382
FXUS20 KWBC 121938
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
237 PM EST Wed Nov 12 2025

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

South American Forecast Bulletin for 12 November 2025 at 1940 UTC

In the southern cone of South America, the region is expected to
become active as an increase in available moisture due to an
atmospheric river and frontal boundaries enter the region over the
next three days. On Wednesday, the exit of a jet stream in the
upper levels and a short wave trough will favor divergence aloft,
while a weak front enters the Austral region from late Wednesday
into early Thursday morning. This will favor maxima of 20-35mm
from south Aysen and north Magallanes on Wednesday. On Thursday,
the exit area of a jet streak enters by the evening hours, while
in the mid levels, the base of a short wave trough encounters the
south Magallanes region, in addition with the advection of
vorticity in the region. In the lower levels, westerly low level
jets are accompanied by the entry of a potent atmospheric river
with precipitable water values estimated to exceed 30-35mm over
most of the Austral region of Chile from Thursday into Friday
morning. This will favor moisture convergence along the terrain,
enhanced by orographic lift, and will receive support from the mid
to upper levels. As a result, expect heavy precipitation on
Thursday, with maxima precipitation ranging from 35-70mm, with
mountain snow, and locally higher amounts are possible. On Friday,
the entrance region of the jet streak has propagated eastward over
the southern Patagonia and ridge axis in the upper levels is
expected over the day on Friday. However, a region of upper
diffluence remains as the jet stream continues to remain over the
Austral region of Chile. From late Friday into early Saturday a
mid to upper level, negatively tilted shortwave trough enters
Austral Chile, favoring conditions for heavy precipitation.
Moisture remains in the region as atmospheric river remains, and a
frontal boundary begins to form an occlusion just west of
Magallanes. High speeds in the lower levels will continue
throughout the forecast area, enhancing moisture convergence and
orographic lift. Expect maxima of 30-60mm in south Aysen and north
Magallanes.

In the central region of the continent, the interaction between an
upper level ridge centered over Brasil and as negatively tilted
upper trough over the south Atlantic Ocean and east of the Rio de
la Plata region will interact over regions of Paraguay, north
Argentina, Uruguay, and south Brasil. On Wednesday, the upper
ridge intensifies, further enhancing the upper level jet and
providing a region of divergence over portions of Paraguay, north
Argentina, and south Brasil. With this, significant troughing is
expected in the lower levels of the atmosphere, as well as the
presence of the South American LLJ, which will transport moist air
from the tropical regions of South America. At the same time, a
frontal boundary becomes stationary over portions of north
Argentina by Wednesday evening. The interactions of all these
systems will persist into Thursday, and propagate slightly
northward into the Chaco region and the Chiquitania region of
Bolivia. The frontal boundary is expected to weaken on Thursday
and Friday as the upper level jet begins to weaken and the upper
level trough in the south Atlantic has left the region. In terms
of precipitation, on Wednesday, expect maxima of 50-100mm in
central Paraguay with a slight risk of severe weather, and maxima
of 30-60mm in west Formosa-Argentina with a marginal risk of
severe weather. The north Chaco region can expect maxima of
20-45mm with a marginal risk of severe weather. On Thursday, north
Paraguay and the south Pantanal region of Brasil can expect maxima
of 50-100mm with a risk of severe weather, while the south Chaco
region can expect maxima of 35-70mm with a marginal risk of severe
weather. The Chiquitania region can expect maxima of 20-45mm,
while the central Bolivia region will see significant moisture
convergence along the Andes Mountains, with precipitable water
values exceeding 55mm, which will favor maxima of 40-80mm. By
Friday, a weaker trough remains in the lower levels, accompanied
by moisture that is converging in north Paraguay and the Pantanal
region. North-central Paraguay can expect maxima of 25-50mm with a
risk of severe weather, while the Pantanal region can expect
maxima of 20-35mm.

In the tropical portion of the continent, diurnal heating and
troughs in the trade winds have dominated the precipitation over
the past few days and it is expected to continue over the next
three days. On Wednesday, the afternoon thunderstorm activity is
widespread, and with it, the release of latent heat into the
atmosphere is assisting in fueling the strengthening of the upper
level ridge over the region. In the lower levels, the LLJ are
transporting moisture to the south along the Andean regions of
Peru and Bolivia. Most of the thunderstorms will remain over
Central and west Brasil, into the Amazon regions of Peru and
Bolivia on Wednesday. This pattern is expected to continue on
Thursday and Friday, with the exception of the presence of low
level jets, due to the lack of presence from the upper level jets
that favor the development of troughs in the lower levels.
However, this will favor the more east to west direction of the
easterly trades, meaning that the moisture being advected in the
trade winds will begin to interact with the Andean regions of Peru
and Bolivia. Friday is expected to be the day with the higher
amount of precipitation, where expect maxima of 30-60mm in
Amazonas-Brasil, and maxima of 20-45mm in the Western Amazon
Basin. The southwest Amazon Basin 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)


$$