Area Forecast Discussion for Hawaii

Area Forecast Discussion   Issued: 04/20/2026 04:26:00 PM UTC

                        
275
FXHW60 PHFO 200626
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
826 PM HST Sun Apr 19 2026

.SYNOPSIS...
An upper level trough passing through the Hawaii region on Monday
will keep high and middle level clouds over the islands with brief
showers and chances for a few thunderstorms. A series of low
pressure systems developing northwest to north of the islands will
weaken the ridge north of the state, producing weak southeasterly
wind flow through Wednesday. The ridge builds back in north of
the state by Thursday, allowing moderate easterly trade winds to
return lasting through the upcoming weekend.

&&

.SHORT TERM UPDATE...
The satellite imagery this evening shows a large Mesoscale
Convective System (MCS) with frequent thunderstorms developing
along a surface trough roughly 200 miles northwest of Kauai.
While chances for showers and a few thunderstorms remain in the
short term forecast through Monday morning as the upper trough
passes over the western islands, the bulk of these heavy showers
and thunderstorms associated with the MCS will likely track north
of Kauai. However, the latest extrapolation track of this system
approaches Kauai by early Monday morning around sunrise. The
current intensity of this system should also weaken over time. We
will continue to closely monitor this feature on satellite for any
changes over the next several hours.

&&

.PREVIOUS DISCUSSION...
Issued at 509 PM HST Sun Apr 19 2026
A surface trough, and an associated shortwave upper level trough
are found a few hundred miles northwest of Kauai this afternoon.
The shortwave is helping to trigger thunderstorms between 200 and
400 miles northwest of Kauai. As the shortwave swings to the east
over the next day, thunderstorm activity will move with it. The
GFS and ECMWF global models are in general agreement with the
timing of these feature, however the GFS is stronger with the
divergence associated with the upper level feature. Thunderstorm
probabilities with the National Blend of Models (NBM), and
climatological based thunderstorm probabilities with both the GFS
and ECMWF keep the bulk of the instability to the north of the
islands, but enough of it sinking southward tonight that its
possible for some thunderstorms to make it into the islands. While
the highest chance for thunderstorms will be over/near Kauai and
Oahu, upslope thunderstorms on the Big Island are also possible. Have
made some adjustments to the thunderstorm areas based on the
latest guidance from the NBM, GFS and ECMWF.

The surface trough is expected to move over the islands behind the
shortwave feature, and will lead to lighter winds Tuesday and
Wednesday. Under this lighter wind regime, we are likely to see
some showers develop over interior areas of the islands during the
afternoon. Trade winds are expected to return to the islands
Thursday, with more typical trade winds showers riding in on the
trades by the weekend. Little change has been made to the PoPs and
associated forecast fields from Wednesday onwards.

&&

.AVIATION...
Issued at 509 PM HST Sun Apr 19 2026
A light easterly background trade wind flow remains over the
state today. The light winds have allowed some land and sea
breezes to develop over terrain sheltered areas. Expect these
winds to remain light and veer more ESE starting tomorrow. Showers
remain possible for windward and mountain areas, and over island
interiors during afternoon hours through tomorrow. An upper level
disturbance moving eastward through the state combined with the
light winds at the surface could increase afternoon shower
coverage statewide tomorrow. MVFR conditions should be expected
within showers, and IFR conditions are possible within heavier
isolated downpours. VFR will occur elsewhere.

No AIRMETs are currently in effect.

&&

.MARINE...
Issued at 509 PM HST Sun Apr 19 2026
A trough of low pressure passing northwest of the state will
act to weaken and veer winds east southeasterly through mid-week
across the local waters. Light flow should allow for afternoon sea
breeze development near the shores. As conditions become
increasingly unstable aloft, isolated thunderstorms will be
possible tonight into Monday over the waters surrounding Kauai
and Oahu. By Thursday, high pressure ridging builds back in north
of the state with fresh northeasterly trade winds returning to the
region lasting through next weekend.

Swell and surf will remain small along all shores through late
Monday.

A small, medium period, northwest swell will move across the
waters Monday night and continue through Wednesday. Another
small, long period, west northwest swell will enter the waters
Wednesday night and provide a slight bump up for surf along west
and northwest facing shores. A moderate, medium to long period,
northeast swell arrives on Tuesday and peaks on Wednesday,
bringing moderate surf to north- facing shores, before slowly
declining through the end of the week.

Surf will remain small along south facing shores through the
period. Surf heights along east facing shores also remain small
due to weaker trade winds until the arrival of the aforementioned
moderate, medium to long period, northeast swell building into the
Hawaii region from Tuesday through Thursday.

&&

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

&&

$$

SHORT TERM...Bohlin
PREVIOUS DISCUSSION...M Ballard
AVIATION...DT
MARINE...Thomas