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FXUS20 KWBC 061753
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
153 PM EDT Mon Oct 06 2025

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

South American Forecast Bulletin from  06 October 2025 at 1800 UTC

Over the next few days, South America is expected to see
increasing precipitation in the southern cone of the continent as
well as in the tropical and central regions. An atmospheric Kelvin
Wave like structure is propagating over the Pacific Ocean on
Monday, forecast to reach north South America by Tuesday. As the
wave continues to propagate, its effects of enhanced precipitation
could be felt throughout the continent over the work week.

In the southern region of South America, the upper level troughing
is expected to remain further south as the Antarctic Oscillation
is presenting signs of a positive phase, meaning the polar jets
associated with the upper troughs are not expected to reach as far
north as has been seen the previous days, blocked by the ridging
in north-central South America. Additionally, the upper troughs
are not progressing as fast over the area, and shorter amplitude
troughs are expected to propagate over the region over the next
several days. On Monday, a weak cold front with precipitable water
values of around 20mm is expected to propagate into Austral and
Sur regions of Chile, with precipitation totals reaching 25mm in
Los Lagos and north Aysen. On Tuesday, a moist atmospheric river
with precipitable water values exceeding 25mm accompanies a
frontal boundary as it enters Austral Chile, with low level
support through enhanced wind speeds and low level jets from the
northwest, interacting with the terrain. In the upper levels, the
polar jet with a jet streak maxima is positioned over the same
region. These conditions will align throughout Tuesday into weary
Wednesday, promoting enhanced moisture convergence and orographic
lift in Austral Chile, and maximum precipitation of 20-40mm in
south Aysen and north Magallanes, with mountain snow expected. By
Wednesday, the system propagates northward into the Sur region of
Chile, with some residual moisture in Austral Chile from the
previous day. The atmospheric river is continuing to favor the
entrance of moisture in the region, while the upper level
conditions with the jet stream persist and enter the same region.
Expect maxima ranging from 35-70mm with a possibility of higher
precipitation locally.

In the central portions of the continent, a cold front extends
from central Bolivia into north Paraguay, entering into south
Brasil, where it exits into the south Atlantic Ocean by Monday
evening. In the upper level, a short amplitude trough embedded in
the larger trough, extends over into south Peru/Bolivia, and into
Paraguay and south Brasil, where it interacts with the upper ridge
over northern South America. This interaction will assist with
maintaining the front as it advances northward into north Bolivia,
west Brasil and into Sao Paulo by Tuesday evening. By Wednesday,
the frontal boundary is expected to weaken over north Bolivia and
as it enters west Brasil, remaining stationary over the Parana
Basin, and advancing into Espirito Santo. The presence of this
front in the region over the next three days  will favor
precipitation at the extremes of the front, in Bolivia and into
central Peru, where they will see moisture convergence along the
foothills of the Andes, and along the Serra do Mar region into
Espirito Santo in Brasil, where moist onshore flow favors the
precipitation. On Monday, expect maxima of 20-40mm in Rio Grande
do Sul and Santa Catarina. North Bolivia and south Peru can expect
maxima of 15-25mm. On Tuesday, moist onshore following the passing
of the front is present in Santa Catarina and Parana, where expect
maxima of 20-40mm with risk of thunderstorms. As the southerly
flow associated with the front encounters the central Peru region
and west Amazonas-Brasil, expect maxima of 20-45mm. By Wednesday,
the strong onshore flow enhanced by low level jet activity will
favor maxima of 25-50mm in Parana and southwest coastal Sao Paulo,
and maxima of 20-35mm north coastal and inland Sao Paulo.

In the northern region of the continent and over the central
Amazon Basin, diurnal processes will dominate over the next
several days. The drier of the three days is expected to be on
Monday, as maxima of 20-35mm are expected in southern Colombia
into the northwest Amazon Basin. By Tuesday, a trough propagating
in the easterly trades enters the Guianas, while most of the
moisture remains in the western Amazon Basin. Expect maxima of
20-35mm in south Colombia and Venezuela. By Wednesday, the passing
of the trough in the trades has favored an increase of moisture
over the Amazon Basin. With the southerly flow associated with the
weakened frontal boundary to the south, moisture convergence is
expected to favor maxima of 30-60mm in south Venezuela and into
Amazonas-Brasil. Peru, as well as the east-central Amazon Basin
can expect maxima of 20-35mm. Totals of 20-45mm in central
Colombia are also expected with the moist flow enhanced by
orographic lift along the foothills of the Andes.

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

Castellanos...(WPC)



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