671
FXUS20 KWBC 171935
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
235 PM EST Mon Nov 17 2025

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

South American Forecast Bulletin for 17 November 2025 at 1930 UTC

For the start of the work week, in the northern portion of the
continent, an upper ridge extends over most of Brasil and is
interacting with an upper level, short amplitude trough that is
positively tilted from south Peru through the Chaco region, and
exits through Uruguay into the south Atlantic Ocean, where it
connects to a potent upper Polar Jet, with a jet streak over the
southern cone of the continent. Accompanying the short amplitude
trough, in the low levels, a cold front extends from north
Bolivia, into west Mato Grosso, the Parana Basin, and exits
through the central Serra do Mar region by Monday evening. As
moisture from the tropical region is interacting with the drier
air moving northward, expect convection along the frontal
boundary. Expect significant moisture convergence along the Andean
foothills along the Yungas region in Bolivia, and the Southwest
Amazon Basin in south Peru and north Bolivia. Another region of
interest is the central Serra do Mar region, as moisture pools
along the coastal region, while southerly onshore flow will favor
heavy precipitation along the coasts from Santa Catarina to Sao
Paulo. These regions will have upper air support due to the mid to
upper level trough interacting with the upper ridge, which will
include the advection of vorticity in the mid levels, plus areas
of diffluence in the upper levels due to the interaction of the
upper trough and ridge. On Monday, south Peru into west Pando and
central La Paz-Bolivia can expect maxima of 40-80mm, while central
Bolivia can expect maxima of 25-50mm. Southwest Sao Paulo and
Parana can expect maxima of 30-60mm, while from north Parana to
south Mato Grosso do Sul, and Santa Catarina cane expect
generalized maxima of 20-40mm. Due to the instability in the
atmosphere, south Brasil can expect a risk of severe weather on
Monday. On Tuesday, the upper ridge over Brasil extends into
portions of Peru and is impacted by the upper trough in the south
that is expected to become disorganized, and potentially split
into two separate troughs by Tuesday. A cut off upper low is
expected over north Chile and extends into the Pacific Ocean,
while a secondary trough becomes a short wave trough over south
Brasil, becoming negatively tilted by Tuesday evening. In the
lower levels, the frontal boundary becomes remnants and troughs
over portions of the southern Amazon into Mato Grosso, while the
front extends from the western Cerrado region of Brasil into Rio
de Janeiro, where it exits into the Atlantic Ocean. Due to the
upper air support, weak cyclogenesis takes place by Tuesday
evening over the Serra do Mar coastal region, and an occlusion
enters into portions of Santa Catarina and Parana, where expect
moist onshore flow to favor moderate precipitation totals of
25-50mm on Tuesday, with a moderate risk of severe weather.
Portions of Sao Paulo and north Parana can expect generalized
maxima of 20-45mm and a slight risk of severe weather. To the
north and west, scattered to widespread precipitation is expected
over central Brasil and the western Amazon Basin with the remnants
of the frontal boundary interacting with the available moisture
and diurnal cycle. The region could expect generalized maxima of
20-45mm, with a potential for locally higher amounts. On
Wednesday, the upper air shortwave trough has weakened and
integrated into the longer amplitude upper trough that extends
from the Southern Ocean into the South Atlantic Ocean. Over
central Brasil, weak upper ridging remains, and it reflects into
the mid levels of the atmosphere. In the lower levels, the frontal
boundary is positioned over south Bahia and Espirito Santo, where
the remnants of the boundary remaining over northeast Brasil into
the southeastern Amazon Basin. This pattern seems to be attempting
at a convergence zone over the eastern parts of Brasil since
Tuesday, similar to the South Atlantic Convergence Zone (SACZ),
however, this does not seem to last for longer than one day and
becomes disorganized overland in Brasil. As onshore flow is
expected over south Bahia and Espirito Santo, the region can
expect maxima of 30-60mm with a slight chance of severe weather.
In the rest of northeast Brasil expect maxima of 20-35mm, with a
chance of locally higher totals. The central Amazon Basin can
expect maxima of 20-45mm.

In the southern region of the continent, light to moderate
precipitation is expected over the next three days, as the amount
of available moisture has decreased. The frontal boundaries that
are entering the region are expected to not be associated with
increased moisture. Although in the upper levels, the southern
regions of Chile and Argentina will be impacted by a potent jet
streak in the Polar Jet Stream, from Monday through Tuesday. This
will reflect in the lower levels, where storm force winds are
expected in the Patagonia region, while in Magallanes-Chile, this
will enhance moisture convergence along elevated regions and favor
maxima of 20-35mm on Monday. On Tuesday, there is a decrease in
moisture expected in the southern cone of the continent, while a
shortwave trough enters by early Wednesday, favoring maxima of
15-25mm in Austral Chile. By Wednesday, a ridge axis extends into
southern Chile from the Pacific Ocean, while in the lower levels
an occlusion and frontal boundaries enter the region, bringing
some moisture into the region. With some upper level support due
to the shortwave trough over the region, expect maxima of 20-35mm
in Los Lagos and north Aysen on Wednesday.

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

Castellanos...(WPC)


$$