South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
304 PM EDT Mon Sep 15 2025

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

South American Forecast Bulletin from 15 September 2025 at 1910 UTC

In Central South America ...

An upper shortwave trough, which currently has an axis between 27S
and 37S and 52W to 55W, is favoring a diffluent flow pattern along
its exit region. This is positively favoring upper divergence
across Uruguay and southern Brasil. A mid - level trough will also
be moving across this region through the day on Monday, where
vorticity advection and destabilization is likely. The low levels
will begin to see a decreasing trend in pressure levels, where
cyclogenesis is anticipated starting Monday evening around 31S and
51W. Also note that precipitable water totals may exceed 32mm
along the northern periphery of the developing progressive low
pressure system. At the surface and low levels, a potent high
pressure system will push the aforementioned low pressure system
to the north and east. Areas of greatest precipitation impact from
Monday morning through Tuesday morning include the Mesopotamia
region of Argentina, northern Uruguay, and southern Brasil, where
a total precipitation maxima of 20 - 35mm is likely with a
marginal risk for severe weather. From Tuesday morning through
Wednesday morning, the area with the greatest precipitation impact
will be along the Serra do Mar region where a total precipitation
maxima of 15 - 25mm is possible. After Wednesday morning, the low
pressure system will begin to lose its definition. Some moisture
convergence may still be likely across Serra do Mar, where a total
precipitation maxima of 15mm is possible.

Another region of interest is the Chaco region. Through the day on
Monday, northerly low level wind flow will be persistent across
northern Argentina, Paraguay, Bolivia. By Monday afternoon, low
level winds will begin to shift from the southeast across
Argentina. The convergence between these opposing wind flows will
favor a cyclonic flow pattern, especially along a pre-existing
stationary front. This wind flow pattern may enhance moisture
convergence across the foothills of the Altiplano in Bolivia,
yielding to a total precipitation maxima of 15mm from Monday
morning through Tuesday morning.

Thereafter, the next time period of interest is after Wednesday
morning. Another upper level trough will begin moving across the
region and will interact with the subtropical ridge to the north,
which will favor the increase in upper level winds. These
conditions will further enhance upper level diffluence and
divergence, especially across Paraguay. In the low levels, there
will be a significant increase in precipitable water, which may
exceed values of 38mm. Moisture convergence will be favorable
along a stationary front that will be located across northern
Argentina. These conditions will yield a total precipitation
maxima of 25 - 50mm across northern Argentina, where a slight risk
for severe weather will be present due to cold mid - levels.
Another region of interest is north - central Argentina. After
Wednesday afternoon, there will be an increase in upper levels
associated with a propagating jet streak max moving from Chile
into the eastern Andes through the day on Wednesday. There will be
a significant increase in upper level divergence associated with
the jet streak max. A warm front will also strengthen across the
La Pampa / Buenos Aires region due to the interaction with the
upper levels, which will assist in the lowering of pressure
values, and the convergence of the tropical air mass to the north,
enhanced low level northerlies, and polar air mass to the south,
enhanced low level southerlies. Moisture convergence and lift will
be abundant along and ahead of this frontal boundary and will
favor total precipitation maxima of 25 - 50mm across La Pampa /
Buenos Aires.


Southern South America...

In Chile, an upper level jet streak max will promote divergence
across southern Chile through Tuesday morning. During this period,
there will also be a long fetch moisture tongue interacting with
the region, increasing precipitable water values up to 25mm. The
upper level enhancement and orographic effects will favor total
precipitation maxima of 20 - 35mm. Some instability will be
present in the region, which will help with the development of
cumulonimbus. Also anticipate mountain snow and risk for
cumulonimbus associated precipitation. After Wednesday morning,
another jet streak max and low level atmospheric science will
begin to promote divergence and increasing precitable water across
central Chile. This will favor total precipitation maxima of 15 -
25mm from Wednesday morning through Thursday morning.


Elsewhere in Tropical South America...

Low level troughs will be a significant driving force for
precipitation across the area. The highest precipitable water
content will remain in the west Amazon Basin. The highest total
precipitation maxima will occur from Wednesday morning through
Thursday morning, when upper level diffluence will be favorable
across this region. Expect total precipitation maxima of 25 - 35mm
to be favorable. Another region to consider is the Pacific side of
Colombia. Low level westerlies will become more dominant,
especially after Tuesday morning. Minimal mid - level shear in the
region may favor the formation of deep convection. Thus, a total
precipitation maxima of 25 - 50mm is likely from Tuesday morning
through Wednesday morning. From Wednesday morning through Thursday
morning, a total precipitation maxima of 35 - 70mm is likely.
Elsewhere, diurnal convection will dominate.


Tinoco-Morales...(WPC)