Area Forecast Discussion for Hawaii

Area Forecast Discussion   Issued: 04/21/2026 08:34:00 AM UTC

                        
051
FXHW60 PHFO 202234
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
1234 PM HST Mon Apr 20 2026

.SYNOPSIS...
An upper level trough passing through the region this morning will
maintain some higher level clouds over the islands with scattered
showers and the chance for a thunderstorm or two, mainly over the
western islands. Lighter winds are expected to spread across the
region Tuesday and Wednesday, leading to some afternoon clouds and
showers. A ridge builds back in north of the state by Thursday,
allowing moderate easterly trade winds with passing windward
showers to return lasting through the upcoming weekend.

&&

.UPDATE...
Thunderstorms have formed over the interior of the Big Island, and
the forecast has been updated to reflect that, with the chances of
thunderstorms continuing into the evening hours.

&&


.SHORT TERM UPDATE...
Issued at 643 AM HST Mon Apr 20 2026
The forecast has been updated this morning to adjust the
PoPs/Wx/Sky based on the latest model guidance. The shortwave
trough swinging through the region continues to spark some
thunderstorm activity, but that activity has remained in the
offshore waters to the north of the islands. The chance for
thunderstorms does remain this morning over/near Kauai and Oahu.

&&


.AVIATION...
Issued at 643 AM HST Mon Apr 20 2026

Light easterlies persist windward areas along with land- sea
breezes over leeward and interior regions. Winds will begin to
veer more ESE late this afternoon into the evening hours. Light
showers possible windward and mauka. Heavier showers likely
develop over island interiors during peak heating this afternoon.
As the upper-level disturbance and associated surface trough
approach the Hawaiian Islands from the west, expect cloud and
shower activity to tick upward. VFR prevails, but MVFR within
heavier showers.

No AIRMETs are currently in effect.

&&

.MARINE...
Issued at 329 AM HST Mon Apr 20 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 wind flow should allow for
afternoon sea breeze development near the shores. Isolated
thunderstorms are expected to continue over the northwestern
waters in the vicinity of the trough through tonight. By Thursday,
high pressure ridging builds back in north of the state with
fresh northeasterly trade winds returning to the region into next
weekend.

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

A small, medium-period northwest swell arrives tonight, bringing
an uptick in surf along north and west facing shores for Tuesday
before peaking on Wednesday. A moderate, medium- period north-
northeast swell arrives on Tuesday and peaks also on Wednesday,
bringing moderate surf to north and northeast facing shores,
before slowly declining through the end of the week.

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. Surf will continue to be small along
south facing shores through the period. East facing shores will
see an increase in surf with the arrival of the aforementioned
north-northeasterly swell despite below-seasonal average trades.

&&

.PREV DISCUSSION...
Issued at 329 AM HST Mon Apr 20 2026

Looking into the infrared satellite imagery this morning we see a
Mesoscale Convective System with moderate to heavy showers and
thunderstorms collapsing as scattered thunderstorms riding in on an
outflow boundary fades as it approaches the western coastal
waters of Kauai. A few of these thunderstorms will likely reach
these western coastal waters early this morning, however the
entire system will likely collapse into a more moderate rain band
by the time it reaches the islands of Kauai and Niihau around
sunrise. Elsewhere the passing upper level trough shown on
satellite water vapor imagery will increase shower trends today
across the state, especially along southeastern slopes of
Haleakala on Maui and the southeast slopes of Mauna Loa on the Big
Island this afternoon. Rainfall and thunderstorm coverage in the
next 24 hours of the statewide forecast was adjusted upward to
account for these smaller scale weather impacts.

An upper level trough will swiftly pass from west to east across
the state today in southeasterly wind flow briefly enhancing
shower activity as it passes over the island chain. An upper
level ridge will build in behind the trough as the trough moves
away from the islands from tonight into Tuesday. Expect drying
trends as the upper level ridge builds stabilizing subsidence,
downward moving warming and drying air, strengthening and lowering
the subsidence inversion heights across the Hawaii region.

Stronger subsidence will build the surface ridge north of the
islands, allowing trade winds to return to the region from
Wednesday night on into the upcoming weekend. Subsidence inversion
heights will range from 5,000 to 7,000 feet during this time
period, high enough for brief passing showers favoring the typical
windward and mountain areas in the overnight to early morning
hours into Sunday.

&&

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

&&

$$

UPDATE...M Ballard
DISCUSSION...Bohlin
AVIATION...JVC
MARINE...Quesada