993
FXUS20 KWBC 311736
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
135 PM EDT Fri Jul 31 2026

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

Forecast Bulletin 31 July 2026 at 1740 UTC:

An upper level trough will continue to move across the Chilean
Andes during the day today. With its arrival into the leeward
side, expect the presence of a potent jet streak max over Buenos
Aires/La Pampa and north-central Argentina through the afternoon
today. At the same time, the South American Low Level Jet (SALLJ)
is being intensified across the region due to the lower pressure
values at the surface because of the upper trough. This is leading
to the increased transport of a moist air mass into the region.
Expect moisture transport as far south as Patagonia, leading to
above average precipitable water values across the entire region.
By the evening, a frontal boundary will be developing across
Buenos Aires/La Pampa/Rio de la Plata/ northern Argentina and
moisture pooling will increase significantly. Deep convection and
severe weather will intensify towards the afternoon and evening
hours of today and continue into Saturday. The mid-level
temperatures will be cool and will help destabilize the
environment. Furthermore, there will be significant vertical speed
shear and support for vertical ascent across the vertical column.
These conditions will favor intense rainfall accumulations,
convection, and hail. The highest rainfall accumulations are
expected across the Rio de la Plata region through Saturday
morning, where a total precipitation maxima of 50 - 100mm is
likely.

On Saturday, this frontal boundary will continue propagating into
south Brasil, Paraguay, and southern Bolivia. However, the
connection with the SALLJ will weaken as the upper trough
continues shifting the system to the east and northeast. Thus,
there will be a reduction in moisture across the region.
Regardless, the environment will be primed for the development of
convection across Uruguay and south Brasil as upper divergence and
mid-level shortwave troughs will still be present across this
region. Moderate rainfall accumulations are likely.

Meanwhile, a projection of the upper trough will also be present
in the subtropical jet stream. Its base will be extending as far
north as north-central Peru. Currently, the Antarctic Oscillation
(AAO) is negative, indicating a weakening in the polar vortex over
the Antarctic continent that is contributing to an equatorward
shift in the axis of the upper level troughs. In short, when the
AAO is negative, expect upper level troughs to extend more
equatorward. For the next three days, expect a highly amplified
upper trough to be moving across Peru, Bolivia, Paraguay, and far
western Brasil. This feature will also help bring in a drier air
mass in the mid-to-upper levels, leading to a stabilizing effect
across the Pacific coasts of Peru and the interior of Bolivia. On
its exit region, expect the presence of speed divergence and
mid-level convergence across Peru, Bolivia, and Paraguay. This
synoptic pattern will favor the development of daily light
thunderstorms across eastern Peru for the next three days.
Precipitation chances will increase across Bolivia, and Paraguay
for Saturday with the arrival of a cold front.

By Sunday, the frontal boundary will weaken and another warm front
will develop across Mesopotamia (Argentina) and south Brasil.
There will be an increase in moisture convergence along this
boundary and another mid-level shortwave trough will be traversing
the region. Expect moderate total precipitation maxima in the
vicinity of this frontal boundary for Sunday. Meanwhile across
Bolivia and Paraguay, light showers and thunderstorms will also be
likely during this period as there will still be moisture
advection into the region.

Another region of interest is Chile. Through the day today, expect
two separate cold fronts to be moving across south and central
Chile respectively. The northern cold front moving into central
Chile is being accompanied by strong low level winds and above
average precipitable water values. At the same time, the
mid-to-upper level dynamic regime will favor precipitation
processes through this evening. Expect the peak precipitation
impacts during the day, diminishing tonight as the upper trough
axis moves east of the Andes. To the south, across south and
austral Chile, precipitation accumulations will be less as
precipitable water values will be lower.

On Saturday, another frontal boundary will be moving into south
Chile and there will be a coupling of an orthogonal upper level
wind flow (jet streak max). This will reinforce upper divergence
across the region. Another moisture plume and frontal boundary
will be moving into the continent by Saturday morning. Total
precipitation maxima will be on the elevated side for this system.
The south-central region of Chile will remain active for Sunday as
the frontal boundaries continue to converge into the region.

In the rest of the tropics, seasonal conditions will persist
through the beginning of next week. There will be various low
level troughs and tropical waves that will be traversing the
region and are expected to increase precipitation chances.
Overall, expect moderate rainfall accumulations in the vicinity of
these features. Along the Pacific coasts of Colombia and Ecuador,
expect an increase in moisture and the intensification of the
Panama trough. It is being reinforced by the passage of tropical
waves and low level troughs across the southeast Caribbean Sea
Basin. Areas to watch are where low level winds (surface to 925mb)
will be converging. Expect the regions of highest precipitation
impacts to be across the northern coast of Ecuador and
south-to-central Colombia for the next three days.


Tinoco-Morales...(WPC)





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