647
FXUS20 KWBC 071831
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
231 PM EDT Tue Oct 07 2025

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

South American Forecast Bulletin from  07 October 2025 at 1830 UTC

On Tuesday, in the southern cone of the continent, the progression
of an upper trough axis extending from the Pacific Ocean and just
south of Chile, is positioned so as the divergent side of the
trough is entering south Chile and Argentina by Tuesday evening.
Accompanied by the polar jet exit region, upper level divergence
is expected to favor heavy precipitation in south Chile on Tuesday
and into Wednesday, as the conditions are expected to continue. In
the lower levels, a frontal boundary with an atmospheric river
with available moisture of 20-25mm in precipitable water is
expected on Tuesday. By Wednesday, the moisture will increase to
precipitable water levels of 25-30mm. As this moist air enters
from the west-northwest, with wind speeds exceeding 30 to 40 knots
in the boundary layer, it will favor orographic lift along the
terrain of Austral Chile. On Tuesday, expect maxima of 35-70mm,
while on Wednesday expect maxima of 30-60mm with snow in the
higher elevations. By Thursday, the amount of available moisture
decreases as the frontal boundary and atmospheric river propagate
northward into north Los Lagos by early Friday, where expect a
maximum of 15-25mm. Westerly flow into the terrain of Austral
Chile will favor some degree of moist onshore flow, with a maximum
of 20-35mm.

To the north, an additional short amplitude upper trough axis is
present over central South America, extending from north Chile,
through Paraguay, and into south Brasil, accompanied by the
presence of a jet over north Argentina and Uruguay on Tuesday.
This system is assisting with maintaining a frontal boundary that
is expected over north Bolivia, along the Pantanal and Parana
Basin, exiting to the Atlantic Ocean through Rio de
Janeiro/Espirito Santo by Tuesday evening. On Wednesday, the
troughing in the upper level weakens, as well as the jet streak in
the region, favoring frontolysis over the central portions of the
continent. However, a shortwave trough in the upper level remains
over the Serra do Mar region, where low-level troughing, as well
as the presence of low level jets will favor moist onshore flow
into the region. The frontal boundary is expected to position over
Espirito Santo and remain a weak stationary front over the Cerrado
region and Parana Basin. This low level onshore flow is expected
to remain over the region over the forecast period, favoring
moderate precipitation each day. On Tuesday, expect maxima of
20-35mm in Sao Paulo, while west Rio de Janeiro can expect maxima
of 15-20mm. On Wednesday, the Serra do Mar can expect maxima of
20-35mm with a marginal risk of severe weather. On Thursday, the
region can expect maxima of 25-50mm from coastal Sao Paulo to
Santa Catarina, while central Parana can expect maxima of 20-40mm.
Inland Sao Paulo can expect maxima of 15-25mm.

In tropical South America, a weak upper ridge pattern is expected
over most of the western side of the continent, while a weaker
troughing is promoted through closed upper lows over the eastern
Amazon Basin and in the Nordeste region of Brasil. These upper
conditions will meander over the region, while in the lower
levels, moisture convergence is expected along the Amazon Basin as
troughs propagate in the trade winds. As these continue towards
the west, they encounter regions where they are stalled over the
western Amazon Basin, and as they encounter the Andes foothills,
precipitation is expected as orographic lift takes effect.
Additionally, the diurnal cycle will play an important role in
activating these thunderstorms in the afternoon and evening hours
over the next three days. With the assistance of a propagating
MJO/Kelvin Wave divergent phase over the Americas this week,
expect regions of precipitation with locally higher amounts of
precipitation. On Tuesday, expect maxima of 25-50mm in the
west-southwest Amazon Basin, while south Colombia and Venezuela
can expect maxima of 20-35mm. On Wednesday, the moderate
precipitation totals migrate northward into the northwest Amazon
Basin with maxima of 20-45mm and maxima of 20-35mm in south
Colombia/Venezuela. By Thursday, moisture converges from Orinoquia
to the Central Amazon Basin with maxima of 30-60mm. While the
western Amazon Basin can expect maxima of 20-45mm, and the eastern
Amazon can expect totals of 20-35mm.

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

Castellanos...(WPC)



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