South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
257 PM EDT Thu Sep 25 2025

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

South American Forecast Bulletin from 25 September 2025 at 1900 UTC

Over the next several days the southern portion of the continent
is expected to progressive movement of mid to upper level troughs,
that are making their way northward into portions of Bolivia and
south-central Brasil. On Thursday, the majority of the south and
central part of the continent is between upper level troughs,
where an upper ridge axis is favoring dry conditions from south
Bolivia into the north Patagonia region. In the lower levels, the
presence of low level jets are expected over most of Argentina as
the next upper level trough and low level frontal boundary is
entering the region through central Chile. In addition, the low
level jets are favoring moisture transport from the north, and the
increase of available moisture over Argentina is seeing
precipitable water values between 25-30mm in north Argentina and
into the Patagonia region by early Friday morning. To the west,
the entrance of the cold front and accompanying atmospheric river,
will favor moisture convergence along the Andes, while there is
interaction with diffluence from the eastern side of the upcoming
upper trough. This will favor precipitation maxima of 20-40mm from
Los Lagos and north Aysen. Generalized maxima of 15-30mm are
expected in the Sur Chile region and into central Chile, with the
chance of mountain snow.

On Friday, the mid to upper level trough axis is expected over
Chile and west Argentina, where it begins to tilt negatively by
the evening hours on Friday. The base of the mid to upper trough
is expected in north Argentina and entering west Paraguay. The
area of diffluence and higher divergence associated with this
trough, is expected over central Argentina, where shortwave
troughs will favor a moderate risk of severe weather. With the
help of the moisture transport of the SALLJ, the amount of
available moisture in the region will favor precipitation maxima
of 30-60mm from Cordoba into Buenos Aires-Argentina, while the
south Cuyo and La Pampa region can expect maxima of 20-35mm. In
central Chile, a new entrance of moisture is interacting with the
terrain, favoring orographic lift, which Aysen can expect maxima
of 20-35mm, while from Los Lagos to Valparaiso can expect maxima
of 15-25mm with mountain snow.

On Saturday, the frontal boundary continues to propagate to the
north and east, where cyclogenesis is expected to occur over the
Rio de la Plata region by Saturday morning. This is due to the
presence of short wave troughs embedded within the large scale
trough that is negatively tilted. In addition, these conditions
will favor severe weather in Buenos Aires and into Uruguay. As the
cold front propagates, it is expected to reach south Bolivia,
enter into south Mato Grosso do Sul, Santa Catarina, Rio Grande do
Sul, and exit into the Atlantic Ocean by Saturday evening. As the
moisture from the north interacts with the propagating cold front,
expect significant lift and the activation of deep convection from
central Bolivia into Paraguay, and into Rio Grande do Sul.
Paraguay is forecast maxima of 25-50mm, while central Bolivia can
expect similar totals, and Rio Grande do Sul can expect maxima of
20-35mm. 

To the north, available moisture pooled in the northwestern
portion of the continent remains in the region, while the trade
winds favor a west-northwest flow towards the Andes region of
Peru, Ecuador, and Colombia. Orographic lift with the presence of
convectively induced low level troughs, expect maxima of 25-50mm
in the aforementioned location and into Amazonas-Brasil.
Similarly, central Brasil and central Peru can expect generalized
maxima of 20-40mm. Towards southeast Brasil, a weakening
stationary boundary over Cerrado region, into the Parana Basin,
and the Pantanal region, extends into a cold front into Espirito
Santo and enters the Atlantic Ocean. The potential presence of a
shear line could be expected into southern Bahia on Thursday into
Friday. As moist onshore flow enters south Bahia and Espirito
Santo, expect maxima of 20-35mm. The Serra do Mar region can
expect maxima of 15-25mm. On Friday, a drying trend begins into
eastern Brasil, as an upper level trough extends into the
northeast region of Brasil. The easterly trade winds are advecting
drier air, and extends into central portions of Brasil. The cold
front continues to propagates northeastward, entering through
north Bahia, where the entrance of moisture will favor maxima of
15-25mm. In the Central Amazon Basin, the diurnal cycle will favor
convection in the afternoon, and precipitation maxima could reach
20-40mm. The central and southern Andean region of Peru can expect
moisture along the foothills of the mountains, where interaction
with the terrain will favor maxima of 20-35mm. On Saturday, the
frontal boundary in north Bahia has become stationary, continuing
to enhance onshore flow into the region and favor maxima of
15-20mm. Drier air continues to enter north and central portions
of Brasil through the easterly trade winds. The central Amazon
Basin could see a decrease in precipitation totals to a range of
15-25mm. However, as the upper level trough in the southern region
of the continent continues to propagate into the Argentina, the
change in pressure that is reflected in the lower levels is
favoring a shift in the low level flow. This will favor moist air
to begin advecting from the central Amazon Basin, southward
towards south Peru and Bolivia. Central and south Peru can expect
maxima of 20-35mm are this advection of moisture experiences
orographic lift.

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

Castellanos...(WPC)