Area Forecast Discussion for Hawaii

Area Forecast Discussion   Issued: 04/19/2026 04:47:00 AM UTC

                        
922
FXHW60 PHFO 181847
AFDHFO

Area Forecast Discussion
National Weather Service Honolulu HI
847 AM HST Sat Apr 18 2026

.SYNOPSIS...
Moderate trades will persist through today, leading to some
showers along windward and mauka areas. An upper-level trough
moving overhead will produce high clouds and may trigger a few
heavy showers and thunderstorms around Kauai and Big Island.
Trades will weaken and veer out of the east-southeast across the
western half of the island chain tonight 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 afternoon and Monday. Light winds
favor chances for mainly afternoon showers Tuesday and Wednesday,
followed by rebuilding trade winds Thursday and Friday.

&&

.SHORT TERM UPDATE...
Issued at 847 AM HST Sat Apr 18 2026

A somewhat unstable moderate trade wind flow remains in place this
morning. A strong 1032 mb surface high centered far north of the
state along 40N latitude is driving the moderate trades. The local
pressure gradient and resulting trade winds are being slightly
weakened by a surface trough parked roughly 300 miles northwest of
Kauai. A weakening negatively tilted upper-level trough
approaching from the west has triggered clusters of thunderstorms
within the trough and along a shallow band of moisture less than
100 miles south of Big Island. Southwest winds aloft just ahead of
the upper trough are spreading thick high clouds from the nearby
thunderstorms over much of the state. This may act to suppress
thunderstorm activity over the islands this afternoon, since weak
ridging below 700 mb is producing a low-level inversion around
4000 ft. If this holds, modest rainfall will remain confined to
mainly windward slopes today. However, in the event that the high
clouds thin and the inversion erodes as the upper-level trough
moves overhead, we have still maintained some chances of
afternoon showers over leeward and interior areas, with
thunderstorms possible near Kauai.

&&

.PREV DISCUSSION...
Issued at 326 AM HST Sat Apr 18 2026

Moderate trades will persist for one additional day as a slow-
moving surface high pressure system north of the Hawaiian Islands
tracks eastward. Simultaneously, a weakening upper-level trough
positioned just west of the state continues to generate high-level
cloud coverage and diminish overall atmospheric stability.
Increasing diurnal heating throughout the morning and afternoon is
expected to heighten instability, potentially triggering another
round of thunderstorms across the interior of the Big Island.

Satellite imagery indicates persistent thunderstorm activity
northwest of Kauai, associated with a developing low pressure
system. Although this system remains relatively weak and is
situated several hundred miles to the west, it may cause trades to
veer southeasterly and weaken. This shift could result in
isolated, potentially heavy showers over leeward sections of all
islands. However, as the upper-level trough moves further away
tonight, the threat of localized heavy rainfall will diminish,
shifting shower activity predominately toward windward and mauka
regions.

A more potent upper-level trough is forecast to approach the
state during the latter half of the weekend, introducing increased
instability from tomorrow night into Monday. Global models
suggest that the majority of moisture and shower activity will
remain concentrated along a quasi-stationary surface trough linked
to the low pressure system northwest of Kauai. Elsewhere,
rainfall will be focused over island interiors, with continued
potential for thunderstorms on the Big Island.

Forecasters are monitoring the possibility of localized heavy
showers anchoring along the Koolau Mountains on Oahu Sunday night.
While current moisture levels do not appear significant, veering
flow with height often facilitates heavy rainfall in this specific
terrain; therefore, close monitoring of this trend is warranted.

By Tuesday, the upper-level trough is expected to drift east of
the islands, followed by the rapid arrival of weak ridging. This
transition should lead to a general decrease of shower activity
statewide. However, model guidance indicates that the surface
trough previously northwest of Kauai will migrate over the western
portion of the state. This movement will likely promote light and
variable winds along with the increased possibility of afternoon
rainfall over island interiors.

&&

.AVIATION...
Issued at 847 AM HST Sat Apr 18 2026

A jet streak at the base of an upper level trough to the west
will continue to supply mid and upper level clouds over the state
with embedded showers through much of the morning and possibly
into the afternoon. In addition, a line of eastward-moving
thunderstorms can be seen about 200 miles northwest of Kauai, but
most model guidance has this line weakening and passing just north
of the area today.

Otherwise, a mix of moderate to locally breezy easterly trade
winds with some sea and land breezes will be in place through the
weekend as the surface trough to the west lingers and VFR
conditions are expected to generally prevail across most airports.
Brief showers will move into windward and mountain areas on the
trades, and afternoon showers and thunderstorms will remain
possible over interior Big Island. Clouds and showers may also
develop over the interior portions of the smaller islands during
the day as sea breezes develop. Any afternoon showers could become
briefly intense due to the upper level trough providing some
instability. These showers would then decrease after sunset as
local land breezes occur.

AIRMET Tango is in place between 100 and FL350 due to the
potential for some moderate mid to upper level turbulence. This
AIRMET will likely be needed through the morning. Light icing will
also be possible between 140 and FL250 within the deep layered
clouds.

&&

.MARINE...
Issued at 326 AM HST Sat Apr 18 2026

A high pressure system far north of the state will slowly drift
eastward this weekend. Easterly trade winds continue today 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 western Hawaii region from Sunday into early
next week.

A small pulse of medium period swell energy from the northwest
should produce a slight bump in north facing surf heights today. 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 Monday
night, peaking from Tuesday to 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 this evening for Alenuihaha
Channel-Big Island Leeward Waters-Big Island Southeast Waters-
Maalaea Bay-Pailolo Channel.

&&

$$

PREV DISCUSSION...Pierce
AVIATION...Farris
MARINE...Bohlin
Update...Wroe