811
FXUS20 KWBC 041902
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
202 PM EST Thu Dec 04 2025

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

South American Forecast Bulletin for 04 December 2025 at 1905 UTC

On Thursday, the central portion of the continent is seeing the
South Atlantic Convergence Zone (SACZ) extend from Espirito
Santo/Rio de Janeiro to the southeastern Amazon Basin in the early
afternoon hours. The SACZ is expected to remain in that region
into the evening hours, favoring heavy precipitation into the
early morning on Friday. The conditions for the SACZ are seen
present in the atmospheric column, where a weak upper level ridge
remains over western Brasil, a mid to upper level trough extending
from the south Atlantic Ocean with its base over the southern
Cerrado region of Brasil, weakening into Friday. In the lower
levels, the 700/850hPa levels show a continuous confluence that
relates to the SACZ, weakening during the day on Friday, due to
the weakening mid level trough that is also propagating eastward,
away from the continent. As such, the position of the SACZ is
expected to remain over the Cerrado region of Brasil, while
moisture convergence and troughing remains over the southeast
Amazon Basin. The rest of Amazon Basin will continue to see
moisture over the region that will be affected by the diurnal
cycle and continued instability from a persistent mid-level trough
extending from Amazonas-Brasil into Bolivia. By Saturday, the SACZ
is expected to have dissipated, leaving behind troughing in the
lower levels and moisture. The mid-level trough is expected to
intensify over Amazonas-Brasil into north Bolivia, favoring a
reflection in the lower levels, where a low level circulation is
present, enhancing lift over the region and a potential for heavy
precipitation. By Saturday, the Bolivian High in the upper levels
is expected to solidify, centered over Bolivia and extending
zonally into the Pacific Ocean and into the Parana Basin in
Brasil. Enhanced upper level divergence from the Madden-Julian
Oscillation, as seen throughout the entire week, will favor deep
convection over the region, and heavy precipitation is expected to
be scattered to widespread over the central and northern regions
of South America. On Thursday, expect maxima of 40-80mm in
Espirito Santo, while a widespread area of heavy precipitation of
30-60mm is expected from the Cerrado region into the eastern
Amazon Basin. Localized heavier totals are expected throughout the
region. On Friday, expect similar amounts from Espirito Santo
through the southeastern Amazon Basin. An increase in
precipitation is expected in Bolivia as the mid-level trough
begins to intensify. By Saturday, the remaining available moisture
and troughing will favor similar maxima in the aforementioned
regions of Brasil. The circulation forming over north Bolivia is
expected to favor heavy precipitation.

From the Chaco region into the Cuyo region of Argentina, there
will be an increasing unstable atmosphere over the next three
days. On Thursday, a cutoff low over the Pacific Ocean, just west
of north Chile, is slowly propagating eastward, and extending its
divergent side from south Peru into north Chile and into the Chaco
region beginning on Thursday. From the southern Peruvian Andes,
into the Altiplano of Bolivia, into the Andean region of northwest
Argentina could see maxima ranging between 20-25mm. On Friday, the
cutoff low moves eastward, while accompanying upper jet stream
intensifies over the continent. With the weakening of the SACZ to
the north, moisture slowly enters the region along the foothills
of the Andes Mountains, aided with the northward movement of a
frontal boundary entering the Cuyo region by Friday evening,
moisture convergence is expected between the Cuyo Region and the
Sierra de Cordoba in Argentina. Due to the divergent conditions
from the upper level cutoff low entering the region, expect a
slight risk of severe weather, as well as maxima ranging from
30-60mm in the northern Cuyo Region. The higher altitudes of the
western Cuyo Region can expect maxima of 15-25mm. In the Chaco
region, expect orographic lift and moisture convergence along the
foothills of the mountains. Precipitation totals are expected to
range from 35-70mm. On Saturday, the cutoff low becomes negatively
tilted as it enters northern Chile, enhancing the risk of moderate
severe weather over northwest Argentina and into the Sierra de
Cordoba region. Moisture continues to slowly increase in the
region, and precipitation totals are expected to range between
30-60mm.

In the southern cone of Chile, an upper level trough extends from
the Southern Ocean and a weakening upper level jet interacts with
the Austral Region of Chile. On Thursday and Friday the levels of
available moisture are not expected to be significant, even with
the presence of the upper level dynamics. As such, the total
precipitation is expected to remain below 30mm in Austral Chile.
By Saturday, a ridge axis extends into the southern cone of the
continent, favoring subsidence. However, an increase in available
moisture from an atmospheric river entering Austral Chile will
favor maxima below 35mm in northern Austral Chile.

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

Castellanos...(WPC)


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