South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
107 PM EDT Tue Sep 09 2025

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

South American Forecast Bulletin from 09 September 2025 at 1700 UTC

In southern South America, a mid to upper level trough enters the
southern cone of South America by Tuesday evening, accompanied by
an enhanced Subtropical Jet over central Chile from Tuesday into
early Wednesday morning. In the lower levels, an atmospheric river
is entering central Chile, with precipitable water amounts ranging
from 25 to 30mm from Los Rios to south Maule. This moisture
transport, as it interacts with the Andes mountains, is also
expected to interact with the divergent side of the upper level
Suptropical Jet, enhancing precipitation in the aforementioned
regions in Chile. In addition, these conditions will favor
instability in the region, favoring a potential for convection
from Bio Bio to north Los Lagos from Tuesday evening into early
Wednesday morning. By Wednesday morning, the amount of available
moisture should decrease as the frontal boundary continues
progressing north and eastward. Maxima of 20-35mm are expected
from Ăuble to north Los Lagos, while south Los Lagos can expect
maxima of 15mm on Tuesday.

On Wednesday, the frontal boundary associated with the atmospheric
river continues east and northward into northern Argentina and
Uruguay. The SALLJ activates over south Bolivia and Paraguay,
favoring moisture to enter north Argentina and portions of
southwest Paraguay, where expect maxima of 15 to 25mm with a risk
of severe weather due to the diffluence in the mid to upper levels
from an upper level trough extending over Bolivia and Paraguay on
Wednesday. To the east in south Brasil, another frontal boundary
progresses from Rio de Janeiro to Mato Grosso Do Sul. However, due
to the low amounts of available moisture, precipitation totals are
expected to remain at around 10mm. Similar amounts are expected in
the Austral and southern regions of Chile as precipitable water
remains at around 20mm.

On Thursday, the aforementioned frontal boundaries are expected to
begin to lose definition from south Bolivia into Sao Paulo, as
well as in Rio de Janeiro. A weak shear line develops over
Espirito Santo, which will favor some statiform precipitation over
the region. Although these conditions are present, the low amount
of moisture in the central portions of South America, will favor
maxima of 15-20mm in Paraguay, 15mm in western Bolivia, as well as
along the northeastern region of Brasil. To the south, an
advancing frontal boundary brings precipitable water values under
25mm to Austral Chile, expected to interact with the terrain on
Thursday. Maxima are expected to remain below 25mm in Aysen and
portions of north Magallanes.

To the north, dry conditions are expected as moisture levels
continue to remain to the north tropical regions of South America.
As troughs propagate over north Colombia and Venezuela over the
forecast period, the low level flow favors a northward propagation
over north Brasil and Peru, and thus moisture is expected to
advect northward and interact with the Andes in portions of
central Colombia and east Peru. Expect maxima of 20-35mm/day in
Colombia and northeast Peru/east Ecuador from Tuesday through
Thursday. While precipitation totals from 15 to 25mm/day are
expected in central Peru and Amazonas-Brasil over the next three
days. Dry air is expected to advect into the Guianas and north
Brasil from the east over the next several days. Due to the lack
of moisture, no significant amount of precipitation is expected in
the region over the forecast period.

Castellanos...(WPC)