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FXUS20 KWBC 231924
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
324 PM EDT Fri Oct 23 2026

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

Forecast Bulletin 23 October 2026 at 1930 UTC:

On Friday, the central sector of the continent will be under the
divergent periphery of the Bolivian High, as it meanders over the
region. To its south, the Subtropical Jet fuels the heavy winds
aloft, adding speed divergence over north Argentina and Uruguay,
where it interacts with a potent trough entering the continent
from the south Pacific. As surface pressures lower in he northern
Argentina region, expect the South American Low Level Jet to
transport moisture from the tropical region along Bolivia,
Paraguay and into south Brasil. All of these conditions are
expected to converge from south Paraguay, through the Parana Basin
and into the southern Cerrado region of central Brasil. This
region will see favorable moisture convergence in the low levels
where troughing is expected, while mid level vorticity will be
present from the mid level troughs, and the divergence available
from the interaction of the Bolivian High and the Subtropical Jet
will ventilate the deep convection that is generated throughout
the day on Friday into Saturday. Expect the region to see slight
to moderate risk of severe weather. To the north, the northern
most region of the Bolivian high is favoring the entry of mid to
upper level weak jets over Amazon Basin. The exit region of the
mid level jet will enhance convergence and favor vorticity and
lift into Ecuador and the west Amazon Basin. As such, the
available moisture that is entering from the Pacific into Ecuador,
and the moisture from the Amazon Basin entering Colombia, east
Ecuador, and north Peru will converge along the Andes mountains
and have the support of the mid to upper levels.

On Saturday, the same regions are expected to continue to see deep
convection from the presence of the Bolivian High and the
Subtropical Jet. Furthermore a cold front is expected to
strengthen and propagate northward into central Bolivia, Paraguay,
and south Brasil, where it will encounter the warm, moist air mass
and induce unstable conditions. At this time, the mid to upper
level trough from the Pacific enters Argentina, favoring the
passage of shortwave troughs along the jet stream, reaching south
Paraguay and Brasil into Saturday evening and early Sunday. Expect
a slight risk of severe weather in Santa Catarina-Brasil and
portions of Parana, with heavy precipitation totals. From Paraguay
through the south Parana Basin expect heavy precipitation. In
northwest South America, a slight decrease in precipitation is
expected as dry air is being advected over the Amazon Basin and
reaching the western sector of the basin by Saturday. Light to
moderate precipitation is expected while to the west, Ecuador
could continue to see moist onshore flow due to troughing offshore
the country.

By Sunday, the orientation of the Bolivian High begins to elongate
zonally over the northern sector of the continent. This will
reorient the regions of maxima divergence and shift where to
expect regions of heavier precipitation and deep convection. As a
result of the progressive cold front entering south-central Brasil
by Sunday evening, expect instability over the southern Cerrado
and Serra do Mar, where expect the heaviest precipitation with a
slight risk of severe weather. A shear line is expected to develop
ahead of this front and produce light precipitation in Minas
Gerais on Sunday into Monday. In the Yungas area, the frontal
boundary will also introduce instability and a chance for heavy
precipitation, with the additional support of short wave troughing
in the mid to upper levels. In western Ecuador and West Colombia,
moist onshore flow is expected to continue into Sunday while
troughing in the low levels remains persistent over Panama.
Increased divergence is expected aloft as the base of an upper
trough in the west Caribbean digs further south, interacting with
the upper level flow over northwest South America. Locally heavy
precipitation along the Sierra of Ecuador could exceed the
forecast totals.

Castellanos...(WPC)



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