877
FXUS20 KWBC 131929
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
229 PM EST Thu Nov 13 2025

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

South American Forecast Bulletin for 13 November 2025 at 1930 UTC

In the southern cone of the continent, a very active end of the
work week is expected with the entry of an atmospheric river with
precipitable water values exceeding 30mm in the Austral region of
Chile. This is accompanied by a potent jet streak, which its exit
region enters into Austral Chile by the afternoon hours of
Thursday. In the lower levels, a cold front with storm force winds
enters over Magallanes by Thursday evening, enhancing moisture
convergence along the terrain. These conditions will be
progressive as the short wave trough in the upper levels will
quickly propagate east by Friday afternoon. The precipitation
totals for south Aysen and north Magallanes are expected to range
from 50-100mm. Because of the progressive pattern in the upper
levels, a second upper level short wave trough is expected to
enter the same region by Friday evening, and this time it will be
a negatively tilted short wave trough, leading to favorable
divergence aloft. In addition, a new pulse of the atmospheric
river is expected to enter Austral Chile with a frontal boundary
and an occlusion. However, the extent of the available moisture
has decreased, and with it, the total precipitation on Friday is
expected to range from 35-70mm from south Aysen to north
Magallanes. By Saturday the axis of a high pressure system in the
lower levels from the Pacific Ocean extends into the Austral
region of Chile, favoring some subsidence that will inhibit the
potential for heavy precipitation. Expect maxima to decrease in
Austral and Sur regions of Chile for Saturday.

Central portions of South America will see the presence of various
short wave troughs in the mid to upper levels that propagate over
portions of Bolivia into Paraguay and south Brasil on Thursday.
The shortwave troughs are propagating along a weak portion of the
Subtropical jet, but due to the negative tilt that model guidance
are suggesting, they are forecast to favor heavy precipitation on
Thursday. With the transport of moisture that was occurring over
the past few days due to the presence of LLJs, a weaker level of
transportation of this moisture is still present. This will
support precipitation totals ranging from 40-80mm in north
Paraguay, 30-60mm in central Bolivia, and 25-50mm in west Paraguay
and portions of extreme southwest Parana Basin in Brasil. On
Friday, there is a weakening in the upper jet stream, with limited
upper level divergent support. The amount of available moisture
decreases on Friday, however, a lesser degree of the conditions
from the previous day remain, and this favoring maxima of 30-60mm
in northeast Paraguay, while the south Parana Basin can expect
maxima of 20-45mm. From east Chiquitania to south Pantanal expect
maxima of 20-35mm. On Saturday, a new upper level trough is
expected to enter the north Argentina region, with ample
diffluence and divergent support. In the lower levels, the upper
level support will favor frontogenesis over portions of
Cordoba-Santa Fe Argentina, while a cold front crosses the Andes
into the northern Patagonia region. The increase in baroclinicity
in the region will favor deep convection over north Argentina,
Uruguay and central Paraguay. Moisture is being transported from
the north with the re-introduction of the SALLJ over Bolivia.
These conditions will favor maxima of 40-80mm from the southern
Chaco region into the Rio de la Plata region. Maxima of 20-45mm
are expected from central Paraguay through Formosa and
Corrientes-Argentina.

The easterly trade winds in the tropical region of South America
continue to provide a moist environment over the Amazon River
Basin and extend into central Brasil. While trough in the lower
levels, and orographic lift along the Andes Mountains will be
important factors in producing precipitation over the three
forecast days, these will be impacted by diurnal heating
processes. In addition, thunderstorms over the region could be
widespread with the presence of a passing of an atmospheric Kelvin
Wave expected to leave the region by the weekend. On Thursday,
expect heavy precipitation in from south Colombia/Venezuela into
Amazonas-Brasil with maxima of 30-60mm. Central Peru can expect
maxima of 20-45mm. On Friday, expect a widespread region of
25-50mm from Colombia and into the Western Amazon basin and along
the eastern foothills of the Andes. By Saturday, the Western
Amazon basin can expect maxima of 30-60mm, while Amazonas-Brasil
can expect maxima of 20-45mm.

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

Castellanos...(WPC)


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