035
FXUS20 KWBC 091901
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
301 PM EDT Thu Oct 09 2025

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

South American Forecast Bulletin from 09 October 2025 at 1900 UTC

In the southern portion of the continent, a weakening atmospheric
river and a frontal boundary entering central Chile that exits
through the northern Patagonia into the southern Atlantic Ocean.
This system is enhanced by an upper level shortwave trough
propagating east and northward on Thursday. Although the amount of
available moisture is not as high as it has been the previous
days, westerly onshore flow in the low levels is expected to
interact with the terrain and orographic enhancement will favor
maxima of 15-20mm in Los Lagos. Similar amounts are expected in
the Austral region of Chile due to westerly flow, with a slight
enhancement in southern Magallanes due to the presence of low
level jets, where expect maxima of 20-35mm. On Friday, the upper
level shortwave trough tries to close over the region, favoring
low level circulation of an occlusion associated with the frontal
boundary over the Sur region of Chile. Increased onshore flow with
low level jets will favor maxima of 20-35mm from Maule to Los
Lagos. To the east, the divergent side of the closing upper level
trough is favoring the development of severe weather from the
Buenos Aires province to the northern Patagonia region. Moisture
is transported from the north, interacting with the frontal
boundary in Patagonia. Expect maxima of 15-25mm, with a potential
for locally higher precipitation totals. As these conditions are
enhanced on Saturday, the SALLJ is transporting moisture from the
north, while the upper level trough becomes negatively tilted, and
the frontal boundary advances north and east into the south Chaco
region in Argentina, and into the Buenos Aires province. Ahead of
the front, a prefrontal trough advances into portions of Uruguay
and just west of Paraguay. These conditions will favor risk of
severe weather over the region and moderate to heavy
precipitation. Expect maxima of 40-80mm from south Paraguay to the
Mesopotamia region of Argentina. Expect maxima of 30-60mm from Rio
Grande do Sul to northwest Argentina, as well as the south Buenos
Aires province.

In the northern portion of the region, a weak upper ridging is
taking effect over portions of northwest South America, while a
weak trough remains over south Brasil. With the presence of a weak
frontal boundary that is favoring moist onshore flow into the
Serra do Mar region on Thursday and Friday. By Saturday, the
frontal boundary has pushed away from the coast and only weak
troughing is expected inland. On Thursday and Friday expect maxima
of 20-45mm in the Serra do Mar region, while a decrease is
expected inland on Saturday, where expect maxima of 15-25mm.

In the Amazon River Basin, moisture remains over the region which
will be interacting with the diurnal cycle over the next three
days. Thursday is expected to be the day with the highest totals
of precipitation, where from south Colombia into the western
Amazon Basin, can expect maxima of 20-45mm, with localized higher
amounts. Otherwise, areas of low level troughing over the central
Amazon Basin will favor daily maxima of 20-35mm per day. Expect a
pocket of dry air to be advected from the east into Eastern
Brasil, where a generalized decrease in precipitation could be
seen on Saturday.

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

Castellanos...(WPC)



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