and Oahu. SIGMET Zulu is
posted for thunderstorms across waters just north of Kauai and
Oahu this morning. This SIGMET may be moved over either/both Kauai
and Oahu later today and tonight as the upper low digs slowly
southward. The upper low is expected to weaken through the day
Saturday, while shifting heavy rain/thunderstorm threat to Maui
and Big Island.

AIRMET Sierra is in effect for tempo mountain obscurations above
2000 feet for entire area of Kauai and Oahu, and for windward and
southeast slopes of the Big Island. It's possible AIRMET Sierra
could expand to portions of Maui County at some point later today,
but likely the areas in effect now will continue into this
evening. AIRMET Tango is posted for tempo moderate turbulence all
islands below 080, and associated with the jet stream from 150 to
FL380. No changed are anticipated with AIRMET Tango today.

&&

.MARINE...
An upper level low and its surface trough near Kauai will turn
weakening winds more east southeast today and this weekend. A
Small Craft Advisory (SCA) remains in effect through 6 AM HST
Saturday for the eastern half of Hawaiian coastal waters. The
close proximity of the upper low, along with disturbances coming
up around the low from the south southwest, will produce
widespread rain with periods of locally heavy rainfall and
scattered thunderstorms across the nearshore waters. Some of these
thunderstorms could have locally strong winds and small hail, so
marine interests should monitor for Special Marine Warnings in the
nearshore waters throughout the day, particularly across the
western waters.

A small, moderate period north northwest (320-330 degree) swell
that peaked last night is declining this morning, but will
produce near to slightly above head high surf along many north-
facing shores today. This swell will gradually decline into the
weekend.

A mix of small, medium to long period south swells will maintain
small surf along south-facing shores into the weekend. A series
of gale lows passing south of New Zealand should send small south
swell into the southern waters through next week. Surf should
lift to summer averages around Sunday and this will hold through
Wednesday of next week. Locally moderate to fresh trades will
maintain rough and choppy surf along east-facing shores.

Tides will peak around 2.5 ft MLLW this weekend. Combined with
ongoing trades and a small south swell bump, water levels will
peak near 3.0 ft as early as tomorrow. Minor overwash of low lying
coastal areas will be possible during times of high tide.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 6 PM HST this evening for Central Oahu-East
Honolulu-Ewa Plain-Haleakala Summit-Honolulu Metro-Kahoolawe-
Kauai East-Kauai Mountains-Kauai North-Kauai South-Kauai
Southwest-Kipahulu-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-Maui County Leeward
Waters-Maui County Windward Waters-Pailolo Channel.

&&

DISCUSSION...Bohlin
AVIATION...Foster
MARINE...Vaughan