659
FXUS20 KWBC 311857
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
256 PM EDT Mon Aug 31 2026

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

Forecast Bulletin 31 August 2026 at 1900 UTC:

On Monday morning, frontal boundary is present from southern
Bolivia through Paraguay and into Rio Grande do Sul, occluding
over portions of northern Uruguay. The system is expected to
strengthen due to the support in the mid to upper levels from a
short wave trough that is tilting negatively. Additionally the
exit region of a jet streak in the Suptropical Jet is located over
northeast Argentina, with its divergent right-exit region supports
the strengthening of the frontal boundary and favors its
organization and northward movement into Brasil by Monday evening.
Additionally, the South American Low Level Jet (SALLJ) is active
and transports moisture from the tropics into central South
America, fueling the moisture gradient and moisture convergence
ahead of the frontal boundary. The instability in the region will
favor a moderate risk of severe weather from south Paraguay,
Pantanal-Brasil, Parana Basin, and into the Serra do Mar region of
Brasil. With the moisture, instability, and mid to upper level
support, expect heavy precipitation in north Rio Grande do Sul,
Santa Catarina and Parana-Brasil. By Tuesday, the frontal boundary
is expected from north Bolivia through Mato Grosso, into the
Cerrado region of Brasil, and exiting the continent from Rio de
Janeiro/Sau Paolo Region. The mid to upper level trough continues
to enter into Brasil over the Serra do Mar region, favoring the
continued northward push from the frontal boundary. Because the
upper trough remains over the area, and the moisture from the
tropical region of the continent continues to converge with the
colder, drier air, expect a moderate risk of severe weather in the
Parana Basin and along the Serra do Mar region for Tuesday. With
these conditions, expect a potential for hail in the region.
Similar convergent conditions are expected in the low levels,
along the foothills of the mountains in south Peru and the Yungas
in Bolivia. Low level moisture convergence will be fueled by the
speed divergence at the periphery of the upper level ridge
interacting with the potent Subtropical Jet. By Wednesday, the
upper level ridge from the Pacific Ocean has entered the continent
and extends into eastern Brasil. The ridge interacts with the
upper trough, maintaining a potent Subtropical Jet over the
central portions of the continent. By Wednesday evening, the
frontal boundary has entered central Brasil, reaching the southern
Amazon Basin. The effects of the ridge in the upper levels will
begin to weaken the front, but along the front, expect
precipitation as moisture converges in the low levels. Expect much
of the precipitation associated with the front over Bahia and
Espirito Santo, while some precipitation is expected from Mato
Grosso and in the Cerrado region, with maxima reaching 15mm. In
the Selva Alta region of Peru, moisture continues to enter the
region, expected to encounter orographic lift due to the mountains
and favor light precipitation.

To the south, upper level troughs continue a progressive pattern
as they enter the continent and quickly propagate over the
Patagonia and into the Atlantic Ocean. On Monday morning, a low
level weak frontal boundary crosses the Andes Mountains over the
Sur region of Chile, favoring precipitation as it interacts with a
potent jet streak in the Subtropical Jet and enhanced by the
divergence at the base of a shortwave upper level trough entering
the Austral Chile region on Monday. A variety of shortwave troughs
in the mid levels will enhance the vorticity and favor lift in the
southern cone of the continent. Expect heavy precipitation in the
higher elevations of Los Lagos, while the rest of the region can
expect light to locally moderate precipitation. As the upper level
ridge over the Pacific Ocean enters the continent, the Subtropical
Jet remains over the central sector of the continent, blocking the
northward movement of upper level troughs to the north. This
favors an eastward progression of the upper troughs over the
southern sector of the continent. As such, from Tuesday evening
into Wednesday, an atmospheric river and frontal boundary enter
the Sur Chile region, where expect maxima to remain from Los Lagos
into Aysen. The frontal boundary enters the Patagonia, where
expect light precipitation. By Wednesday, the frontal system
reaches central Chile, where expect maxima precipitation of
20-35mm with heavier precipitation in the higher elevations of the
Andes Mountains. While the rest of the front continues into the
Rio de la Plata region, where expect maxima below 35mm.

Castellanos...(WPC)





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