160
FXUS20 KWBC 251818
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
118 PM EST Tue Nov 25 2025

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

South American Forecast Bulletin for 25 November 2025 at 1820 UTC

On Tuesday, the northern region of the continent is experiencing
the development of the South Atlantic Convergence Zone (SACZ),
over portions of central Brasil and exciting through south
Bahia/Espirito Santo by early Tuesday morning. The region is
seeing a prominent mid to upper level trough with its base near
Mato Grosso, while a weak ridging in the upper levels extends over
most of the Amazon Basin on Tuesday morning. In the lower levels,
at around 700hPa, there is a convergence zone located around north
Mato Grosso and extends along the Cerrado region, where it exits
the aforementioned region of south Bahia/Espirito Santo. In
addition, there is the entrance of an atmospheric Kelvin Wave over
tropical South America, which will assist with upper level
divergence as widespread thunderstorms are expected in Brasil and
extending into portions of the western Amazon River Basin. By
Tuesday evening, the SACZ is expected to remain over the
aforementioned regions, and is expected to be accompanied by a
frontal boundary to the south, located from the Pantanal region
and exiting through Espirito Santo. These conditions are expected
to remain over the region through early Friday morning. However,
the SACZ is expected to begin weakening by Thursday morning, were
the extent of the SACZ is expected mostly over south Bahia and
extends into the Atlantic Ocean. Due to the SACZ remaining to the
east of South America, the flow of moisture is expected to
continue along the Andes Mountains in portions of Peru and
Bolivia. Since the amount of available moisture is expected to
remain high over the next three days, orographic lift will favor
moderate precipitation along the foothills of the mountains over
the forecast period. This will enhance convection due to the
presence of a mid level trough that extends over portions of
Bolivia into south Peru from early Wednesday through early Friday.
Expect moderate to heavy precipitation over the region for the
next three days. On Tuesday, expect maxima of 40-80mm from south
Bahia through Espirito Santo, along the regions with the SACZ,
expect widespread precipitation, with maxima ranging from 30-80mm
from the Cerrado region through the central Amazon Basin. The
southwest Amazon Basin can expect maxima of 20-45mm. On Wednesday,
the onshore flow into south Bahia and Espirito Santo has increased
moisture associated with it, and will bring heavier precipitation
in the region as this persistent flow continues throughout the
day. They can expect maxima of 60-125mm. In inland Bahia into east
Minas Gerais, expect maxima of 35-70mm. A decrease in
precipitation over the northeast Mato Grosso/Tocantins region is
expected as the SACZ begins to weaken, with maxima of 30-60mm.
With the northeasterly trade winds advecting moisture over the
central Amazon into Peru, expect the region to see a widespread of
thunderstorms with maxima of 35-70mm. By Thursday, moisture
remains over most of central Brasil and extends to northern Peru,
which will assist with moderate to heavy precipitation in the
region. Expect generalized maxima of 30-70mm from south
Bahia/Espirito Santo, along the Cerrado region, and into the
southern Amazon Basin in central Brasil. North Peru and into west
Amazonas-Brasil can expect maxima of 25-50mm.

In the southern portion of the continent, drier conditions are
expected as the presence of the SACZ is debilitating the moisture
transport into southern South America on Tuesday and Wednesday. In
addition, in the upper levels, the presence of a ridge axis over
north Argentina, followed by the divergent side of a negatively
tilted shortwave trough over Chile by Tuesday night will favor
some showers over the north Patagonia region and portions of the
northwest Cuyo region in Argentina. Especially in the north
Patagonia, expect maxima of  15mm with a marginal risk of severe
weather with the instability present on Tuesday. In Austral Chile,
the presence of the Polar jet stream over the region will favor
some divergence aloft while a weak atmospheric river and frontal
boundary enter the region. This will favor maxima of 20-35mm from
south Aysen through north Magallanes. On Wednesday, the presence
of the upper level shortwave trough entering central Argentina, is
expected to reflect in the lower levels as pressures begin to
drop, favoring troughing, as well as the entry of the frontal
boundary in Central Patagonia. Since moisture remains low over the
region, expect precipitation totals to range between 15-25mm over
the Cordoba region, and into the northern Patagonia. However,
instability increases with the shortwave trough present, and thus
the aforementioned regions can expect a slight to moderate risk of
severe weather. Less moisture is available along Austral Chile,
and thus no significant precipitation totals are expected on
Wednesday. By Thursday, the upper level shortwave trough
propagates eastward, weakening over central Argentina. By
Thursday, moisture transport from the north is enhanced with the
presence of the South American Low Level Jet (SALLJ), meaning this
different air mass will interact with the northeasterly
propagating frontal boundary over portions of Buenos Aires, and
central Argentina. With the increase in moisture, expect Buenos
Aires to see maxima of 20-35mm with a slight risk of severe
weather, while the Cordoba/Santa Fe region can expect maxima of
10mm.

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

Castellanos...(WPC)


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