South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
129 PM EDT Fri Sep 12 2025

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

South American Forecast Bulletin from 12 September 2025 at 1730 UTC

On Friday, a potent upper level trough is propagating eastward
into Chile, where it is interacting with a ridge axis located over
Argentina, and enhancing divergent conditions over portions of
central and south Chile. In the low levels, a frontal boundary is
also making its way into Chile, extending from the Pacific Ocean,
entering through Araucania and Bio Bio, entering the Patagonia
region in Argentina, and exiting through Austral Chile by Friday
evening. Precipitable water levels nearing 25mm are expected to
accompany the passing of this front as this moisture converges
along the high terrain of the region, favoring moisture
convergence and precipitation maxima of 15-30mm from south Los
Lagos and Aysen. This region can expect a risk of cumulonimbus due
to an unstable conditions expected with the colder air aloft. As
the system continues propagating east and north, expect a decrease
in available moisture and precipitation maxima of 15-20mm from
north Los Lagos to Valparaiso. On Saturday, expect a decrease in
precipitation as the frontal boundary is further east over central
Argentina and into the Atlantic Ocean. A weaker and less moist air
mass enters south and Austral Chile, favoring maxima below 15mm.
On Sunday, a weak shortwave trough axis associated with a bigger
upper level trough pattern extending from the Southern Ocean,
enters Chile, while it interacts with zonal upper level winds in
central Chile and Argentina, enhancing the upper level North Polar
Jet. A jet steak is expected to perpendicularly interact with the
Andes Mountains over Austral Chile, where divergence is enhanced.
Moisture ahead of the front is expected to converge in the lower
levels, favoring precipitation maxima of 15-30mm, with mountain
snow in Aysen. Magallanes can expect maxima of 15mm.

In the central portions of the continent, a weak upper trough
interacting with the subtropical ridges in the region, has
sustained a weak stationary boundary over Paraguay and south
Brasil on Friday. Generalized available moisture ranges from
35-40mm in precipitable water from Bolivia, into north Argentina,
and extending into southwest Brasil. Due to the diurnal cycle,
expect maxima of 15-25mm in Paraguay, while south Brasil can
expect maxima of 15mm on Friday. Along Rio de Janeiro and Espirito
Santo, the stationary front and associated shear line will favor
generalized maxima ranging from 15-25mm. As the aforementioned
upper level trough enters the continent, with its base in north
Argentina, it is expected to interact with the strengthening upper
level ridge located in the northern portion of the continent. A
more zonal upper level set up is expected, however, in the mid
levels, shortwave troughs propagate along the base of the mid to
upper level trough in north Argentina, while in the lower levels,
moisture transport from the north is enhanced through the SALLJ,
favoring maxima of 15-25mm with a marginal risk of severe weather
in the Chaco region. To the east, Espirito Santo-Brasil can expect
some increase in moisture along the frontal boundary, with
precipitation maxima of 15-30mm. On Sunday, the upper level
systems are expected to continue interacting, increasing the upper
level flows, and favoring the propagation of shortwave troughs in
the mid levels over north Argentina, south Bolivia, and Paraguay.
In the lower levels, the frontal boundary that had propagated over
Chile and Argentina has entered north Argentina into Uruguay,
interacting with a secondary frontal boundary over the Atlantic
Ocean. Precipitable water levels have increased along north
Argentina, particularly along the Chaco Region to amounts of about
45mm. As the pressures decrease in response, troughing will favor
easterly flow in North Salta-Argentina and Tarija-Bolivia, and
favor moisture convergence along the foothills of the Andes. That
and the propagating mid to upper level shortwave troughs will
favor maxima of 15-30mm with a slight risk of severe weather.
Similarly in north-central Argentina, increased amount of moisture
along the stationary frontal boundary and the assistance of the
shortwave troughs, will favor similar precipitation totals and a
slight risk of severe weather. Localized higher amounts are
possible.

In the northern portion of the continent, the propagation of low
level troughs and available moisture are favoring precipitation
over the north and eastern part of the continent throughout the
forecast period. To the west, generalized drier conditions are
expected as drier air continues to advect from the east. While in
the upper levels, the Subtropical Ridge meanders over north South
America. On Friday, from east Colombia into the western Amazon
Basin, expect moisture convergence along the Andes and generalized
maxima ranging from 20-45mm. Elsewhere over north Brasil and
central Peru expect maxima of 15-25mm. On Saturday, a decrease in
available moisture will favor maxima of 15-25mm from Ecuador
through the west and central Amazon Basin. On Sunday, a
propagating trough enters into east Colombia and west Venezuela,
where expect maxima of 20-35mm. The west and central Amazon Basin
can expect maxima of 15-25mm.

Castellanos...(WPC)