064
FXUS20 KWBC 291651
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1250 PM EDT Wed Jul 29 2026

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

Forecast Bulletin 29 July 2026 at 1700 UTC:

In the mid-latitudes and subtropics...
Currently, an upper level trough is moving across Rio de la Plata
and south Brasil and its axis is expected to exit the continent by
the afternoon. During the day, expect lingering thunderstorms
across south Brasil as the environment will remain moist and
unstable. This evening and into the overnight hours, another upper
level shortwave trough will be moving across Bolivia and Paraguay.
Upper diffluence will develop over the Pantanal and Parana Basin
(Brasil) briefly during the late evening. A weak mid-level
shortwave trough will also be present in the region, which will
further destabilize the environment and provide vertical ascent.
Thus, expect light rainfall accumulations with these conditions in
Pantanal and the Parana Basin (Brasil) through Thursday morning.
On Thursday, expect lingering light showers and thunderstorms to
persist in Pantanal (Brasil) through the day. Dry conditions will
return for Friday.

Another system of interest is an incoming upper trough. Its axis
is currently near 103W and its exit region will begin to influence
the dynamic regime across Chile starting on Thursday. Ahead of
this system, expect a broad upper level ridging pattern across
Chile and an upper jet streak max to be affecting the southern
regions of the continent. The upper trough axis will be moving
into the continent by Friday afternoon/evening. The upper trough
is also currently sustaining a broad frontal system whose warm
front is anticipated to move into southern Chile during Wednesday
evening. The cold front will arrive in southern Chile by Thursday
morning. With the eastward advancement of this frontal system,
expect an increase in precipitable water values in the region,
which will reach 25mm. Low level north-northwesterly wind speeds
will also be exceeding 40 knots, leading to the enhancement in
moisture convergence across south Chile for Thursday.

From Thursday morning through Friday morning, expect elevated
total precipitation maxima across Nuble, Bio-Bio, Araucania, and
Los Rios. The warm front and occluded front will drive the
precipitation processes across southern Chile, yielding moderate
total precipitation maxima. This occluded front will meander off
the coast of southern Chile thereafter, still yielding moderate
total precipitation maxima across southern Chile for Friday.
Meanwhile across central Chile, expect an uptick in precipitation
on Friday as the cold front continues to move into the region.
Precipitable water values will begin to decrease as the
accompanying moisture plume weakens. Enhanced total precipitation
maxima are still likely in this region through early Saturday
morning.

Meanwhile across the leeward side of the Andes, ahead of the
arrival of the upper air trough axis, a low level ridge will be
developing across Rio de la Plata (Argentina/Uruguay), which will
promote the increase in northerly low level winds. This, in turn,
will favor the increase in moisture into Patagonia. On Wednesday,
the presence of a jet streak max over the region will also support
the development of mid-level shortwave troughs and in turn the
development of low level troughs and weak surface boundaries.
Expect surface convergence to be enhanced in the vicinity of these
features and thus the increase in precipitation accumulations. By
Thursday, the arrival of the exit region of the aforementioned
upper trough will support the lowering of  surface pressure levels
and the increase in moisture in the region. Expect the South
American Low Level Jet (SALLJ) to extend into Buenos Aires/ La
Pampa (Argentina). Across south-central Patagonia, there will also
be a resurgence in easterly low level winds on Thursday, which
will help transport moisture into the region. This low level wind
regime is being influenced by the development of a low level
trough off the coast of Patagonia. These conditions will favor an
increase in precipitation chances across Patagonia.

By Friday, the upper trough axis will be moving east of the Andes
and frontogenesis will occur across north-central Argentina. This
frontal boundary will quickly advance northward into Uruguay, and
south Brasil by Saturday morning. Moisture pooling will be
significant along the frontal boundary. Furthermore, in the
mid-levels, expect convergence across south Brasil and Uruguay due
to the interaction between the incoming mid-to-upper trough and a
mid-level ridge across east Brasil. This will reinforce vertical
ascent and moisture pooling. The dynamic surface-to-upper level
conditions will favor the development of deep convection in the
region and there will be a risk for severe weather. Regions to
watch for convective activity are La Pampa/Buenos Aires
(Argentina), Rio de la Plata (Argentina/Uruguay), Mesopotamia
(Argentina), Cordoba/ Santa Fe (Argentina), and south Brasil. Rain
activity will begin during the day on Friday across Buenos
Aires/La Pampa and continue moving northward into Saturday
morning.

In the tropics...
Seasonal conditions will prevail through the basin for the next
three days. Precipitation regions of interest will be in the
vicinity of eastward propagating tropical waves, though the
greatest precipitation impacts will stay confined to the northerly
extremes of the continent. Otherwise, expect prevailing low-to-mid
level easterly winds across the basin, which will favor moisture
pooling across the northwest Amazon. Daily moderate rainfall
accumulations are likely with the passage of tropical waves.
Otherwise, diurnal thunderstorms will dominate across the region.



Tinoco-Morales...(WPC)





$$