047
FXZS60 NSTU 210128 CCA
AFDPPG

Area Forecast Discussion...CORRECTED
National Weather Service Pago Pago AS
228 PM SST Mon Jul 20 2026

.Synopsis...
There are three key features of note observed in latest satellite
imageries that will drive the weather pattern across American
Samoa over the next 7 days:

First is the high pressure ridge to the south and southeast that
has driven the trade wind pattern over the past several days.
This can be seen in latest ASCAT satellite wind estimates as a
counterclockwise circulation to the south of the Cook Islands.

Second is a developing surface low pressure system just west of
the aforementioned ridge. This surface low is developing beneath
an upper-level trough in the subtropical jet stream and is
clearly evident by a large area of heavy cloud cover and
clockwise circulation observed via ASCAT between Fiji and New
Zealand.

Third is a surface trough to the north, currently positioned near
the Tokelau island chain. This trough has been held to the north
of American Samoa over the past several days by the
aforementioned high pressure ridge.

&&

.Short term [tonight through Wednesday night]...
00Z RAOB upper-air observation from this afternoon revealed that a
strong stable trade wind inversion is still in place above a
shallow moist layer. This limits convective cloud growth to this
shallow moist layer, below the stable temperature inversion. This
will keep rainfall activity limited to scattered shallow, brief,
and mostly light showers while the trade wind pattern remains.

The high pressure driving the trade winds has weakened slightly
since its peak, but winds should continue to remain gusty through
at least this evening. The high pressure system will then be
gradually pushed out as the developing low pressure system moves
eastward and overtakes it. This will lead to further decreases in
wind speed tonight into Tuesday night. By Wednesday as the low
passes to the south of American Samoa, it will completely cut off
the islands from the easterly trade winds, bringing light and
variable wind conditions.

As the weather pattern changes Wednesday, the trade wind
inversion will rapidly erode. Associated with the upper-level
system will be a cold front extending from the surface low into
the tropical South Pacific. This cold front will approach
American Samoa from the south by late Wednesday. Additionally,
the surface trough to the north of the islands will begin to
drift southward and closer to the American Samoa islands as the
high pressure system is pushed out of the area, but is expected
to weaken significantly as it does so. Erosion of the stable
inversion layer, lift along the cold front, the surface trough
approaching from the north, and upper-level forcing from the jet
stream will all combine to increase rain chances by late
Wednesday. Numerous to widespread showers are expected by this
time frame. Heavy uncertainty remains regarding whether there
will be any threat for flash flooding, with most recent model
runs not very bullish for heavy rainfall.

.Long term [Thursday through Monday]...
Rain chances will remain elevated on Thursday as the active
weather system remains in the area. Model data indicates that
American Samoa will be on the cold side of the frontal boundary
by Thursday morning. Therefore, temperatures are likely to be
well below average on Thursday as southerly winds transport cool
air from near New Zealand into the tropics.

By late week through the weekend, yet another high pressure
system will take over to the southwest of the territory, turning
winds back out of the east to southeast. Models indicate that
this high pressure system will be considerably weaker than the
one that is currently in the area, depicting trade winds at only
a modest 5 to 15 mph.

&&

.Marine...
Observations from PacIOOS Aunu`u buoy indicates wave heights
remaining in the 9 to 12 foot range driven by breezy trade winds
and southerly swells. A new, very long period, south swell
reached the shorelines last night, increasing peak period times
to over 20 seconds. As a result, the High Surf Warning remains in
effect for today as these peak period times continue to favor
surf near the 15 foot threshold. Sea and surf conditions will
begin to gradually improve starting tonight and continuing through
midweek as winds and swell strength both diminish.

The low pressure system is forecast to develop into a formidable
midlatitude cyclone as it passes south of American Samoa. While
the system will pass well to the south and not cause any direct
impacts on the territory, it will generate yet another south
swell. This swell is forecast to reach coastal waters by late
week into the weekend and build seas back to at least 7 to 9 feet.

&&

.PPG WATCHES/WARNINGS/ADVISORIES...
High Surf Warning remains in effect.
Small Craft Advisory remains in effect.

&&

$$

Barton