South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
156 PM EDT Mon Sep 08 2025

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

South American Forecast Bulletin from 08 September 2025 at 1800 UTC


... In Mid - Latitude South America...

In Chile...

In the upper levels, the right exit region of a potent jet streak
affiliated with a vertically stacked upper trough, that currently
has an axis of from 27W to 43W, will begin to promote upper
divergence across central Chile starting Monday afternoon. Upper
divergence will remain favorable through Tuesday evening as the
aforementioned upper troughâ€s axis crosses the Andes. Also
starting on Monday afternoon, a mid - level shortwave trough is
anticipated to move across south and central Chile. In the low
levels, north - northwesterly flow will exceed 50 knots between
Coquimbo and Los Lagos through Tuesday morning. During this
period, precipitable water values may reach 20mm across Los Lagos,
yielding to total precipitation maxima of 20 - 35mm with the risk
for cumulonimbus from Monday morning through Tuesday morning.
Meanwhile, between Los Rios and Coquimbo, precipitable water
values may reach 38mm and may favor a total precipitation maxima
of 20 - 45mm from Monday morning through Tuesday morning.

Starting Tuesday morning, the mid - level trough will become more
negatively tilted, favoring an increase in instability across
south - central Chile. In the low - levels, a more west - north
westerly flow will dominate between Los Rios and Coquimbo, with
the wind speeds continuing to exceed 40 knots through midday
Wednesday. Thereafter, expect a decrease in both wind speed and
precipitable water. The combination of upper level divergence,
enhanced moisture convergence, and instability will favor total
precipitation maxima of 25 - 50mm between Los Rios and Coquimbo
from Tuesday morning through Wednesday morning. Meanwhile, expect
total precipitation maxima of 15 - 25mm in southern Chile.
Mountain snow is possible as well with this system.


In Central Argentina...

A cold front will begin to develop early Tuesday morning across
northern Patagonia. By Tuesday afternoon, it will be located
across the Rio Negro Province. By Wednesday afternoon, the front
will be located across from the Santiago del Estero Province and
into the Entre Rios Province of Argentina and extending into
central Uruguay.

In the upper - levels, a potent jet streak and affiliated upper
trough will begin to promote upper divergence across La Pampa -
Buenos Aires Province beginning early Wednesday morning through
Wednesday afternoon. During this period, a mid - level trough and
increase in moisture in the low levels will favor the potential
for precipitation across La Pampa - Buenos Aires between Tuesday
morning and Thursday morning. A daily total precipitation maxima
of 15mm is possible in this region.


In Argentina, South Brasil, Paraguay, and Uruguay...

An upper trough, that currently has an axis at 63W, will continue
to favor divergence in its exit region through Tuesday morning
across the region. It is accompanied by a weakening jet streak max
that will further sustain divergence across southern Brasil and
north Uruguay. In the mid - levels, a shortwave trough will favor
destabilization, especially across sectors of Paraguay and far
northern Argentina. At the low levels, expect cyclogenesis, and
frontogenesis, to occur Monday afternoon across Uruguay and
Southern Brasil. The developing cold front will be located across
Paraguay and south Brasil by Monday evening. Along and ahead of
the cold front, expect enhanced moisture convergence.

The combination of the cold mid - levels, which will promote
instability, upper level divergence, and an increase of moisture
from the tropics will yield to total precipitation maxima of 20 -
45mm across southern Brasil, where a slight risk for severe
weather is likely. Across the borders of Argentina, Brasil, and
Paraguay, expect a total precipitation maxima of 20 - 35mm with a
moderate risk for severe weather. Across Central Paraguay, expect
a total precipitation maxima of 15 - 25mm with a moderate risk for
severe weather.

The aforementioned cold front will continue moving northward into
Brasil. By Tuesday evening, the front will be located between
Serra do Mar and Espirito Santo and into the Cerrado region of
Brasil. By Wednesday evening, the decaying cold front will be
located across Mato Grosso, the Cerrado region, and Espirito
Santo.


Elsewhere in Eastern Brasil...

A series of low level moisture plumes will continue to impact
parts of Espirito Santo Bahia, and Brasilian Nordeste from Monday
morning through Wednesday morning. Total precipitation maxima of
up to 15mm are possible in this region. A shear line is expected
to develop after Wednesday morning, extending across Bahia and
yielding a total precipitation maxima of 15mm.


In northern Paraguay...

From Wednesday morning through Thursday morning, an upper level
shortwave trough will help favor a more diffluent pattern across
northern Paraguay. This will positively contribute to the
sustenance of upper divergence. In the mid - levels, a shortwave
trough may help trigger precipitation in this sector and
contribute to the destabilization of the vertical column, which
could promote severe weather. A significant increase in low level
moisture and cyclonic flow will favor total precipitation maxima
of 15mm with a slight risk of severe weather.


... In Tropical South America ...

In Ecuador and Peru...

An increase in low level easterlies will favor moisture
convergence and orographic effects through the forecast cycle. A
gradual increase in moisture in the low levels is anticipated from
Tuesday morning through Thursday morning. From Tuesday morning
through Wednesday morning, a total precipitation maxima of 20 -
40mm is possible, and from Wednesday morning through Thursday
morning, a total precipitation maxima of 20 - 35mm is possible.

Elsewhere in tropical South America, local effects coupled with
diurnal processes will help drive precipitation patterns. Low
level troughs will also continue to propagate across the region
and will be accompanied by moisture convergence, helping yield to
higher total precipitation maxima.



Tinoco-Morales...(WPC)