679
FXUS20 KWBC 011920
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
320 PM EDT Thu Oct 01 2026

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

Forecast Bulletin 01 October 2026 at 1920 UTC:

In northern South America, a deep upper level trough extends a
trough axis over the northwest sector of the continent, while a
potent upper level ridge reaches the region from the Central
Atlantic Ocean on Thursday. Throughout the day on Thursday and
into Friday, the upper trough begins to close as the ridge axis
continues to extend westward. With these systems interacting over
Colombia, Ecuador, and the west Amazon Basin, expect enhanced
upper level divergence. In the lower levels, propagating troughs
over the central Amazon Basin will take advantage of the available
divergence in the mid to upper levels to fuel deep convection
overnight from Thursday to Friday, especially from south Venezuela
into east Colombia. In west Ecuador and Colombia, mid level
moisture is expected over the region and into the higher
elevations of the Andes Mountains. Over Central America, a
tropical wave is propagating over the region, modulating the low
level winds over the east Pacific Ocean, where enhanced moisture
is expected. As these winds shift to westerlies for an extended
distance on Thursday, they will be entering the western coasts of
Colombia and Ecuador, favoring long fetch moist onshore flow. With
orographic enhancement, expect heavy precipitation to reach the
high elevations of Ecuador and southwest Colombia on Thursday into
Friday. On Friday, the ridge over the central Atlantic retrogrades
into northern South America, while the periphery of the ridge
enters the east Pacific. This ridge encounters a disorganized
upper level pattern over Central America and the west Caribbean,
however diffluence along the periphery of the ridge could enhance
some lift along northwest South America. The winds from the west
in the low levels are expected to continue into Ecuador and
Colombia, favoring deep convection entering the region. To the
east, dry air from the central Atlantic enters the region,
favoring dry conditions on Friday and Saturday. By Sunday, the
synoptic conditions are expected to continue and fuel deep
convection in west Ecuador and Colombia. With the easterly trade
winds entering over the western Amazon Basin, expect moisture to
advect and converge over the western Amazon Basin and favor
moderate precipitation totals.

The deep mid to upper level trough over South America is
maintaining the presence of a frontal boundary over southern
Brasil As moisture enters the central portions of the continent
from the north, expect moderate precipitation totals in the Serra
do Mar region and into the Pantanal and Mato Grosso. As the upper
level trough encounters the retrograding ridge entering the
continent from the central Atlantic, the frontal boundary begins
to weaken and transition to remnants over Central Brasil. From
Minas Gerais to Espirito Santo and Rio de Janeiro, the frontal
boundary becomes stationary, where persistent moisture interacts
with the terrain, fueling deep convection and heavy precipitation
on Friday. On Saturday, the frontal boundary encounters a new cold
front that is approaching from the south. The remnant moisture is
expected to fuel moderate precipitation totals on Saturday in
Minas Gerais, Rio de Janeiro and Espirito Santo. To the south, the
new frontal boundary is encountering enhanced upper level
divergence with the shortwave mid to upper level troughs passing
over Paraguay and southern Brasil. Expect heavy precipitation,
primarily from Saturday night into Sunday from southern Paraguay
through northern Argentina and into Rio Grande do Sul. The
activation of the South American Low Level Jet (SALLJ) will advect
increased moisture into the region, becoming an important factor
in the heavy precipitations expected. In addition, the dynamics
associated with the mid to upper level trough will favor
instability over the region, with a moderate risk of severe
weather from northeast Argentina through southern Brasil and
Paraguay.

Chile will see the entry of an extra-tropical system in
association with the potent mid to upper level trough. This system
also brings with it an atmospheric river with moisture that is
expected to enter into the continent over the next several days.
The Austral and Sur regions can expect the upper level trough to
be accompanied by vorticity, providing some lift as low level
moist flow interacts with the terrain, affected by orographic
lift. Expect moderate precipitation from central Chile to Los
Lagos. The Austral region can expect light to locally moderate
precipitation. On Friday, the frontal boundaries continues
propagating to the north and east, weakening in amount of moisture
levels and in wind speeds in the low levels. Expect some
orographic enhancement to favor moderate precipitation through the
central portions of the country. Argentina will begin to see
moisture enter from the north as the SALLJ activates in the
continent. They can expect light to locally moderate precipitation
totals on Friday. By Saturday, the remnants of the occlusion enter
the Sur region, while enhanced by the mid to upper level short
wave troughing that is making its way into the region on Saturday.
The mountain regions can expect enhanced orographic lift that will
favor heavy precipitation primarily in Los Lagos and north Aysen.
The central and Austral region could expect moderate precipitation
totals on Saturday.

Castellanos...(WPC)





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