866 
FXUS20 KWBC 291911
PMDSA 

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
311 PM EDT Wed Oct 29 2025

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

South American Forecast Bulletin for 29 October 2025 at 1900 UTC

Over the next three days the most significant rainfall is forecast
over the Amazon Basin and across portions of Southeast Brasil.
This will be due to low level moisture convergence over the Amazon
Basin, and also ahead or north of a frontal boundary that is
expected across Central South America that will extend from the
Atlantic to Southeast Brasil into northern Paraguay and Bolivia.
Some of the thunderstorms forecast along the frontal boundary
could be strong at times.

At the synoptic scale, the mid and upper levels are vertically
stacked to a decent degree. There is a slight difference in where
the jets are in the sense that there is an upper-level jet that is
not too impressive in the mid levels. That said, today in the mid
to upper levels, there is a shortwave trough over central Chile, a
low pressure in the southwestern Atlantic just east of Argentina,
and a trough in the southeastern Pacific that is entering Austral
Chile and Argentina. By Saturday, there will be a trough in the
southwestern Atlantic just east of Argentina, extending to the
northwest in a positively tilted trough north of 45S from Central
Argentina to Central Chile and the Pacific near 30S. There will
also be plenty of diffluence in the mid levels across eastern
Argentina near the Buenos Aires province.

In terms of mid and upper level jets, there will be an upper jet
today over northern Argentina, Paraguay and southeast Brasil, as
well as one entering the Austral Chile and Argentina. In the mid
levels there will be a jet entering Austral Chile and Argentina
today, while other areas does not appear to have impressive jets
for the region. By Saturday, there will still be an upper jet over
northern Argentina, northern Chile, Paraguay and Southeast Brasil,
as well as an upper jet over Austral Chile and Argentina. In the
mid levels, there will be a jet entering Austral Chile and
Argentina by Saturday, but no significant mid-level jet prior to
that, at least not of 70 kt or higher.

The available moisture will be above normal ahead of the frontal
boundary reaching central South America in the location mentioned
above. The Amazon Basin will also have above normal moisture for
the next three days. In terms of rainfall, this above normal
moisture will have a direct impact in the rainfall locations and
amounts. That said, the mid to upper level dynamics will play a
role when combining with the moisture convergence ahead of the
frontal boundary over central South America. This combination will
cause moderate to strong showers and thunderstorms, especially
across portions of southeastern Brasil, where the upper level
divergence is strongest. Across southeastern Brasil, daily max
values of rain could be in the order of 30-60mm today and Friday,
decreasing slightly on Saturday with values closer to 20-45mm.
Across the Amazon Basin, the moisture will combine with the
diurnal heating and local effects and develop showers and
thunderstorms each day. The daily max rainfall totals will be
anywhere between 15-45mm across the region, though the latest
model guidance does suggest a small area of southern Peru that
could observe up to 60mm today.

Chile will have some bands of moisture moving in over the next few
days, and when combined with the upper troughs that are also
moving in, it could cause some rain showers in central and
southern Chile, with mountain snow each day. 

For the latest available charts please visit:
https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml

Alamo...(WPC)



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