South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
252 PM EDT Thu Oct 30 2025

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

South American Forecast Bulletin for 30 October 2025 at 1855 UTC

On Thursday, the northern extend of the South American continent
has an upper level ridge extending from Peru to east Brasil, and
its most southern extent is located over north Bolivia, where it
meets with an upper trough axis, embedded in a large trough,
centered just south of Uruguay. An upper level jet extends from
north Chile into south Brasil, while its exit region is located
over the South Atlantic Ocean. In the lower levels, cyclogenesis
has occured over the south Atlantic Ocean, where a cyclone extends
with its cold front into the north Serra do Mar region of Brasil,
while the stationary portion of the front is expected from south
Cerrado region through the Pantanal, and into the Chiquitania
region of Bolivia by Thursday evening. As this fronts begin to
become stationary in Rio de Janeiro, expect moist southerly
onshore flow extending into south Espirito Santo and southeast
Cerrado-Brasil. With the assistance of the upper level jet, expect
maxima of 30-60mm in the aforementioned region, with a slight risk
of severe weather on Thursday. The north Parana Basin can expect
to see some of the moisture entering, and favor maxima of 20-45mm.
From the Serra do Mar coastal region into the south Parana Basin,
expect lower totals of 20-35mm as less moisture is expected in
that region. On Friday, a slight decrease in the total amounts of
precipitation in the region, as the upper level ridge digs
southward, and the amounts of available moisture along the
stationary frontal boundary in the Serra do Mar region will favor
the decrease in totals. Due to the interaction between the upper
level trough and ridge, expect some support for divergence in the
upper levels, and such localized higher amounts of precipitation
could be expected. Maxima of 20-35mm are forecast from the Serra
do Mar coastal region to the Parana Basin. On Saturday, conditions
in the upper levels change again, with the approaching upper level
shortwave trough associated with a closed upper low that crossed
over the Andes Mountain in north Chile and Argentina on Friday.
The upper jet is enhanced as this upper trough interacts with the
upper ridge, enhancing divergence aloft downwind from south
Paraguay into south Brasil and along the Serra do Mar region. As
vorticity advection is expected in the mid to upper levels,
troughing is reflected in the lower levels over south Paraguay and
into southern Brasil. In addition, the South American Low Level
Jet activates over Bolivia, favoring the transport of moisture
from the tropical region of the continent. This enhanced
instability in the atmosphere, coupled with the jet dynamics, and
available moisture will favor heavy precipitation from south
Paraguay and through south Brasil. From south Paraguay through the
southwest Parana Basin, expect maxima of 60-125mm, with a moderate
risk of severe weather. From central Paraguay through the central
Parana Basin, expect maxima of 40-80mm with a moderate risk of
severe weather. Along the Serra do Mar region into the east Parana
Basin, expect maxima of 30-60mm. The surrounding regions in south
to central Brasil, north Paraguay, and the south Mesopotamia
region of Argentina can also expect moderate amounts of
precipitation with a risk of severe weather.

In the northern tropical region of the continent, the diurnal
cycle will favor afternoon thunderstorms over the Amazon Basin,
into Mato Grosso, and extending northeast into Colombia,
Venezuela, and into Peru over the forecast period. The
northeastern side of Brasil can expect to remain relatively dry
over the next three days as available moisture remains low in the
region. The regions that will expect the highest amount of
precipitation over the next three days are the Western Amazon
Basin into the Selva Alta region of Peru, as well as south
Venezuela. The Western Amazon region can expect moisture to
transport along the Andes Mountains as the easterly trade winds
are transporting moisture from the east and turning southward once
they reach the foothills of the mountains. This will favor
orographic enhancement along the north and central foothills of
Peru, where expect maxima of 30-60mm on Thursday in the central
region, and maxima of 20-35mm in the north and central regions of
Peru on Friday and Saturday.

In the southern portion of the continent, a mid to upper trough
pattern is expected to continue over the next three days. A long
amplitude trough extends from the Southern Ocean, with various
trough axis in the northern base of the trough, expected in
portions of central Chile by Thursday evening. This pattern will
favor the north and east propagation of various frontal boundaries
that enter Chile and propagate into north and central Argentina
over the next three days. On Thursday, a frontal boundary is
expected in central Chile and into the Patagonia region of
Argentina, exiting east into the South Atlantic Ocean. With the
moisture associated with this front propagating from the west, it
will go through orographic enhancement from Los Lagos through
north Magallanes, where expect maxima of 20-35mm with a chance of
mountain snow. As this front continues northward, by Friday
evening it is expected to enter the Cuyo and Cordoba region of
Argentina, and will be met with the moist airmass propagating from
the north through the presence of low level jets. With the
assistance on upper level trough, instability is expected in the
atmospheric column, favoring the risk of severe weather around the
Sierra de Cordoba, with maxima of 20-35mm east of the Sierra de
Cordoba, and maxima of 15-20mm to the west of the Sierra. In
southern Chile, new frontal boundaries enter the region and favor
maxima of 15-25mm in Magallanes. On Saturday, the cold front is
expected from Northwest Argentina through Cordoba, and enter the
Buenos Aires province before it enters the south Atlantic Ocean.
At this time, the closed low has entered the region, enhancing
divergence in north Argentina. Expect maxima of 20-35mm from
Northwest Argentina into the Cordoba region, with a slight chance
of severe weather, while the south Chaco region can expect maxima
of 15mm.

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

Castellanos...(WPC)