489
FXUS20 KWBC 011933
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
232 PM EST Mon Dec 01 2025

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

South American Forecast Bulletin for 01 December 2025 at 1930 UTC

In the northern part of South America, a weak upper level
subtropical ridge is present between two areas of troughing,
extending from Peru to central Brasil on Monday afternoon. In the
mid levels, a weak trough extends from Bolivia into east Paraguay,
expected to remain stationary from Monday into Tuesday. These mid
to upper level conditions will favor the presence of divergence
aloft, particularly along the central portions of Bolivia and
west-central Amazon Basin. In the lower levels, troughing embedded
in the trade winds with moist air is propagating over the Amazon
River Basin, and interacting with the upper air conditions of the
aforementioned regions. Over Paraguay a weak jet with embedded
shortwave troughs will favor troughing in the lower levels, where
expect ventilation for convection on Monday. In addition, the
diurnal cycle will enhance the chance of widespread thunderstorms
over Brasil and into Bolivia and into Paraguay. Expect generalized
maxima of 20-45mm along central Bolivia and into the Chaco region,
as well as in central Paraguay, along the low level troughs. On
Tuesday, the base of an upper trough extends into portions of
Paraguay and south Brasil, enhancing frontogenesis in northern
Argentina, while the jet to the north of the base of the trough is
enabling the presence of troughs in the lower levels from Bolivia
to the Serra do Mar region of Brasil. With this set up over the
central portions of the continent, weak moisture transport from
the north is present along the Selva Alta region of Peru into the
Yungas region of Bolivia, interacting with the terrain and
favoring orographic lift in the aforementioned regions. This will
extend into the north Chaco region in Paraguay. As the upper ridge
enters the Serra do Mar region on Tuesday, the area of divergence
increases the potential for frontogenesis and troughing in the
lower levels just east of Serra do Mar over the Atlantic Ocean.
This set up continues into Wednesday, where the upper level ridge
remains weak over the north-central portion of the continent, a
mid to upper level trough extends from the southern Atlantic Ocean
and enters southern Brasil, and moisture pools along a generalized
area of troughing that extends from north Bolivia into Rio de
Janeiro by Wednesday evening. Looking further into the latter part
of the work-week, this may be the genesis of a developing South
Atlantic Convergence Zone. On Tuesday and Wednesday, the maxima in
precipitation totals are expected to remain in Bolivia and north
Paraguay, as well as in the Serra do Mar/south Espirito Santo
regions over the two days. An area of moderate precipitation
totals are expected to connect them on both days. On Tuesday,
expect maxima of 40-80mm from southern Selva Alta region of Peru
into the northern Yungas region of Bolivia. Central Bolivia and
north Paraguay can expect maxima of 30-60mm. South Minas Gerais
into the Serra do Mar region can expect maxima of 20-45mm, with a
marginal risk of severe weather. On Wednesday, expect maxima of
40-80mm in Rio de Janeiro and south Minas Gerais, with a risk of
severe weather. Similar maxima are expected along the Selva Alta
of Peru into central Bolivia.

With the set up mentioned above in the northern portion of the
continent, available moisture is not being advected as far south
into the central regions of Argentina and Chile. They can expect
relatively dry conditions over the next three days. In the Austral
and Sur regions of Chile, the propagation of frontal boundaries
will favor precipitation in the region over the next three days,
however, due to the fast progression of upper and low level
systems, and the lower amount of available moisture, expect
precipitation to remain light to moderate over the next three
days. On Monday, expect maxima of 15-25mm in north Austral Chile,
while the southern portion of Austral Chile can expect maxima of
15mm. On Tuesday, expect north Austral Chile to see maxima of
20-35mm, with a chance of mountain snow, while the Sur region and
the southern Austral region can expect maxima of 15mm. On
Wednesday, expect maxima of 20-45mm in Austral Chile, while from
Los Lagos to north Aysen can expect maxima 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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