681
FXUS20 KWBC 261714
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1213 PM EST Wed Nov 26 2025

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

South American Forecast Bulletin for 26 November 2025 at 1730 UTC

Note: Due to the observance of the Thanksgiving holiday, there
will be no production of South American charts and discussions for
27 November and 28 November, 2025. Products will resume on 1
December 2025.

In the subtropics...

Across northeast Brasil, a potent upper trough propagating across
the region will promote an enhancement of upper divergence. This
upper trough will reflect in the mid-levels and will favor a
cooling of the mid-level temperatures, aiding in the
destabilization of the environment. In the lower levels, expect
the presence of the South American Convergence Zone (SACZ) to
persist through Thursday. Precipitable water values will begin to
decrease Thursday afternoon but still expect enhanced moisture
convergence across the Bahia region through Friday morning due to
ongoing onshore low level flow. Thus, the greatest precipitation
impacts in this region will be on Wednesday with the presence of
the SACZ. Precipitation accumulations will gradually decrease with
passing days.

In the western Amazon Basin, upper diffluence will dominate across
the region prior to the arrival of an upper level trough that is
currently located in the Pacific. This trough will begin to move
into the continent Wednesday evening and will become negatively
tilted as it moves inland. At the same time, a mid-level trough
will be developing across northern Bolivia and southern Peru,
further enhancing vertical ascent. In the low-levels, troughing
will be present across the same region on Wednesday with
precipitable water values exceeding 50mm. This will favor
increased moisture convergence that may be further enhanced by
orographic effects. A total precipitation maxima of 40 - 80mm is
expected across northern Bolivia and southern Peru from Wednesday
morning through Thursday morning. The aforementioned upper trough
will lose its definition quickly once it crosses east of the
Andes. However, mid-level troughing will still be present in the
region for Thursday. The low level wind direction will also become
more northerly early Thursday morning and this wind regime will
remain through the weekend. Moisture convergence will be enhanced
across the foothills of the Andes in Bolivia. After Thursday, a
mid-to-upper ridging pattern will dominate across the interior of
the western Amazon, limiting support for deep convection. Thus,
the convection that does develop will likely be affiliated with
the presence of low level troughs, moisture convergence being
enhanced by orographic effects, and the diurnal cycle. Elsewhere
in the subtropics, expect seasonal conditions to prevail. The
presence of low level troughs will enhance moisture convergence
and vertical ascent.

In the mid-latitudes...

A mid-to-upper level shortwave trough, that currently has an axis
near 72W, will continue moving east of the Andes, arriving into
central Argentina by early Thursday morning. This feature will
help lower pressure values across north-central Argentina, which
will assist in the development of a surface cold front in this
region. Moisture convergence is anticipated across this boundary,
yielding a total precipitation maxima of 15 to 20mm. There will
also be a risk for severe weather due to the presence of cooler
mid-level temperatures. Some lingering precipitation is
anticipated across Rio de la Plata on Thursday morning as the cold
front and and the mid-to-upper level trough exit the region.
Accumulations during this period will range from 15 to 20mm.

Thereafter, an upper jet streak max will develop across central
Argentina ahead of the passage of another upper level shortwave
trough that is anticipated to move into the continent on Saturday.
The presence of the jet streak max will favor the development of
shortwave troughs in the mid-levels that may contribute to the
initiation of convection across central and northern Argentina on
Friday. At the surface, expect the development of meandering
surface troughs on Friday and Saturday. The highest precipitation
impacts on Friday are expected across the La Pampa and Buenos
Aires provinces. Note that there is model disagreement on the
location of the surface troughs on Friday across central
Argentina. Because the GFS is resolving a more well-defined upper
ridge across Bolivia, it is also resolving more intense upper wind
speed divergence across northern Argentina and developing a
surface trough south of the Chaco region of Argentina where
moisture convergence is expected to be most intense. However, both
models are indicating the presence of a surface trough across Rio
de la Plata and extending into Cordoba/Santa Fe. Considering the
aforementioned conditions, we are leaning more towards the ECMWF
for the onset of this event. By Saturday, the mid-to-upper level
trough will begin to move into north-central Chile and still
expect the generation of mid-level troughs east of the Andes due
to the ongoing presence of the upper jet streak max. For Saturday,
the area of greatest precipitation impact will be across Rio de la
Plata and the Mesopotamia regions of Argentina. Low level troughs
will be present in this region and there will be a series of mid
level troughs moving across this region. With respect to moisture,
the South America Low Level Jet (SALLJ) is anticipated to gain
definition starting on Thursday and will help transport moist
tropical air into Argentina. Expect above normal precipitable
water quantities across the region through the weekend.

Elsewhere in the mid-latitudes, on Friday, a weak moisture plume
will be converging across southern Austral Chile which may yield
total precipitation maxima of 15 - 20mm. Otherwise, expect
seasonal conditions to prevail across Chile and southern Argentina
through Sunday morning.

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

Tinoco-Morales...(WPC)


$$