South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
333 PM EDT Mon Aug 03 2026

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

Forecast Bulletin 03 August 2026 at 1935 UTC:

An upper level trough extends from north Argentina into north Peru
and Ecuador early Monday, and it propagates eastward and begins to
tilt negatively overnight into early Tuesday morning. Embedded in
the trough are mid to upper level shortwave troughs that are
making their way from Paraguay through south Brasil, fueling
instability over the region. In the low levels, a frontal boundary
remains stationary from north Argentina and extends into Uruguay
and south Brasil. As moisture from the tropical regions reaches
the frontal boundary, expect enhanced moisture convergence and
lift, fueling moderate to heavy precipitation, particularly in
southern Mesopotamia-Argentina and into northern Rio Grande do
Sul. Eastern Uruguay can expect moderate precipitation on Monday.
From south Paraguay through Rio Grande do Sul, expect a slight
risk of severe weather. On Tuesday, the upper level trough is
fortified by the Subtropical Jet stream. The divergent side of the
trough remains over central South America, while mid to upper
level shortwave troughs enhance the instability from south
Paraguay through southern Brasil. In the lower levels there is a
reorganization of the stationary frontal boundary and a weakening
of the South American Low Level Jet that is transporting moisture
into the region. This decrease in moisture will favor light
precipitation from Paraguay through south Brasil on Tuesday. By
early Wednesday morning, the SALLJ beings to strengthen with the
enhanced jet streak of the Subtropical Jet. Enhanced moisture
transport is expected to being again, but its effect in enhancing
heavy precipitation will not come until until later in the work
week. On Wednesday expect light to moderate precipitation from
Paraguay through Uruguay/Rio Grande do Sul. There is a potential
for higher precipitation totals, dependent on the timing of short
wave trough activity entering the region on late Wednesday into
early Thursday.

For the southern cone of the continent, the entry of moisture
alongside frontal boundaries in the Sur and Central sectors of
Chile will bring light precipitation on Monday and Tuesday. The
lower amounts of precipitation are in part due to zonal flow in
the upper levels, while an upper trough remains to the south and
west of the continent. This pattern changes on Wednesday when a
moderately strong atmospheric river with precipitable water levels
exceeding 30mm in some portions enters Central Chile and is
enhanced by the exit region of the jet streak over Chile in the
upper levels. Expect moderate precipitation with a chance of snow
in higher elevations.

In the tropical sector of the continent, much of the same dry
pattern continues over central Brasil, while the precipitation
activity remains in northern South America, along the foothills of
the Andes Mountains and west Amazon Basin, and over the eastern
coasts of Brasil. Low level perturbations and remnants of fronts
are embedded in the easterly trade winds, fueled by orographic
lift as they reach the coast of Brasil. Expect trace to light
total precipitation over the next three days. Over the western
Amazon Basin, moisture is being transported due to the presence of
the SALLJ, interacting with the foothills of the Andes, while
being fueled by the mid to upper level trough over the region.
Expect trace to locally light precipitation totals.

Castellanos...(WPC)