846
FXUS20 KWBC 021913
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
213 PM EST Tue Dec 02 2025

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

South American Forecast Bulletin for 02 December 2025 at 1915 UTC

The north and central regions of the continent are seeing heavy
precipitation across the region with the potential development of
South Atlantic Convergence Zone event across the region in the
middle of the work week. The conditions for this event are
starting to show become present in the model guidance as we
approached the second half of the week. On Tuesday, a shortwave
trough embedded in a mid to upper level trough is strengthening
over the south Atlantic Ocean with its base over Paraguay, Uruguay
and south Brasil. This trough will interact with the weak ridging
over central Brasil that extends into Peru. In the lower levels,
the presence of this shortwave trough in the mid to upper levels,
and a weak jet over Paraguay and south Brasil is ventilating the
presence of troughs and fueling the advancing cold front expected
from the southern Chaco region, through the Mesopotamia region of
Argentina, and exiting through Santa Catarina-Brasil, where an
occlusion is expected to develop offshore by Tuesday evening. The
presence of these shortwave troughs over south Brasil will favor
instability and potential for marginal risk of severe weather
along the Serra do Mar region of Brasil. Troughing over central
Bolivia and increased orographic lift along the Andes Mountains
into central portions of Peru will have the support of a mid level
trough that extends from Bolivia into southern Peru. On Wednesday,
these conditions are expected to remain over the region. The mid
to upper level trough strengthens and continues to propagate east
and north, extending into Mato Grosso do Sul and Goias, and
generating some instability for south Brasil. The cold front is
expected from the central Chaco region along central Paraguay, and
through the Parana Basin, exiting Rio de Janeiro by Wednesday
evening. As the western part of the front begins to interact with
the upper ridge, it is expected to weaken by Thursday, but ample
troughing remains in the region, while an increase in available
moisture over the west and southern Amazon River Basin will favor
heavy precipitation. To the east, the presence of a convergence
zone begins to show in the 700 to 850 hPa levels from Rio de
Janeiro/Espirito Santo and enters as far northwest as Mato
Grosso-Brasil. Heavy precipitation is expected over Rio de Janeiro
and Espirito Santo, with moderate precipitation over central
Brasil. However, locally higher precipitation totals are expected
over the region. By Thursday, the convergence zone in the 700-850
hPa level remains from Espirito Santo through the Mato Grosso
region, where heavy precipitation is expected to continue from
Thursday into early Friday morning. Moisture is expected to
continue to pool over the central Amazon Basin, where moderate
precipitation is expected, while troughing in the lower levels
continue in the region. A slight decrease in precipitation in
central Peru and Bolivia is expected on Thursday as available
moisture is expected to decrease during the day. In the upper
levels, the trough over southern Brasil begins to move eastward
over the Atlantic Ocean, with its base in the southern Cerrado
region of Brasil.

As the conditions in the tropical and subtropical regions of the
continent continue to activate, the southern regions of South
America are expected to remain relatively dry over the next three
days, with the exception of the Austral and Sur regions of Chile.
On Tuesday, the polar jet stream is expected over the Southern
Cone of the continent, with the presence of an upper trough
extending into the region from the Southern Ocean. In addition, a
variety of frontal boundaries enter the region, with some
available moisture that interacts with the terrain. The Austral
region of Chile can expect moderate precipitation on Tuesday. On
Wednesday, a weak moisture plume enters the aforementioned region
and generates light precipitation from Los Lagos through north
Magallanes, with a chance of mountain snow. The lighter amount of
precipitation coincides with the weakening of the upper jet over
the region. By Thursday, another moist plume enters the region,
and it will be accompanied by a mid to upper level shortwave
trough that will enhance the precipitation over the Austral
region. Expect moderate precipitation with a potential for
mountain snow. In northern Chile, a cutoff low has been present
over the Pacific Ocean over the forecast period, slowly inching
near the north Chile coast. By Thursday, the eastern side of the
cutoff low is expected to enter the region, where enhanced
divergence will be available aloft. Although the presence of the
frontal boundary over the Chaco Region and central South America,
as well as the absence of the South American Low Level Jet has not
allowed for moisture transport from the north, there will be some
available moisture along the foothills of the mountain regions as
well as in the Altiplano region expanding from Bolivia, into north
Chile and northwest Argentina. The region could see a potential
for light to moderate precipitation on Thursday, with a chance of
mountain snow.

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

Castellanos...(WPC)


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