South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1103 AM EST Fri Jan 23 2026

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

Forecast Bulletin 23 January 2026 at 1630 UTC:

A potent upper trough in the subtropical jet stream will be
situated across southeast Brasil on Friday. It will begin to lose
its structure during the day on Saturday and into Sunday. This
upper feature will project into the mid-levels, where a potent
trough will be located off the coast of Sao Paulo and Rio de
Janeiro on Friday. The mid-level trough will start to become
disorganized in structure as it moves farther into the Atlantic.
Regardless, still expect weak mid-level troughing to be present
across southeast Brasil on Saturday and Sunday. In the low levels,
the South Atlantic Convergence Zone (SACZ) that has been present
over the region throughout the past couple of days will begin to
lose its structure on Friday. Nonetheless, low level troughing
will still persist through the forecast cycle. There will be a
gradual decrease in precipitable water values across the region
except along the coasts of Sao Paulo and Rio de Janeiro, where
values still will be exceeding 44mm through the weekend. With
respect to precipitation impacts, the highest daily total
precipitation accumulations will occur on Friday when there will
still be enhanced mid-to-upper level support. Thereafter, expect a
decreasing trend in daily accumulations and rainfall coverage
across this region.

Meanwhile in the central, southwest, and western Amazon, there
will be a broad low level trough and wind confluence that will
remain in this region on Friday and Saturday. With the weakening
of the Bolivian High and the SACZ, expect the low level troughing
to weaken starting on Sunday. There will be a return of enhanced
north-northwesterly low level winds. The greatest precipitation
impacts on Friday and Saturday will be along the aforementioned
low level troughing and confluence. By Sunday, the change in the
low level wind flow will favor increased orographic lift across
the southwest Amazon and in turn, increased precipitation chances.
Note that low level troughs will also be moving across the basin
and will favor increased precipitation chances throughout the
forecast cycle. Upper level diffluence will also be present
throughout the weekend across the central Amazon, which will help
with the development of deep convection.

Along the Pacific coast, a low level trough will begin to
redevelop off the coast of Ecuador and Colombia on Sunday. This
trough will be accompanied by weak low level winds. Hence, low
level moisture convergence will be limited. If any convection and
showers develop, they will be reinforced by the diurnal cycle and
orographic effects. The area with the most intense precipitation
accumulations will be along the southern Pacific coast of Colombia
where the low level wind flow will be orthogonal to the local
topography. There will be speed and directional shear present in
the region that will inhibit the development of deep convection on
Sunday as well.

Along the Atlantic coast, precipitable water values will remain
above 50mm for the next three days. During this period, expect the
development of daily showers and convection that will be
reinforced by the diurnal cycle and local effects. During the
weekend, a mid-level ridge will begin to gain definition and will
be situated across the Guianas and the Amazon Delta. This feature
will limit the development of deep convection.

Across central Argentina, a series of mid-to-upper level shortwave
troughs will be traversing the region on Friday and Saturday.
These features will trigger the development of weak frontal
boundaries and low level troughs that will promote a brief
increase in moisture convergence and precipitation chances. Expect
daily precipitation across the northern regions of northern
Patagonia, Cuyo, and La Pampa/Buenos Aires. By Sunday evening, a
mid-to-upper level shortwave trough that has been semi-stationary
off the coast of central Chile will finally begin moving across
the Andes. Expect the development of mid-level shortwave troughs
during this period across central Argentina and a surface cold
front. Thus, the period with the greatest precipitation chances
will be overnight Sunday into early Monday morning across Cordoba,
Santa Fe, and northwest Argentina.

In austral and southern Chile, a potent upper trough that
currently has an axis near 108W will continue to move eastward. It
is projected to move into the continent Sunday evening. This
trough will be accompanied by a strengthening upper jet streak max
that will begin to affect the upper level dynamic pattern of
austral Chile on Saturday and Sunday. At the same time, there will
also be vorticity advection. A long fetch moisture plume will
extend into the region during this period and it will gradually
shift northward with passing days. On Friday, the moisture plume
will be primarily impacting austral Chile. By Saturday, it will
impact the northern regions of austral Chile. Low level wind
speeds will exceed 40 knots and will be from the north-northwest.
Thus, expect enhanced low level moisture convergence. With respect
to precipitation impacts, the highest accumulations on Friday will
be across southern austral Chile. Precipitation accumulations will
increase significantly for Saturday across north-central austral
Chile and by Sunday, the area will extend northward into southern
Chile. Note that there will be a decrease in precipitable water
for Sunday into Monday as the mid-to-upper level trough axis moves
overhead.


Tinoco-Morales...(WPC)