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FXUS20 KWBC 101854
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
153 PM EST Wed Dec 10 2025

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

South American Forecast Bulletin for 10 December 2025 at 1900 UTC

The northern portion of the continent is seeing the presence of an
organized Bolivian High centered in central Bolivia on Wednesday,
extending into the Pacific Ocean, the Amazon Basin, central
Brasil, and into north Argentina/Paraguay. To the southeast, a
negatively tilted shortwave upper trough reaches south Brasil and
Uruguay, and exits into the south Atlantic Ocean. This will favor
the risk of severe weather in the Serra do Mar region of Brasil on
Wednesday. In the lower levels, the presence of a frontal boundary
extends from south Bolivia, along the Parana Basin, and exits
through the Serra do Mar region, connecting to an occlusion to the
south of Brasil. To the southwest of the Bolivian High, another
shortwave trough is entering the continent over Central Chile,
favoring enhanced divergent conditions on the right side of the
trough, from the Chaco region through the Cuyo region of Argentina
on Wednesday into Thursday morning. Wednesday is the day with the
lowest amount of available moisture in the region, in comparison
to the rest of the forecast period.

On Thursday, the upper level shortwave trough over south Brasil
has exited the region and into the Atlantic Ocean, decreasing the
potential for heavy precipitation in southern Brasil on Thursday.
The Bolivian High over the central portions of the continent
becomes weaker over the region, as convection has decreased over
the last few days. From the Amazon Basin into the Parana Basin,
expect a generalized decrease in precipitation. An upper level jet
from Paraguay through south Brasil provides the assistance to
maintain a stationary front that extends from east Paraguay along
south Brasil, but as available moisture continues to remain low in
the region, expect precipitation to remain low. In the contrary
over the Chaco Region and south Paraguay, the shortwave upper
level trough enters the region and with its enhanced upper
divergence, favors the decrease of pressures in the region. The
development of the Chaco Low, activates the South American Low
Level Jet (SALLJ) over Bolivia and into the Chaco Region,
increasing the amount of available moisture coming from tropical
South America. As lift is present in the lower levels, increased
amounts of available moisture, divergence aloft is present, expect
the Chaco Region, northwest Argentina, and into central Paraguay
to see a chance of severe weather with heavy precipitation on
Thursday.

By Friday, the upper level shortwave trough is weakening as it
enters north Argentina, but the jet streak that remains, although
weak, continues to support upper level divergence from Paraguay
into south Brasil. This will continue to support the weak
stationary frontal boundary in the region, while the SALLJ
continues to bring moisture from the north into Paraguay, the
south Parana Basin and into coastal Sao Paulo. Along the frontal
boundaries and troughs, expect heavy precipitation for Friday into
early Saturday, with the risk of severe weather, primarily from
south Paraguay through Sao Paulo.

In the southern cone of the continent, generally low amount of
available moisture are favoring less significant precipitation in
the region over the next three days. On Wednesday, a weak frontal
boundary enters the region, with some moisture that will favor
generalized maxima below 20mm in Austral Chile. On Thursday, a
strong cold air mass is expected to push into the region with an
atmospheric river in Austral Chile. This will favor precipitation
extending from the Sur region into Austral Chile, although upper
divergence due to the exit region of an upper level jet stream
enters the region, expect maxima to remain below 35mm, as
conditions are trending to the drier end. The chance of mountain
snow is expected in higher elevations. On Friday, the northward
push of the cold air mass enters central Chile, however, the
decrease in available moisture as the atmospheric river effects
weaken in the region, expect maxima to remain below 20mm with a
chance of mountain snow, from south-central Chile into 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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