193
FXUS20 KWBC 201741
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1240 PM EST Thu Nov 20 2025

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

South American Forecast Bulletin for 20 November 2025 at 1740 UTC

A mid-to-upper level trough will be moving across central
Argentina on Thursday and will favor the decrease in surface
pressure values and an increase in baroclinicity. These conditions
will favor the development of two cold fronts on Thursday
afternoon across central Argentina. The first cold front will be
located across La Pampa/Buenos Aires while the second cold front
will be located across northwest Argentina, Cordoba/Santa Fe, and
central Uruguay on Thursday evening. By Friday evening, these two
frontal boundaries will combine into one and will drape across
Paraguay, and southeastern Brasil.  The cold front will remain
across Paraguay and southeastern Brasil on Saturday evening. Note
that another fast moving cold front will begin to gain definition
across el Chaco region on Friday morning and will have a fast
propagation speed. By Friday evening, this cold front will be
located across south-central Bolivia. This front will transition
into a low-level trough by Saturday. The development of this cold
front is attributable to the presence of mid-to-upper level
shortwave troughs moving across this region.

At the low levels, expect increasing northerly winds into central
Argentina during the day on Thursday. As the cold front develops,
moisture pooling will increase along the boundary due to
increasing low-level wind convergence. This will help increase
precipitable water values across the region. On Thursday, the area
of greatest precipitation impact will be Rio de la Plata, where a
total precipitation maxima of 35 - 70mm is expected. Precipitation
chances will begin to increase late Thursday afternoon and
continue until early morning Friday.  Thereafter, the cold front
will continue moving northward but precipitation accumulations
will begin to decrease across Uruguay and southeastern Brasil due
to decreasing moisture across the region and weakening low-level
wind speeds.

Another region of interest is the Chaco region. Starting Friday,
the aforementioned cold fronts will drive the enhancement in
low-level moisture convergence and vertical ascent across this
region. Furthermore, a shortwave mid-level trough and the presence
of a diffluent upper level wind pattern will help sustain deep
convection. Thus, we expect total precipitation maxima of 35 -
70mm across southern Bolivia and northern Paraguay and another
maxima of 60 - 125mm across the oriental foothills of the Andes in
north-central Bolivia for Friday.  On Saturday, easterly low-level
winds will prevail across northern Bolivia and eastern Peru and
this will promote an enhancement in moisture convergence. Thus, we
expect total precipitation maxima to exceed 60mm in this region
for Saturday. There will also be weak mid-level troughing in the
region that will help enhance vertical ascent and upper level
diffluence that will positively contribute to the sustenance of
upper divergence.

Another region of interest in the mid-latitudes is austral Chile.
A series of mid-to-upper level troughs will be moving across the
region through the forecast cycle. There will be an increase in
precipitable water each day with the arrival of the cold fronts
into the coast on Thursday and Saturday. However, the period with
the greatest precipitation impact will be on Saturday, when a
well-defined cold front will begin to move inland.  A long fetch
moisture tongue that will be extending from the subtropics will
also accompany this front and will increase moisture significantly
in the region. Precipitable water values are expected to surpass
25mm. Low-level wind speeds will approach 50 knots, but the
direction of the winds will be from a west-northwest direction,
limiting moisture convergence across the region. These conditions
will favor total precipitation maxima of 15 - 25mm across the
region.

In the subtropics, a region of interest is eastern Brasil. A
stationary front is currently converging across northern Bahia and
will remain in the area until Friday. Thereafter, this front will
begin to transition into a low-level trough. Also, a weak upper
jet streak max will help sustain upper divergence in this region
through Friday. Thus, the greatest precipitation impacts will
occur on Thursday, where a total precipitation maxima of 75 -
125mm is possible across northern Bahia and another maxima of 35 -
70mm is possible across the Interior Nordeste region of Brasil.
Total precipitation maxima will decrease by Friday in both regions
but accumulations will still exceed 60mm. Across central Brasil, a
mid-to-upper level trough will be moving across the area through
the next three days, enhancing upper divergence across its base.
This trough will also be interacting with upper level ridging in
the subtropics and this interaction will favor an increasing
upper-level diffluent wind pattern across central and east Brasil
for Friday and Saturday, further enhancing upper divergence. With
respect to precipitation impacts, expect total precipitation
maxima of 20 - 45mm across central Brasil for Friday and it
decreasing to 20 - 35mm for Saturday as moisture will begin to
decrease and low-level wind speeds will weaken across this region.

Elsewhere in tropical South America, low-level troughs will drive
precipitation initiation across the region. Expect daily diurnal
convection to be enhanced by orography and local effects. In
general, there will be ample moisture across the Amazon Basin and
the Pacific coasts of Colombia and Ecuador through the forecast
cycle.

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

Tinoco-Morales...(WPC)


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