Area Forecast Discussion for Hawaii

Area Forecast Discussion   Issued: 04/19/2026 11:52:00 AM UTC

                        
289
FXHW60 PHFO 190152
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
352 PM HST Sat Apr 18 2026

.SYNOPSIS...
Moderate trade winds will weaken and veer out of the east-
southeast across the western half of the island chain tonight as
low pressure deepens several hundred miles northwest of Kauai.
Shower activity should be low and focused mainly over windward
areas as high clouds briefly diminish. An upper-level trough will
will bring increased high clouds and renewed chances for isolated
heavy showers and thunderstorms Sunday afternoon and Monday.
Light winds favor chances for mainly afternoon showers Tuesday and
Wednesday, followed by rebuilding trade winds Thursday and
Friday.

&&

.DISCUSSION...
An upper-level trough passing overhead has brought extensive high
clouds and instability aloft, but moderate easterly trade winds
and a weak low-level ridge have kept shower activity rather
minimal and largely confined to windward and mauka areas so far
today. A robust 1033 mb surface high is centered far north of the
state along 40N latitude, but the local pressure gradient has been
slightly weakened by a diffuse surface trough sitting several
hundred miles northwest of Kauai, resulting in the moderate
easterly trade winds. Afternoon soundings revealed an inversion
between 4000 to 5000 ft produced by a weak low-level ridge, giving
way to instability above 700 mb generated by the upper-level
trough. The axis of the upper trough recently triggered a couple
of briefly heavy showers over Kauai, and given the shallow stable
layer, we still could see a heavy shower flare up as the upper-
level trough advances eastward.

Easterly trade winds will ease slightly tonight as developing low
pressure several hundred miles north of Kauai continues to erode
the surface pressure gradient. The upper-level trough will flatten
as it swings northeast of the islands, and a weak ridge at 700 mb
will remain in place. As a result of this tenuous stability,
showers will likely remain modest and largely confined to windward
slopes. That said, we still cannot rule out a brief and isolated
heavy shower anywhere. In addition, expect a short-lived decrease
in high clouds.

On Sunday and Monday, trade winds will weaken further and will
shift out of the east-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 the surface trough northwest
of Kauai, but isolated heavy showers or a thunderstorm remain
possible around Kauai. Elsewhere, expect 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 and Wednesday.
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 over most
areas with higher rainfall chances over island interiors during
the late morning and afternoon hours. Trade winds are expected to
rebuild Thursday and Friday.

&&

.AVIATION...
A mix of moderate to locally breezy easterly or east-
southeasterly trade winds with some sea and land breezes will
remain in place through the weekend with high pressure to the far
northeast and as a surface trough lingers to the west. Overall,
VFR conditions are expected to generally prevail across most
airports through tomorrow.

Brief showers will move into windward and mountain areas on the
trades, and afternoon clouds and showers may also develop over
the interior portions of the Hawaiian Islands during the day as
sea breezes develop in the slightly veered flow. However, mid to
high clouds with embedded light showers streaming in from the
southwest from a nearby jet streak have helped to minimize daytime
heating and sea breeze development so far today.

AIRMET Tango is in place between 100 and FL400 due to the
potential for moderate mid to upper level turbulence. This AIRMET
will likely be needed for the eastern half of the state through
the early evening. Light icing will also be possible between 140
and FL250 within the deep layered clouds.

&&

.MARINE...
A high pressure system far north of the state will slowly drift
eastward this weekend. Easterly trade winds diminish tonight as a
trough to the northwest of the islands develops into a weak low
pressure system by Sunday. The resulting weakness in the ridge
causes winds to veer southeasterly and diminish over the western
Hawaii region from Sunday into early next week. Late next week,
east to northeasterly trade winds begin to redevelop with weak
ridging re-establishing north of the islands.

A small pulse of medium period swell energy from the northwest
should produce a slight bump in north facing surf heights tonight
into Sunday morning. 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
arrives Tuesday and peaks on Wednesday, bringing moderate surf to
north-facing shores, before slowly declining through the end of
the week.

A medium-period south swell will continue to fade tonight,
allowing surf along south shores to return to background levels by
Sunday. A series of overlapping small south swells will move into
the region into next week. Surf heights along east-facing shores
remain on the smaller side due to weaker trade winds until the
arrival of the aforementioned northeast swell on Tuesday. Small
background energy from the west will be possible, especially later
in the week, from Severe Tropical Storm Sinlaku.

&&

.HFO WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

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