680
FXUS20 KWBC 281855
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
254 PM EDT Mon Sep 28 2026

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

Forecast Bulletin 28 September 2026 at 1800 UTC:

Heavy precipitation is expected today over the Rio Grande do Sul
region in Brazil, Uruguay, and portions of Argentina, as a series
of frontal boundaries in the region increases low-level
convergence, particularly along Uruguay and Brazil. Additionally,
the presence of low-level jets will aid deep moisture advection
from the tropics. The combination of converging strong low-level
winds, moisture advection, and upper-level divergence, due to the
presence of strong winds aloft, will create favorable conditions
for the development of precipitation and thunderstorms in the
area. From today through Tuesday morning, precipitation will range
from 35-70mm over Rio Grande do Sul in Brazil and northern
Uruguay. A moderate risk of severe weather exists in the region.
Over Argentina and Uruguay, totals are forecasted to be around
20-35mm.

Then, from Tuesday through Wednesday, a cold front is anticipated
to reach Paraguay, southern Brazil, and portions of Bolivia.
Combined with an increase in upper-level divergence as an
upper-level trough approaches the region, this will result in
heavy precipitation and thunderstorms. The heaviest precipitation
is expected over Paraguay and Bolivia, with totals ranging from
40-80mm. Another 20-45mm of rain is anticipated along the border
between Paraguay and Brazil.

Heavy rainfall will continue from Wednesday through Thursday as
the cold front propagates, as lower-level conditions will continue
to promote low-level convergence and moisture advection. The
heaviest precipitation is expected over Bolivia, with
accumulations around 45mm. With the frontal boundary extending
into Peru and the western Amazon region in Brazil, alongside
enhanced upper-level divergence, rainfall is also forecasted for
the region with totals around 15-30mm.

In the tropical region of the basin, heavy rainfall is expected
each day along the Pacific coast of Ecuador and Colombia due to
westerly low-level flow advecting moisture from the Pacific into
the area. Additionally, the propagation of tropical waves and the
presence of low-level troughs will cause rainfall and
thunderstorms over Colombia, Venezuela, and portions of the Amazon
each day of the forecast period.





Rivera-Torres...(WPC)





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