South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1134 AM EDT Fri Oct 10 2025

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

South American Forecast Bulletin from 10 October 2025 at 1530 UTC

On Friday, an upper level shortwave trough is entering the central
region of Chile throughout the day, with the divergent jet stream
exit region over central Argentina by Friday evening. In the lower
levels, a frontal boundary is located over central Chile, with an
occlusion offshore Los Lagos, and continues over the northern
Patagonia region, as it exits into the southern Atlantic Ocean.
Low level jets from the north will be transporting moisture into
central Argentina, favoring low level moisture convergence along
the boundary of the cooler air mass.  The instability expected
from these conditions will favor a chance of moderate severe
weather over the central regions of Argentina, extending from the
north Patagonia, into south Cordoba and into the Buenos Aires
province on Friday into early Saturday. Precipitation totals are
expected to remain below 25mm. On Saturday, the upper level
shortwave trough becomes negatively tilted over north and central
Argentina, with the divergent exit of the jet stream extending
from Paraguay to Uruguay and portions of Buenos Aires. In the
lower levels, the remnants of the occlusion associated with the
frontal boundary also crosses over into central Argentina and is
expected to have the upper level support from the divergence of
the shortwave trough and the jet stream for cyclogenesis over
Buenos Aires by early Sunday morning. The cold front extends from
the south Chaco region in northwest Argentina, entering into
Uruguay, and exiting into the Atlantic Ocean just east of the Rio
de La Plata Delta and Buenos Aires by Saturday evening. With a
prefrontal trough extending from Paraguay into Rio Grande do Sul.
Increased levels of available moisture along the frontal boundary
from south Paraguay to Rio Grande do Sul will favor maxima of
35-70mm. The southeastward onshore flow in the Buenos Aires and
north Patagonia region will favor maxima of 30-60mm. On Sunday,
the shortwave trough in the upper level attempts to close off, as
it propagates to the east, entering the Atlantic Ocean. However,
the upper jet stream to the north also propagates east and north,
with its divergent exit region over Paraguay, Mato Grosso and
reaching the Serra do Mar region of south Brasil. With the
conditions similar from Saturday, with an increase in available
moisture, expect the region to see a slight risk of severe weather
over the south Parana Basin into Santa Catarina, where they can
also expect maxima of 40-80mm. From south Mato Grosso to the Serra
do Mar region expect maxima of 15-25mm. To the south, the
occlusion associated with the cold front will continue to favor
moist onshore flow into Buenos Aires, where expect maxima of
20-35mm.

In tropical South America, the presence of troughs in the lower
levels will be under the effect of diurnal processes over the
forecast period. While these troughs propagate along the easterly
trade winds, they will be influenced by the amount of available
moisture that will favor moderate precipitation totals over the
weekend and into the next work week. On Friday, expect maxima of
20-35mm over the Amazon Basin into Mato Grosso. On Saturday, a
slight decrease in precipitation is expected over the region, as a
pocket of dry air is advected over the Guianas and east Brasil.
Most of the Amazon Basin can expect maxima below 25mm. The
exception is Mato Grosso region of Brasil, as it begins to feel
the effects of the incoming upper level trough from the south.
They can expect maxima of 20-45mm. By Sunday, moisture is expected
to pool over south Colombia/Venezuela into Amazonas-Brasil,
enhanced by a weak upper trough over Colombia, where expect maxima
of 15-30mm. The southeastern Amazon Basin, into Tocantins and Mato
Grosso will see a weak upper level trough axis, enhancing the low
level troughs in the region. This interaction will favor maxima of
25-50mm on Sunday.

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

Castellanos...(WPC)