South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
157 PM EDT Thu Sep 11 2025

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

South American Forecast Bulletin from 11 September 2025 at 1800 UTC

On Thursday, a potent upper level ridge over the Pacific Ocean
extends from west of Ecuador/Peru and into the Chile, and a ridge
axis enters into portions of central Argentina. To the south and
west of this ridge, a high amplitude upper level trough extends
over the Pacific Ocean and is interacting with the upper ridge,
enhancing the upper level jets over Chile on Thursday into early
Friday. In the lower levels, a frontal boundary is accompanied by
an atmospheric river positioned west of Chile, blocked by the
presence of the low level ridge over Argentina. Precipitation
values are expected to remain below 20mm in Austral Chile on
Thursday. By Friday, the upper level trough advances to the east
and enters the continent by Friday evening, favoring the passage
of the frontal boundary and atmospheric river in Chile from Friday
into Saturday morning. The upper level jets continue to position
over Chile, where enhanced divergence is expected to interact with
the aforementioned frontal boundary with precipitable water levels
ranging from 20-25mm. As this moisture interacts with the terrain,
expect precipitation maxima of 20-35mm in the Zona Sur of Chile,
while Central Chile can expect maxima of 15-25mm. By Saturday, the
upper level trough continues to interact with the upper ridge and
ridge axis that are expected to restrain the north and eastward
movement of the trough further into the continent. By Saturday
evening, the base of the upper trough is expected into north
Argentina/Paraguay, while the eastward most extent of the trough
is expected to enter the Atlantic Ocean, just east of Argentina.
Due to the interaction between these systems, the base of the
trough will see the presence of mid to upper level shortwave
troughs propagating over portions of north Argentina and Paraguay,
where the potential for cyclogenesis is expected with the entry of
the frontal boundary over the Chaco region by Saturday evening
into Sunday. Additionally, the area could see a risk of severe
weather with precipitation maxima of 15-25mm. Shortwave troughs
are expected to enter in Austral Chile, while moisture from the
passing frontal boundary favors precipitation maxima of 15-20mm
with a risk of cumulonimbus clouds embedded in the system.

To the north, an additional upper ridge extends over east Brasil,
weakening the presence of an upper trough over Bolivia. The trough
however, is enhancing convection in Bolivia and Paraguay, which is
interacting with a pocket of moisture and a stationary front from
south Bolivia to Mato Grosso do Sul on Thursday. There is a
potential for severe weather associated with the afternoon storms
over the region. Additional moisture is expected in Colombia,
Peru, and Amazonas-Brasil, where low level moisture convergence
will favor convection and precipitation maxima ranging from
15-30mm on Thursday. On Friday, the region can expect generalized
maxima of 20-45mm as moisture continues to converge from south
Colombia into Ecuador and north Peru. A low level trough is
propagating west over Para-Brasil and is expected to bring
moisture to the Guianas and north Brasil on Saturday. In the upper
levels, a ridging pattern is enhanced, as the western ridge
meanders into the continent, interacting with the eastern ridge
and strengthening over north South America. This strengthening
will continue into Saturday, as convection strengthens over north
Brasil and into south Colombia/Venezuela by Saturday due to the
propagating trough. The area can expect maxima of 15-30mm.

Castellanos...(WPC)