Area Forecast Discussion for Hawaii

Area Forecast Discussion   Issued: 04/18/2026 11:36:00 AM UTC

                        
775
FXHW60 PHFO 180136
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
336 PM HST Fri Apr 17 2026

.SYNOPSIS...
Moderate trade winds will hold through Saturday, bringing modest
showers to windward slopes. An upper level trough will move
overhead, producing some high clouds and triggering a few heavy
showers and thunderstorms interior Big Island and around Kauai.
Trades will weaken and veer out of the southeast across western
half of the island chain late Saturday and Sunday as low pressure
deepens several hundred miles northwest of Kauai. Another passing
upper level trough will bring renewed chances for isolated heavy
showers Sunday and Monday. Light winds favor chances for mainly
afternoon showers Tuesday into Thursday.

&&

.DISCUSSION...
A slightly unstable trade wind flow is in place this afternoon.
Surface high pressure passing well north of the area has boosted
trades to moderate strength this afternoon, something we have not
seen for some time. Little organized moisture within the trade
wind flow has led to scant rainfall across most areas today. An
upper level trough approaching from the west is producing
increased high clouds and a decrease in stability aloft. Combined
with afternoon heating, this instability was sufficient to
trigger a couple of thunderstorms across the high interior terrain
of the Big Island, though deep convection elsewhere has remained
confined along a weak surface trough 150 to 200 miles northwest
of Kauai.

Thunderstorms will diminish by sundown on the Big Island, but a
few could flare up west of, and possibly over, Kauai overnight as
the upper level trough inches closer to the islands. Elsewhere,
continued moderate trades will focus modest rainfall across
windward slopes.

Only subtle changes are expected on Saturday. The upper level
trough will weaken as it swings over the islands, likely
maintaining enough instability to trigger another round of
showers and thunderstorms over the Big Island interior.
Developing surface low pressure centered several hundred miles
northwest of area should keep the threat of widespread heavy
showers and thunderstorms northwest of Kauai. However, the
developing low may cause the trades to weak and shift south of due
east by afternoon, which could allow for isolated showers, some
potentially heavy, to form over leeward terrain of all islands.
The upper level trough should move off to the northeast overnight,
diminishing the threat for localized heavy showers and focusing
modest rainfall over windward areas.

On Sunday and Monday, trade winds will weaken everywhere and will
shift southeasterly across the western half of the island chain,
while another upper level trough brings instability, mainly Sunday
night into Monday morning. For now, it appears that the deepest
moisture and highest chances for widespread rainfall will remain
along a surface trough northwest of Kauai, and while there will be
some potential for heavy rainfall around Kauai, a Flood Watch
does not seem warranted. Elsewhere, expect additional rounds of
afternoon interior showers and possibly a thunderstorm on Big
Island, and another item to monitor will be a chance for localized
anchored heavy showers along the Koolau Mountains of Oahu Sunday
night. Even though moisture does not appear to be significant on
Oahu, the expected veering flow with height has a tendency to
produce heavy rain events on the Koolau.

Chances for heavy rainfall look to diminish Tuesday into Thursday.
The upper level trough should move off to the east late Monday,
allowing a weak ridge aloft to settle over the state. The GFS and
ECMWF are hinting that the surface trough that had been lingering
northwest of Kauai will be pushed over the western end of the
island chain. This would favor light and variable winds with
higher rainfall chances over island interiors during the late
morning and afternoon hours.

&&

.AVIATION...
Moderate to locally breezy easterly trade winds filled in earlier
today and may linger into tomorrow. However, model guidance
differs on the predominant direction of the wind field for
Saturday. Most of the short term guidance indicates a slight
veering of the winds as a trough approaches from the northwest,
which would lend to a more sea breeze driven pattern, while the
remaining models have easterly trades persisting. With these model
discrepancies, confidence on the wind forecast for Saturday is a
bit lower than normal.

Otherwise, VFR conditions are expected to generally prevail. Brief
light showers will move into windward and mountain areas on the
trades, and afternoon showers and thunderstorms will remain
possible over interior Big Island again on Saturday. If the winds
veer slightly, clouds and showers may also develop over the
interior portions of the smaller islands during the day on
Saturday as sea breezes develop.

No AIRMETs are in effect.

&&

.MARINE...
A weak trough northwest of the state will lift north and away
from the area through the weekend. High pressure then builds
northeast of the area, which will allow trade winds to gradually
strengthen tonight. Moderate to fresh trade winds are expected
across area waters, with locally strong trades over the windier
waters around Maui County and the Big Island. A Small Craft
Advisory (SCA) is in effect for the windier waters through
Saturday. The trough to the north develops into a weak low by
Sunday, with a trailing trough remaining to the west of the main
Hawaiian Islands. This will cause trades to weaken slightly and
veer to the east-southeast. Gentle to moderate east-southeast
winds are expected Sunday into early next week, though winds could
be southerly near the trough axis in the northwest waters for
Monday and Tuesday.

North shores are nearly flat with a few spots seeing tiny surf this
afternoon. A small pulse of energy from the north should produce more
widespread (tiny) surf on Saturday. A storm force low near Attu
Island, Alaska will bring the next larger pulse for north-facing
shores. This will be a small to moderate, medium-period northwest
swell arriving Monday night and filling in by mid-week. A gale
force low advancing south along the west coast of the US next week
also produces a moderate, medium period northeast swell that
arrives Tuesday and peaks around the middle of next week.

A small to moderate, medium-period SSE swell will begin to subside
tonight and fade through the weekend. Surf is expected to return to
background levels along south facing shores by early next week. Surf
along east-facing shores remains below the seasonal average as trades
remain light. The aforementioned northeast swell also boosts surf
along east-facing shores mid-next week. Small background energy from
the west will be possible this weekend into next week from Typhoon
Sinlaku, but confidence remains low.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
Small Craft Advisory until 6 PM HST Saturday for Maalaea Bay-
Pailolo Channel-Alenuihaha Channel-Big Island Leeward Waters-Big
Island Southeast Waters.

&&

$$

DISCUSSION...Wroe
AVIATION...Farris
MARINE...Quesada