South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
158 PM EST Mon Nov 24 2025

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

South American Forecast Bulletin for 24 November 2025 at 1900 UTC

In the beginning of the work week, instability is located over
Brasil and into the Amazon River Basin on Monday afternoon. In the
Amazon River Basin, the region will see persistent troughing in
the low levels that will interact with the diurnal cycle and
activate scattered to widespread thunderstorms over the next
several days. There is a persistent short wave upper level trough
extending from the south Atlantic, with its base over south
Paraguay and south Brasil. As this trough interacts with the upper
level ridging over central Brasil, a weak upper level jet streak
is located over Paraguay, and into the Pantanal and Parana River
Basin in Brasil. This upper level short wave trough is expected to
tilt negatively by the afternoon/evening hours of Monday, and the
regions from Bahia to Serra do Mar, and inland into the Cerrado
region of Brasil can expect to see divergence aloft from Monday
into Wednesday morning. In the lower levels, the presence of a
frontal boundary extends into the aforementioned regions, where an
occlusion is forming on Monday evening just off-shore Sao Paulo.
This will favor moist onshore flow along the Serra do Mar region
from Monday into early Wednesday morning. This area will continue
to see persistent troughing in the lower levels, with assistance
of the mid to upper level trough that remains in the region from
Monday through early Thursday morning. As the low level frontal
boundary goes through cyclogenesis over water, it will favor
continuous moist onshore flow from Rio de Janeiro to Espirito
Santo on Tuesday, The weakening boundary over land will persist to
favor precipitation into the Cerrado region of Brasil on Tuesday
and the remnant troughs will remain in the region on Wednesday.
This area of troughing is expected to remain persistent over the
region and moderate to heavy precipitation is expected over the
next three days. On Monday, with the assistance of the upper air
conditions, expect maxima of 50-100mm in Parana and southwest
coastal Sao Paulo. The rest of coastal Sao Paulo can expect maxima
of 40-80mm. Along the frontal boundary in Minas Gerais, and Rio de
Janeiro, expect maxima of 25-50mm, while an area of low level
troughing favors maxima of 30-60mm in Espirito Santo, south Bahia,
and east Minas Gerais. As moisture is expected in the northern
portion of the Amazon Basin, expect maxima of 25-50mm from south
Colombia/Venezuela, into the northwest Amazon Basin, as moist air
interacts with the Andes in north and central Peru. Over the
eastern Amazon Basin, expect maxima of 20-35mm. On Tuesday,
southeast Minas Gerais and Espirito Santo can expect maxima of
35-70mm, with a slight risk of severe weather. South Minas Gerais
and Rio de Janeiro can expect a decrease in maxima of 20-35mm,
with a marginal risk of severe weather. The Cerrado region can
expect maxima of 20-45mm. The central Amazon Basin can expect
maxima of 30-60mm, while 20-45mm are expected from
Venezuela/Colombia into Amazonas-Brasil. Expect moist air to
advect southward along the Andes in south-central Peru, with
maxima of 20-35mm. By Wednesday, the upper level trough is
expected over the Atlantic Ocean, weakening and thus decreasing
its effects over the region. However, due to the amount of
available moisture and the persistent troughing over the region,
expect coastal Bahia to see maxima of 40-80mm, while central Bahia
can expect maxima of 25-50mm. More moisture is expected in the
northern portions of Bolivia and south Peru, where expect maxima
of 30-60mm, while the west and central Amazon can expect maxima of
20-45mm.

In the mid latitudes of South America, a ridge axis extends over
Argentina, while a cut-off low present over the South Pacific
Ocean is approaching north-central Chile by Monday evening. The
Polar jet remains to the south, over Austral Chile and the south
Patagonia region, where they will see the persistent entry of
frontal boundaries from Monday into Thursday morning. Due to the
lack of moisture transport from the tropical region of the
continent due to the lack of low level jets over Bolivia and north
Argentina, drier conditions are expected over portions of
Argentina, the Chaco region of Paraguay, and Uruguay from Monday
through early Wednesday morning. This pattern will change on
Wednesday, as the upper level cut-off low enters as a shortwave
trough, embedded in a larger amplitude upper level trough that
enters south Chile by Wednesday morning. These conditions will
assist in lowering the pressures in the low levels of the
atmosphere, activating low level jets of south Bolivia and north
Argentina, and thus transporting moisture in the region. Although
with these conditions in the region, of interest is Austral Chile,
where they will see the greater entrance of moisture over the next
three days as weak atmospheric rivers accompany frontal
boundaries, encountering the terrain and favoring orographic lift
in Austral Chile. In terms of precipitation, on Monday, expect
maxima of 15-25mm in south Magallanes, while similar amounts are
expected in west Rio Negro-Argentina with the presence of a
frontal boundary. On Tuesday, Austral Chile can expect maxima of
20-35mm in south Aysen into north Magallanes, while south
Magallanes, extreme southwest Buenos Aires province, and the north
Cuyo region of Argentina can expect maxima below 20mm. On
Wednesday, the same regions can expect maxima below 15mm.

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

Castellanos...(WPC)