South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
134 PM EDT Thu Sep 18 2025

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

South American Forecast Bulletin from 18 September 2025 at 1745 UTC

In the upper levels...

A potent upper trough, that currently has an axis across 33S 98W
and 41S 86W, will continue to interact with the potent subtropical
ridge to the north, which is currently centered at 13S 80W. These
two features will promote the general increase in wind speeds,
which will help sustain a broad jet streak max that is expected to
impact the northern half of Chile through Saturday morning, when
the upper trough axis is anticipated to cross east of the Andes.
An ample diffluent wind flow pattern will develop downwind of the
aforementioned trough and ridge, which will further sustain
divergence across north - central Argentina from Thursday morning
through Sunday morning. Note that after Saturday morning, the
aforementioned subtropical ridge will begin to weaken as it moves
over tropical South America and will quickly lose its definition.
However, still expect a generalized diffluent wind flow pattern
and the formation of jet streak maxes across the region. After
Saturday evening, another jet streak will begin to develop over
Uruguay and southern Brasil. These upper level features will favor
the sustenance of upper divergence across central South America
for the forecast cycle as well.

In the mid - levels ...

A potent trough, that is currently centered at 40S 84W, will
continue to favor the development of shortwave troughs that will
be accompanied by vorticity maxima for the entire forecast cycle.
These troughs are expected to propagate across south - central
Chile and north - central Argentina through Sunday morning. The
main axis of the potent mid - level trough will begin to move in -
land between Saturday afternoon and early Sunday morning. General
impacts associated with these features include destabilization of
the atmosphere and the enhancement of upward motion.

In south - central Chile ...

In the upper levels, a jet streak will be embedded within the exit
region of the aforementioned upper trough. Expect upper divergence
to be favorable through Saturday afternoon in southern Chile,
though upper divergence may remain favorable across the northern
half of Chile after Saturday afternoon due to jet dynamics
associated with a potent jet streak affecting that region. In the
mid - levels, a series of shortwave troughs that will be
accompanied by vorticity maxima will continue to propagate
southeastward ahead of the arrival of the trough axis by late
Saturday evening. At the low levels, the highest precipitable
water values will be found across central Chile, but a north -
northwesterly low level flow will limit orographically enhanced
moisture convergence. Precipitable water should begin to decrease
in central Chile after Saturday morning, but overall, the values
across south - central Chile should remain above normal through
the entire forecast cycle. Low level wind speeds will be moderate
and from the north - northwest during this period as well. From
Thursday morning through Friday morning, expect a total
precipitation maxima of 15 - 20mm across southern Chile. Though
precipitable water is highest in this period, the mid - level
support will be more favorable after Friday morning. From Friday
morning through Saturday morning, expect a total precipitation
maxima of 20 - 35mm across south - central Chile. A total
precipitation maxima of 20 to 45mm is possible from Saturday
morning through Sunday morning.

In Central South America ...

Enhanced low level southerly flow will continue to dominate the
interior of Argentina through the forecast cycle, mostly being
influenced by the dynamic upper level pattern. At the surface, a
series of fronts will develop and will favor an increase in
moisture convergence ahead and along these boundaries. The first
front to impact the region will be a stationary front that will
become more defined starting mid - day Thursday and will be
located across La Pampa - Buenos Aires and the Cuyo provinces on
Thursday evening. By Friday evening, the front will begin
propagating northward and will be situated across Rio de la Plata
and Cordoba. Starting Saturday afternoon, expect cyclogenesis in
this region. A cold front should extend across Rio de la Plata and
Cordoba by Saturday evening and an occluded front will be located
across La Pampa - Buenos Aires. The total precipitation maxima for
the next three days will be synonymous with the location of the
front, where enhanced moisture convergence and vertical ascent
will be most favorable. The combination of upper divergence,
influenced by the aforementioned upper level pattern, mid - level
shortwave troughs accompanied by vorticity maxima, increase in
moisture, and vertical ascent may significantly increase severe
weather potential across north - central Argentina and Uruguay.
Thus, from Thursday morning through Friday morning, expect a total
precipitation maxima of 25 - 50mm across the Cuyo and La Pampa -
Buenos Aires provinces with a slight risk for severe weather. From
Friday morning through Saturday morning, a total precipitation
maxima of 30 - 60mm with a moderate risk for severe weather is
possible across La Pampa - Buenos Aires. Thereafter, expect the
greatest total precipitation maxima across La Pampa - Buenos Aires
and Rio de la Plata, where the low level low pressure system is
expected to develop. From Saturday morning through Sunday morning,
a total precipitation maxima of 35 to 70 mm with a risk for severe
weather is likely across  this region.

Another region of interest is north Argentina, Uruguay, and
southern Brasil. In the upper levels, an elongated shortwave
trough, that currently has an axis across 5S 62W and 23S 52W, is
promoting a diffluent upper level wind pattern in its wake. This
diffluence is will also be sustained from the interactions of the
aforementioned subtropical upper ridge and potent upper trough to
the west of Chile. As the subtropical upper ridge moves into
tropical South America, the aforementioned jet streak max
associated with these upper level features will begin to affect
this region when it moves east of the Andes. Jet dynamics is
expected to dominate through Sunday morning. In the mid and low
levels, a mid - level shortwave trough will propagate across the
region through Friday morning. During this period, precipitable
water totals should be exceeding 38mm, especially near the borders
of Uruguay, Argentina, and Brasil. These conditions will favor a
total precipitation maxima of 20 - 35mm from Thursday morning
through Friday morning. From Friday morning through Saturday
morning, a series of mid level troughs will be impacting this area
alongside the northward propagation of the aforementioned frontal
boundaries. Expect moisture pooling in the low levels to the north
of a developing low level low pressure system that will be located
across La Pampa - Buenos Aires provinces. This surge in moisture
and the mid - level troughs may favor the development of
convection. Thus, a total precipitation maxima of 20 - 40mm is
likely with a slight risk for severe weather. After Saturday
morning, precipitation initiation processes will be mostly
influenced by frontogenesis and cyclogenesis. Expect a total
precipitation maxima of 40 to 80mm front Saturday morning through
Sunday morning. A risk for severe weather is also possible.

In Tropical South America â€

A series of low level troughs will be the main driving force for
precipitation initiation. Ample moisture exists in the region.
However, the aforementioned subtropical ridge will begin to move
over the region through Saturday morning, losing its definition
thereafter. The main effects from this upper level feature include
subsidence, leading to stabilization, across its center. Along its
northeast periphery, some upper diffluence may be favorable due to
the interaction between this upper level feature and an
aforementioned elongated shortwave upper trough. Overall, areas of
greatest precipitation impact include the eastern regions of the
Amazon Basin, where the greatest instability and moisture will be
located. A total precipitation maxima of 20 - 40mm is possible
from Thursday morning through Friday morning, and a maxima of 20 -
35mm is likely from Friday morning through Saturday morning. By
Saturday morning through Sunday morning, a general decrease in
total precipitation maxima is expected across the entire basin as
a broad ridge develops.


Tinoco-Morales...(WPC)