Area Forecast Discussion for Hawaii

Area Forecast Discussion   Issued: 04/18/2026 04:20:00 PM UTC

                        
432
FXHW60 PHFO 180620
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
820 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.

&&

.SHORT TERM UPDATE...
Issued at 818 PM HST Fri Apr 17 2026

Latest satellite and radar imagery depicts a light band of
moisture associated with an upper-level trough moving
northeastward across Kauai and extending into Oahu this evening.
Increased precipitation probabilities over Oahu a bit to show
this feature, extending the line of isolated to scattered showers
more eastward to cover both islands. Otherwise, the remainder of
the forecast remains on track showing moderate trade winds holding
throughout Saturday, bringing some decent shower activity to
windward slopes before weakening and veering for the remainder of
the weekend period.

&&

.PREV 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...
Issued at 818 PM HST Fri Apr 17 2026

A mix of moderate to locally breezy easterly trade winds with
some land and sea breezes will be in place through the weekend. Most
of the short term guidance indicates a slight veering of the
winds as a trough approaches from the northwest tonight through
tomorrow morning, 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 and Saturday night is on the lower end.

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. Clouds and
showers may also develop over the interior portions of the smaller
islands during the day on Saturday as sea breezes develop.

AIRMET Tango is in place between FL230 and FL320 due to the
potential for some moderate mid level turbulence. This AIRMET
will likely be needed through the night and potentially tomorrow
morning.


&&

.MARINE...
Issued at 818 PM HST Fri Apr 17 2026
A high pressure system far north of the state will slowly drift
eastward this week. Easterly trade winds continue through Saturday
with Small Craft Advisory conditions in effect for the windier
waters and channels near the Big Island and Maui. A series of low
pressure systems developing northwest of the islands will weaken
the ridge over the state, causing weak southeasterly winds to
develop over the Hawaii region lasting into early next week.

A small pulse of swell energy from the north should produce a
slight bump in north facing surf heights on Saturday. A small to
moderate, medium period northwest swell will move into the region
by Monday night and continue through Wednesday. A moderate,
medium period northeast swell will arrive by Tuesday, peaking
early Wednesday, and then slowly declining through Thursday.

A series of overlapping small south swells will move into the
region into next week. Surf heights along east facing shores
remains on the smaller side due to weaker trade winds.


&&

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


&&

$$

SHORT TERM UPDATE...Pierce
DISCUSSION...Wroe
AVIATION...Tsamous
MARINE...Bohlin