South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
236 PM EDT Fri Sep 26 2025

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

South American Forecast Bulletin from 26 September 2025 at 1840 UTC

On Friday, a progressive pattern of mid to upper level troughs
continues over the southern portion of South America. On Friday
morning, an upper level trough is making its way into the
continent, partially negatively tilted, and with its divergent
right side over Chile, and later enter Argentina by the evening
hours. Embedded within the trough, mid to upper level shortwave
troughs are propagating along the base of the trough, expected to
impact central Argentina Friday evening into early Saturday. As
this cooler mid to upper level air mass enters the central
Argentina region, the setup for an unstable atmosphere is expected
from Friday into Saturday. In addition, the change of air mass
associated with the frontal boundary entering the region in the
lower levels, will favor moisture transport associated with the
South American Low Level Jet, and moisture from the northern
region of the continent will increase to around 45-50mm in
precipitable water. As such, precipitation totals of 35-70mm from
Cordoba to Buenos Aires region of Argentina, with a moderate risk
of severe weather. The surrounding areas, including the Cuyo
region, north Cordoba/Santa Fe, and the Rio de La Plata Region can
expect generalized maxima of 15-35mm with a moderate risk of
severe weather. These conditions may favor cyclogenesis from early
Saturday into Sunday, as the frontal boundary continues to
propagate into central Bolivia, along Paraguay, and Rio Grande do
Sul, with a potential occlusion over Uruguay by Saturday evening.
There is still the presence of the shortwave troughs in the mid to
upper levels that will be located over portions of north Argentina
and Uruguay, favoring diffluence over Paraguay and south Brasil
late on Saturday and into early Sunday. Precipitable water is
expected above 50mm in portions of Paraguay, where expect maxima
of 20-45mm that extends into the the north Mesopotamia region of
Argentina and into Rio Grande do Sul. A moderate risk of severe
weather is forecast for the region. Similar precipitation totals
are expected in central Bolivia, as the presence of the cold front
favoring ascent along the western Chiquitania region encounters a
moist atmosphere. Localized higher amounts of precipitation are
possible. By Sunday, the upper shortwave troughs push eastward
into the Atlantic Ocean, while a more zonal upper level
environment enters by Sunday evening. In the lower levels, the
frontal boundary has weakened into a stationary front over east
Paraguay and into Santa Catarina. Precipitable water at around
45mm remains over the region, however, much of the upper air
support becomes weak for deep convection and heavy precipitation.
Expect a decrease in totals throughout the region on Sunday.

In Chile, the progressive pattern of the upper levels is favoring
the fast propagation of the upper trough over the continent, as a
second upper level trough will enter the country by Saturday. On
Friday, the first upper level trough extends into the northern
portion of Chile, and the divergent right side will be located
over the Andean regions. In the lower levels, a series of frontal
boundaries, in association with an atmospheric river will interact
with the terrain, increasing moisture convergence from Los Lagos
into north Magallanes, where expect precipitation maxima of
20-35mm with a risk of cumulonimbus formation. From north Los
Lagos to Coquimbo can expect maxima of 15-20mm. On Saturday, the
heaviest precipitation is expected in the Sur and Austral regions
of Chile. As a secondary upper level trough enters the region, the
jet stream associated with this trough enters over the south
Chile, favoring divergent conditions aloft. In addition, the
previously mentioned atmospheric river is expected to sustain
higher amounts of moisture entering the southern region, with
precipitable water values ranging from 25-30mm, aligning with the
divergence aloft. Another cold front enters the region along with
the atmospheric river, favoring ascent into the region. These
conditions will favor maxima of 40-80mm from Los Lagos to north
Aysen, and maxima of 25-50mm in south Aysen. There is a risk of
cumulonimbus formation along Sur and Austral regions of Chile. On
Sunday, a weaker upper level trough approaches the region quickly.
However, a decrease in available moisture is present southern
Chile, favoring a decrease in total precipitation for Sunday.
Expect maxima of 20-35mm in Los Lagos and Aysen, while Magallanes
can expect maxima of 15-20mm.


In the northern portion of the continent, diurnal processes will
the dominating pattern over the forecast period. In the upper
levels, a ridging pattern is present, extending from the Pacific
Ocean into portions of the western Amazon River Basin. To the
east, a trough axis is present over eastern Brasil, weakening and
meandering over northern Brasil by Sunday into Monday. The
activation of the moisture transport into south and central
portions of the continent, as mentioned above, is favoring
moisture to advect westward over the central and western Amazon
Basin, while convectively induced troughs activate in the
afternoon hours of each day. A drier pattern is expected in
eastern Brasil, as the upper level trough weakens over Brasil, and
a ridge over the Atlantic Ocean strengthens, favoring an increase
in low level jets along the Brasilian North Coast. On Friday,
expect a generalized maxima of 20-45mm from the eastern Amazon
Basin into Amazonas-Brasil and into north Bolivia, while north
Peru can expect maxima of 20-35mm. The Central and Southern
Peruvian Andes are expected to see an increase in moisture in the
region, and maxima of 15mm could occur, with mountain snow. On
Saturday, moisture decreases in the Amazon Basin, favoring maxima
of 15-30mm in the central Amazon Basin, while regions of Peru can
expect generalized maxima of 15-20mm, as moisture remains in the
region. On Sunday, generalized maxima of 20-45mm are expected from
southern Venezuela and into the western Amazon Basin.


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

Castellanos...(WPC)