South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
245 PM EST Wed Jan 07 2026

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

South American Forecast Bulletin for 07 January 2026 at 1945 UTC

On Wednesday, the Bolivian High is centered just off-shore south
Peru and extends into western Brasil and to north Argentina, with
its periphery interacting with the broad periphery of the Cavado
do Nordeste in northeast Brasil, and in the south with a trough
axis that is exiting south Brasil. And additional upper level
shortwave trough is entering Chile by Wednesday evening that will
be interacting with the southern periphery of the Bolivian High
from south Peru/Bolivia into north Chile/Argentina. With the
presence of progressive shortwave troughs over Chile and
Argentina, the Chaco region and Northwest Argentina region have
seen the presence of the semi-permanent low pressure systems in
the lower levels of the atmosphere, favoring the advection of
moisture from the north. With these moist air mass reaching the
foothills of the Andes Mountains and the Sierra de Cordoba,
orographic lift is expected. In addition, a cold air mass is
approaching the northwest Argentina region, interacting with the
moist and warm air mass being advected from the north. With the
upper level support of the shortwave trough, expect heavy
precipitation maxima ranging from 40-80mm in the north Cuyo Region
with a moderate risk of severe weather, and maxima of 25-50mm in
the Northwest Argentina region with a slight risk of severe
weather. The southern Cuyo region can expect maxima of 20-35mm
with a moderate risk of severe weather. The amount of upper level
support extends to Uruguay and south Brasil, favoring a moderate
risk of severe weather from the Mesopotamia region of Argentina,
Uruguay an into Rio Grande do Sul-Brasil. To the north, the
abundance of available moisture is interacting with the periphery
of the Bolivian High and the Cavado do Nordeste, favoring deep
convection over the Amazon River basin and from the Selva Alta
region of Peru into the Yungas region of Bolivia. Expect maxima of
35-70mm in the Yungas Region and the Central Amazon basin, and
moderate to heavy precipitation over Tocantins, and the Selva Alta
region.

On Thursday, the upper level shortwave trough continues to
propagate over into central Argentina, with its divergent side
over north Argentina and Uruguay. This will favor lift from the
lower levels, as the Chaco Low continues over the region, and the
activation of the South American Low Level Jet from Bolivia
advects more moisture from the north. As such, the moist easterly
flow is expected to interact with the Andes Region in the south
Chaco, Northwest Argentina, and the Sierra de Cordoba regions,
favoring orographic lift. As these conditions persist from
Thursday evening into early Friday morning, expect heavy
precipitation from the south Chaco region into Cordoba/Santa Fe,
with maxima ranging from 75-125mm with a moderate risk of severe
weather. From the Northwest Argentina Region into the Sierra de
Cordoba region, expect maxima of 35-70mm with a moderate risk of
severe weather. The surrounding regions of north Argentina are
expected to also see a moderate risk of severe weather as the
event progresses overnight. To the north, the Cavado do Nordeste
is retrograding inland over northeast Brasil, extending into the
eastern Amazon basin, favoring drier conditions over these regions
on Thursday with the advection of drier air from the east.
Furthermore, with this easterly flow, the moisture over the Amazon
Basin is being advected to the west into the western Amazon Basin,
where expect orographic interaction as the moist flow shifts from
east to south from Peru into Bolivia, where expect maxima of
25-50mm.

On Friday, as the upper level shortwave trough continues into
northeast Argentina, it begins to tilt negatively, enhancing its
divergent effects over the lower level systems developing in the
region. This shift in conditions will favor the development of
frontal boundaries over north Argentina and Paraguay and
cyclogenesis over the Rio de la Plata region by Friday evening. As
the moisture transport from the north is expected to continue, the
amount of available moisture is expected to bring heavy
precipitation from the eastern Chaco Region into Uruguay and Rio
Grande do Sul. In addition, expect a moderate risk of severe
weather that extends over the aforementioned regions. To the
north, drier conditions will continue to expand inland Brasil, as
the Cavado do Nordeste continues to retrograde further inland on
Friday. The moisture over the Amazon Basin will continue to pool
to the west, and interact with the diffluence associated with the
upper level periphery of the Bolivian High, as the subtropical
high meanders over the Pacific Ocean, west of south Peru. Expect
moderate to localized heavy precipitation from north Peru into the
Selva Alta region of Peru.

Castellanos...(WPC)