South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
221 PM EDT Wed Sep 17 2025

GFS data at ftpprd.ncep.noaa.gov/data/nccf/com/gfs/prod/

South American Forecast Bulletin from 17 September 2025 at 1830 UTC

In the upper levels, a potent positively tilted vertically stacked
upper trough, located to the west of south - central Chile, will
continue to interact with the strengthening subtropical ridge to
the north. Expect an increase in upper level wind speeds across
the northern periphery of the upper trough and the southern
periphery of the subtropical ridge. This will help maintain and
strengthen a broad jet streak max that will have significant upper
level impacts across the entire continent.

Another upper level feature of interest is a shortwave upper level
trough that is beginning to interact with the aforementioned upper
level subtropical ridge. The axis of the shortwave trough is being
amplified to the north. As the trough moves eastward, the eastern
periphery of the subtropical ridge will begin to move farther
inland into the northwest region of South America. Expect an
increase in divergence across central South America with the
passage of the highly amplified upper shortwave trough. The
interaction between the subtropical ridge and the upper troughs
will help promote a diffluent upper level wind flow pattern across
much of central South America as well, with greatest impacts
observed primarily north of 40S. 

In south - central Chile ...

In the upper levels, ahead of the aforementioned jet streak, the
exit region of the upper trough will promote upper divergence
through Thursday morning. Thereafter, jet dynamics will begin to
affect the area, especially across central Chile. After Friday
afternoon, the upper trough will begin to develop a negative tilt
as it begins to propagate over and to the east of the Chilean
Andes. During this period, upper divergence will be favorable
across and north of central Chile. In the mid - levels, a series
of shortwave troughs, which are associated with a broad mid -
level trough that is being sustained by the aforementioned potent
upper trough, will be moving across south - central Chile for the
next three days. These shortwave troughs will be accompanied by
vorticity advection pulses, which will help enhance ascent across
the area. The axis of the broad mid - level trough is anticipated
to move east of the Andes after Saturday afternoon. In the low -
levels, an extended area of north-northwesterly winds will begin
to converge in-land starting late Wednesday night, when we also
expect an increase in moisture convergence. Precipitable water
values should quickly decrease thereafter across southern Chile,
but will remain relatively high across central Chile through
Friday evening, with values up to 20mm possible through the entire
forecast cycle. After Friday evening, as the upper trough begins
to move across the Andes, so will a low level low pressure system.
Expect an increase in moisture across the south - central region
throughout this passage. The risk for cumulonimbus related
precipitation is possible for the next three days. From Wednesday
morning through Thursday morning, a total precipitation maxima of
15 - 25mm is likely across southern Chile. From Thursday morning
through Friday morning, a total precipitation maxima of 15 - 20mm
is likely. From Friday morning through Saturday morning, a total
precipitation maxima of 20 - 45mm is possible across central
Chile.

In north - central Argentina...

In the upper levels, speed divergence will dominate through
Wednesday evening. Thereafter, the aforementioned jet streak maxes
will begin to move eastward, and the interaction between both the
polar upper trough and upper subtropical ridge will help develop a
diffluent upper level wind flow pattern across north - central
Argentina through Friday afternoon. Afterwards, the upper levels
will be mostly dominated by jet dynamics. Overall, expect
significant upper level support in the entire region for the
entire forecast cycle.

The chaotic nature of the upper levels will promote the lowering
of pressure levels at the surface and will enhance low level
northerly winds into the region. This will drive an increase in
low level moisture as well. On Wednesday evening and Thursday
evening, a stationary front will be located across Cuyo and La
Pampa - Buenos Aires provinces. A mid - level trough will also be
moving across this region on Wednesday, helping provide further
ascent. The increase in moisture convergence and ascent will favor
total precipitation maxima of 25 - 50mm with a marginal risk for
severe weather from Wednesday morning through Thursday morning.
Starting Thursday afternoon, the upper level wind flow will become
more diffluent and may positively contribute to the sustenance of
upper divergence. Also, another shortwave trough will be moving
across the area Thursday afternoon. Thus, a total precipitation
maxima of 30 - 60mm is possible and will be accompanied with a
moderate risk for severe weather.

After Friday morning, the aforementioned polar trough will begin
to move east of the Andes. Jet dynamics and diffluence may help
contribute to the sustenance and enhancement of upper divergence
across the region. At the surface, cyclogenesis may occur across
La Pampa - Buenos Aires province. The chaotic nature of the upper
level may contribute to multiple mid - level shortwave troughs
across the region. These troughs may enhance ascent across the
region and may contribute to further destabilizing the
environment. Thus, we expect a total precipitation maxima of 35 to
70mm with a risk for severe weather across La Pampa - Buenos Aires.

Farther north, the aforementioned upper level shortwave trough
will contribute to the sustenance of upper divergence across
northern Argentina. A shortwave mid - level trough will help
trigger convection across this sector through the day on
Wednesday. At the low levels, there will be a significant increase
in moisture. A mid - level shortwave trough may help trigger
convection across the far northwest corner of the Chaco region of
Argentina on Wednesday afternoon. A total precipitation maxima of
20 - 40mm with a marginal risk for severe weather is possible in
this sector. By Wednesday evening, a brief period of enhanced
cyclonic rotation in the low level wind flow may help trigger
convection across the Cordoba and Chaco regions of Argentina. A
total precipitation maxima of 25 - 50mm is likely and will be
accompanied with a slight risk for severe weather due to the cool
mid - levels, increased divergence, and substantial moisture
content. As the upper level and mid - level shortwave troughs move
eastward, the total precipitation maxima will also move farther
west with passing days. From Thursday morning through Friday
morning, a total precipitation maxima of 20 - 45mm with
accompanied slight risk for severe weather is possible along the
border of Argentina and western Uruguay. After Friday morning, any
precipitation in the northern portions of Argentina will be driven
by the cyclogenesis event. This may yield a total precipitation
maxima of 35 - 70mm.

In Tropical South Americaâ€

A series of low level troughs will help promote ascent and
moisture convergence across the entire basin. A region of interest
includes east Peru and west Brasil, where we will see a confined
region of greater precipitable water. Expect daily total
precipitation maxima of 20 - 35mm in this region for the next
three days. Along the Pacific coast of Colombia, high precipitable
water values will remain in the region for the next three days.
However, moisture convergence will become less favorable with
passing days as the low level wind flow will begin to weaken from
Wednesday to Thursday and become more southeast in direction after
Friday. If any precipitation does develop across the Pacific coast
of Colombia, it will primarily be influenced by local and diurnal
effects. Elsewhere in the region, diurnal and local effects will
be the driving force for precipitation initiation.


Tinoco-Morales...(WPC)