868
FXUS20 KWBC 021719
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
118 PM EDT Fri Oct 02 2026

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

Forecast Bulletin 02 October 2026 at 1720 UTC:

Edit: There is an event in central and northern Chile that we are
watching for days 4 and 5. Please refer to an update to the
written discussion and the daily graphics on Monday 05 October for
the most up to date information.

A potent mid to upper level trough extends over Chile and
Argentina from the Pacific Ocean and digs a trough axis into the
north-central sector of South America. This helps position the
Subtropical Jet north over the northern sector of Chile. As the
exit region of the jet stream enters northern Chile on Friday,
expect an enhancement of troughing in the low levels favoring the
activation of the South American Low Level Jet (SALLJ) and fueling
frontal boundaries in the Chaco region. Additionally, shortwave
troughs are expected to travel along the periphery of the trough
as they strengthen due to the interaction with a potent upper
level ridge over northern South America. These conditions will
favor the enhancement of divergence, increase in available
moisture, and increased instability over much of northern Chile,
south Bolivia, north Argentina, Paraguay, and south Brasil from
Friday and into the weekend. Northern Chile will see enhanced
divergence aloft with the exit region of the Subtropical Jet and
moisture entering in the mid to lower levels from a remnant of a
frontal boundary that will interact with the terrain. Much of the
moisture is spread out along the Andes Mountains from the Sur
region of Chile through the high elevations of the Norte Chico
region on Friday. Expect light to moderate precipitation with a
chance of mountain snow accumulations in the higher elevations. In
north and central Argentina, expect moisture to advect from the
north as low level jets increase the amount of available moisture.
Precipitation totals are expected to remain light to moderate on
Friday. By Saturday, the Subtropical Jet continues to enter in the
northern portions of Chile and extends into the Chaco region to
the east of the Andes Mountains. At the same time a trough axis
embedded in the broad mid to upper level trough is tilting
negatively, fueling enhanced divergence over Paraguay and
northeast Argentina. At the same time, the exit region of the
Subtropical Jet over Argentina will couple with the entrance
region of the entrance region of a second portion of the
Subtropical Jet, fueling even more divergence in the region on
Saturday. As such, the SALLJ will strengthen and advect more
moisture from the north, a cold front advances quickly over
northeast Argentina, while mid to upper level short wave troughs
propagate along the periphery of the upper trough over Paraguay
and south Brasil. These combination of these conditions will favor
an increased risk of severe weather in central South America, as
well as heavy precipitation. By Sunday, these conditions are
expected to continue to propagate from Paraguay through south
Brasil, where heavy precipitation will continue.

The central and southern portions of Chile will also see the
effects of the mid to upper level trough over the next several
days. On Friday, an extra tropical cyclone is located over the
southern Pacific and into the Southern Ocean, while a cold front
associated with the cyclone has entered Chile by the morning
hours. An atmospheric river joins these conditions and brings
moisture into the elevated regions of the Andes Mountains over the
Sur and Austral regions of Chile and propagates northward
throughout the day on Friday into Saturday. Expect moderate
precipitation along the Andes from the Sur region into Central
Chile with mountain snow in the higher terrain. On Saturday, the
frontal boundary is expected to weaken, however the occlusion
associated with the extra tropical cyclone remains over the ocean
and propagated eastward into the Sur region of Chile. Expect the
presence of a weak upper level jet and positive vorticity
advection in the mid levels in association with the circulation of
the closed low, now projected throughout the entire atmospheric
column on Saturday. Expect enhanced orographic lift to favor heavy
precipitation in the higher terrain in the Sur region of Chile. To
the north, remnant moisture continues to advect into land,
favoring moderate precipitation into the northern sector of Chile.
On Sunday, a jet streak in the upper levels provides the dynamics
to enhance the lift over the northern region of Chile, favoring
the moist convergence that continues. Expect moderate
precipitation in north Chile. The remnants of the circulation of
the extra tropical storm continues to enter the southern Chile
region, where precipitation totals are expected to remain moderate
and locally heavy in higher elevations, primarily in Los Lagos.

In northern South America, northerly upper level winds are
favoring diffluence over much of the northwest sector, while
ridging extends over much of the central and eastern portion. In
the lower levels, propagating troughs over the central Amazon
Basin will take advantage of the available diffluence in the mid
to upper levels to fuel deep convection overnight from Friday into
Saturday, especially from west 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 the westerlies extend from a
long distance on Friday, 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 Friday.
On Saturday, the ridging retrogrades further 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
Saturday. By Sunday, the areas of higher amounts of available
moisture extend from the central South America region into the
west and central Amazon Basin. The eastern portion of the basin
continues to see the advection of dry air from the central
Atlantic Ocean. As such, moisture convergence is expect over
Colombia and Venezuela, where expect light to moderate
thunderstorms on Sunday. Over the east Pacific, the winds begin to
shift to a northerly direction, decreasing the long fetch moisture
advection entering the region the previous two days. Expect a
decrease in total precipitation in west Ecuador and Colombia on
Sunday.

Castellanos...(WPC)





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