South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
145 PM EDT Fri Oct 31 2025

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

South American Forecast Bulletin for 31 October 2025 at 1745 UTC

On Friday, the central region of the continent will see the
presence of various upper level short wave troughs associated with
a large amplitude upper level trough that is extending from the
south. This system is expected to interact with the upper level
ridge over the northern region of the continent at its periphery,
where an upper jet stream will be located from Paraguay through
southeastern Brasil, becoming an important factor in the heavy
precipitation event over the weekend. On Friday, the first short
wave trough is located east of Brasil and the jet stream is
expected over south Brasil, where it will be impacted by the upper
ridge, creating a divergent region over south Brasil. In the lower
levels, the advection of moisture from the tropical northern
region of South America will favor a moist environment, while
troughing in the lower levels is present from the Pantanal region
of Brasil to Serra do Mar.These conditions will be present over
the next few days, increasing the chance of heavy precipitation
and severe weather over the next three days. On Friday expect
maxima of 20-45mm with a risk of severe weather. on Saturday, the
increase in available moisture entering the region due to
advection from the north will favor maxima of 60-125mm from
central Paraguay to the southern Parana Basin, while maxima of
40-80mm from the southern Parana Basin to the central Serra do Mar
coastal region. These regions can expect a moderate risk of severe
weather on Saturday. On Sunday, there is a decrease in moisture
entering the region as the low level jets associated with the
moisture advection have weaken by Sunday. This will bring a slight
decrease in heavy precipitation, however due to the upper air
dynamics still present, expect maxima of 30-60mm along the Serra
do Mar region into the southern Parana River Basin and into the
Cerrado region. In addition, heavier precipitation totals of
40-80mm are expected from the Pantanal into the Parana Basin.
Isolated higher amounts are possible on Sunday. Expect a risk of
severe weather to continue into Sunday evening an the early
morning hours of Monday.

To the north, troughing in the lower levels and available moisture
will interact with the diurnal cycle throughout the weekend to
produce precipitation over the Amazon River Basin and along the
foothills of the Andes Mountains in Peru and Bolivia. The
southerly advection of moisture due to the presence of low level
jets over Bolivia, will favor orographic enhancement along the
foothills of the mountains, where the heaviest precipitation is
expected in central Peru and central Bolivia. On Friday, central
Peru could expect maxima of 20-35mm as moisture encounters the
terrain. On Saturday, the SALLJ is active and bringing additional
moisture, now expected in central Bolivia, where maxima of 25-50mm
could be observed. Central Peru can expect maxima of 20-35mm. By
Sunday, the low level jet has weakened over the region but the
amount of available moisture can favor maxima of 20-45mm in
central Bolivia, and maxima of 20-35mm in central Peru.

In southern Chile, another upper level trough axis enters the
region on Friday, and with it, a frontal boundary that will
interact with the terrain from Southern to Austral Chile. This
will favor moisture to go through orographic enhancement, and
maxima of 20-35mm are expected in Magallanes, while maxima of
15-25mm is expected from Los Lagos through Aysen. The chance of
snow is expected in the region. On Saturday, similar amounts are
expected in Magallanes, as a new frontal boundary enters the
region. With the passing of the trough axis in the upper levels,
some subsidence is expected with a ridge axis entering into
central and southern parts of Chile on Saturday. On Sunday, a more
zonal upper level pattern is expected over most of Chile, while
frontal boundaries remain in the Austral region of Chile. This
will favor most of the precipitation to remain there while most of
the country remains dry. Expect maxima of 20-35mm in south
Magallanes.


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

Castellanos...(WPC)