South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
101 PM EDT Wed Sep 10 2025

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

South American Forecast Bulletin from 10 September 2025 at 1700 UTC

In the central portion of the forecast region, the upper level
Subtropical Ridge extends from the eastern Pacific Ocean and
extends over the western coast of the continent into northern
Chile. A weaker ridge axis is extending into northern Brasil from
the Atlantic Ocean and it is expected to continue expanding over
Brasil over Wednesday into Thursday. A weak upper level trough
builds over west Brasil and portions of Bolivia, favoring
interaction between the upper level systems and providing
diffluent conditions in north Peru and Amazonas-Brasil. On
Thursday, the Subtropical Ridge over the Pacific Ocean extends
into Bolivia, deepening the upper trough over south Brasil. In the
lower levels on Wednesday and Thursday, a frontal boundary
extending from Mato Grosso do Sul and into Espirito Santo remains
stationary, but weakens over the region by Thursday, and advancing
into the Atlantic Ocean. A secondary frontal boundary advances
from north Argentina and Uruguay into south Bolivia and through
south Brasil by Thursday evening. These systems are enhanced by
the upper level trough that is deepening. Favoring the potential
for precipitation in south Paraguay and portions of south Brasil.
However, due to amount of precipitable water remaining below 40mm,
precipitation totals are expected to remain below 20mm. A risk of
severe weather is expected over Paraguay on Wednesday and Thursday
as shortwave troughs propagate over Paraguay in the mid to upper
levels, associated with the upper level trough in the region. On
Friday, the upper level trough over east Brasil continues to
deepen as it is embedded between enhanced upper level ridges. The
base of this upper trough is expected to interact with a low level
trough that has been propagating over northeast Brasil since
Wednesday and expected over the eastern Amazon Basin by Friday
evening, where it will favor enhanced convection in the region,
favoring maxima of 15-25mm.

To the northwest, pressures are expected to lower over south
Central America, favoring low level flow towards the Andes region
in Colombia and north Peru over the forecast period. This
northwestward flow is advecting moisture to the aforementioned
regions, favoring moisture convergence along the Andes mountains
in Colombia and Peru. The enhanced moist low level flow is
expected to increase by Friday. In terms of precipitation, expect
generalized maxima of 20-35mm from central Colombia into east
Ecuador and north Peru. Generalized maxima of 20-45mm are expected
in the same region on Thursday and Friday.

In the southern regions of South America, drier conditions are
expected on Wednesday and Thursday as frontal boundaries in the
lower levels are to the west of Chile and being blocked by the
strengthening low level ridge to the east. In the upper levels, an
upper trough is advancing just west of Chile, but will not enter
the region until late Thursday into Friday, when enhanced upper
level jets will favor precipitation over the southern region of
Chile. The frontal boundaries finally propagate over the Andes
Mountains and are expected to enhance the moisture convergence in
the region. Maxima of 15-25mm with mountain snow is expected from
Los Rios into Aysen, while maxima below 15mm are expected in
Magallanes and central regions of Chile, as moisture associated
with the atmospheric river/frontal boundary passage is forecast to
remain below the 25-30mm range in precipitable water.

Castellanos...(WPC)