South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
144 PM EDT Wed Sep 24 2025

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

South American Forecast Bulletin from 24 September 2025 at 1745 UTC

On Wednesday, in the northern region of the continent, a weakening
frontal boundary extends from central Peru along the southwest
Amazon River Basin, Mato Grosso and Cerrado in Brasil, exiting
through Espirito Santo and into the Atlantic Ocean by Wednesday
evening. The presence of this boundary has favored multiple areas
of moisture convergence along the boundary and to the north. Deep
convection has been present over the past few days, with the
presence of an upper level ridge centered over south Peru, and
extending into central Brasil, where it interacts with the base of
a propagating upper trough that extends from the southern Atlantic
Ocean and reaches over Argentina, Uruguay and south Brasil. This
interaction will favor upper level divergence in central Brasil,
favoring maxima of 15-25mm from Mato Grosso and the Eastern Amazon
Basin, through Cerrado region of Brasil. However, the potential
for higher precipitation totals is expected. Along southern
Espirito Santo, maxima of 20-35mm are expected as moist onshore
flow associated with the frontal boundary interacts with the
terrain. In the Amazon Basin, assisted by diurnal heating, and the
presence of available moisture and prefrontal troughs, expect
these regions to develop strong thunderstorms on Wednesday. The
northwest and western Amazon Basin can expect maxima of 30-60mm,
while the Central and Southwest Amazon Basin can expect maxima of
20-45mm. These regions can expect localized higher precipitation
totals. On Thursday, the frontal boundary continues to remain
stationary from south Bahia-Brasil, into the Cerrado Region and
south Mato Grosso in Brasil. Low level troughs have remained in
the northwest Amazon River Basin, as well as a pool of moisture
that will favor thunderstorms in the afternoon and into Friday
morning. Expect maxima of 30-60mm in north Peru and east Ecuador,
while south Colombia can expect maxima of 20-35mm. The Central
Amazon Basin can expect maxima of 20-45mm. In Espirito Santo,
moist onshore continues to interact with the terrain, and can
expect 20-35mm, while the surrounding regions of Bahia and Serra
do Mar can expect generalized maxima of 15-25mm. By Friday, a
noticeable decrease in available moisture in the tropical region
is expected to assist with decreasing precipitation totals along
the Amazon River basin as well as the advection of dry air along
the easterly trade winds over eastern and central Brasil. With the
entrance of an upper level trough into northern portions of
Brasil, the upper ridge meanders over northwest South America and
into the Pacific Ocean. This will weaken the ridge over the area,
and regions of upper divergence will decrease. Precipitation
maxima of 20-35mm are expected in the central Amazon Basin and
into central Peru, while maxima of 15-25mm are expected in central
Brasil and from south Colombia into north Peru and Ecuador.
Central Bahia can expect to see the onshore flow associated with
the frontal boundary, with maxima of 20-35mm.

In the southern region of the continent, expect an increasing
trend of precipitation starting in Chile on Wednesday and
expanding into Argentina by early Saturday morning. The region is
seeing a progressive movement of upper level troughs, assisting
with fast moving systems over the forecast period. On Wednesday,
Argentina and Chile are expected to remain in an upper ridge axis
between two upper troughs, favoring dry conditions over most of
the region. The exception is south Austral Chile, as the entrance
of frontal boundaries with precipitable water levels near 25mm is
expected by Wednesday evening. This may favor maxima of 15-20mm.
On Thursday, the upper level trough enters Chile from the west,
with the divergent side of the trough interacting with the low
level fronts entering Central Chile. The accompanying atmospheric
river is expected to have moisture at around 25mm, where it will
favor moist onshore flow, interacting with the terrain. Expect
maxima of 15-30mm from South Chile to the Taitao Peninsula, and
generalized maxima of 15-20mm in Central Chile and Austral Chile.
In addition to the cold front, moisture transport is enhancing
over Argentina, as the pressure is decreasing with the propagation
of the upper level trough. By Friday, the upper trough axis is
expected to have entered the continent, with the divergent side of
trough over north and central Argentina by the evening on Friday.
Along the base of the mid to upper level trough, short wave
troughs are expected to propagate over regions of Cordoba, Cuyo,
and La Pampa in Argentina, while the South American Low Level Jet
(SALLJ) continues to transport moisture from the north. The
frontal boundary that entered through central Chile on Thursday,
is expected over central Argentina by Friday evening. These
conditions will interact from Friday evening into early Saturday
morning, where a moderate risk of severe weather is expected. The
heaviest precipitation is forecast along the Cordoba Sierra and
into Buenos Aires province with maxima of 35-70mm, while the
surrounding areas of central Argentina  and into the southern
Chaco region can expect maxima of 20-35mm. The north Patagonia
region can expect maxima of 15-25mm with a moderate risk of severe
weather.


For the latest available charts please visit:
https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml

Castellanos...(WPC)