869
FXUS20 KWBC 031924
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
224 PM EST Wed Dec 03 2025

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

South American Forecast Bulletin for 03 December 2025 at 1930 UTC

On early Wednesday morning, the presence of the South Atlantic
Convergence Zone (SACZ) is seen on satellite and model guidance is
providing confidence that this event will last into late Friday
into early Saturday. On Wednesday, the upper level conditions are
disorganized over central portions of Brasil into Peru, but
generalized ridging is expected to continue over the next several
days. Two upper level troughs are present to the east and west of
the ridging over the continent. A closed upper low is located west
of Peru and Chile, over the Pacific Ocean, while a short wave
trough extends into south Brasil and Paraguay from the southern
Atlantic Ocean. In addition, there is an upper divergent setup due
to the presence of the Madden Julian Oscillation and an
atmospheric Kelvin wave, enhancing precipitation over the region.
As the jet stream between the upper level trough strengthens into
Thursday and Friday, the upper ridge is expected to strengthen
over over west Brasil, Peru and into Bolivia and north Paraguay.
In the mid levels, a significant trough extends into south Brasil
and Paraguay, providing mid-level support for the frontal boundary
that is present over the same region on Wednesday and into
Thursday. This trough will also assist in increasing instability
and favor marginal risk of severe weather on Wednesday into
Thursday from the southern Parana Basin, along the Serra do mar
region, and into Rio de Janeiro. The frontal boundary is expected
to extend from the southern Atlantic Ocean and enter the continent
through Rio de Janeiro and extend as a weakening stationary front
from the southern Parana Basin and into central Paraguay by
Wednesday evening. Scattered to widespread thunderstorms are
expected with heavy precipitation over the large area of coverage.
The areas with the heaviest precipitation are expected over the
Central and southwestern Amazon Basin, with maxima of 35-70mm, and
in Rio de Janeiro and Espirito Santo with maxima of 40-80mm.
Amounts exceeding these maxima are expected locally. On Thursday,
the extent of the SACZ is expected to decrease as the mid level
trough begins to weaken over southern Brasil and propagate
eastward into the Atlantic Ocean. The heaviest precipitation is
expected in Espirito Santo and Rio de Janeiro and extend inland
into southeast Mato Grosso.  As dry air begins to advect from
Colombia/Venezuela, expect precipitation totals to decrease on
Thursday. Over northern Chile and south Bolivia, the western
cutoff low in the upper level begins to enter the continent, as
its divergent side is located over the Altiplano of Bolivia and
along the western Chaco region. Although no significant moisture
transport has occurred due to the lack of the South American Low
Level Jet (SALLJ), the presence of the divergence aloft,
converging winds along the foothills of the mountain range, expect
maxima of 20-35mm in the higher elevations of northwest Argentina
and into the southern Altiplano of Bolivia. On Friday, the
conditions surrounding the region of the SACZ decrease, as the mid
level trough has propagated to the east away from the coast and a
weak shortwave trough remains as its base by Friday evening in
southern Brasil. The main component of the SACZ is located over
eastern Minas Gerais and Espirito Santo. Heavy precipitation is
expected to remain over the eastern portions of Brasil and along
the area of the SACZ on Friday into early Saturday morning. Areas
of troughing and increased moisture is expected to remain over the
Amazon Basin and will be affected by the diurnal cycle. Moderate
to heavy precipitation is expected in scattered to widespread
thunderstorms over the region. The continued presence of a mid
level trough over Bolivia will reflect in the lower levels, as
significant troughing is expected on Friday over central Bolivia,
which will be accompanied by moisture. Heavy precipitation is
expected in the region. In southern Bolivia into north Argentina,
the upper trough continues to enter the region slowly and favor
diffluence over the region. As moisture remains over northwest
Argentina and into the Altiplano region of Bolivia, expect
moderate precipitation on Friday.

In the southern region of the continent, southern Chile is
expected to see significant precipitation over the next three
days. The entry of frontal boundaries with some moisture
associated with them will favor precipitation in the Austral
region of Chile. In the upper levels, an upper trough with a jet
streak over southern Chile will assist with divergence aloft of
Wednesday and Thursday, as shortwave troughs in the mid levels
provide additional support for lift in the region. On Wednesday,
expect a potential for 15-25mm of total precipitation from Los
Lagos to north Magallanes, while on Thursday, expect maxima of
25-50mm, as moisture converges along the terrain, and assistance
from a negatively tilted shortwave trough in the mid-levels
enhances the conditions. By Friday, the entrance region of the
upper jet enters southern Chile, followed by a ridge axis,
favoring subsidence throughout the day. Expect maxima to remain
below 35mm in Austral Chile.


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

Castellanos...(WPC)


$$