947
FXUS20 KWBC 041520
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1020 AM EST Wed Nov 04 2026

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

Forecast Bulletin 04 November 2026 at 1525 UTC:

A potent jet streak max will continue to be present across central
South America for the next three days. At the same time, a
subtropical upper ridge will be centered across Bolivia and will
support the intensification/sustenance of the fast upper level
wind speeds in the subtropical jet stream. The interaction between
these two features will lead to the sustenance of an area of upper
level wind diffluence across the Mesopotamia region (Argentina)
and Parana/Santa Catarina/Rio Grande do Sul (Brasil). Thereafter,
expect the development of an upper level trough across the Rio de
la Plata region and south Brasil beginning late Wednesday. This
dynamic upper level pattern will support a surface frontal
boundary, which is expected to extend across south Brasil. This
boundary will continue to move eastward while a stationary front
will be meandering northward through the day Wednesday into
Thursday. Additionally, a northerly low level wind flow will be
dominating in the region, helping transport moist tropical air
into the region. As the frontal boundary and other developing low
level troughs settle in the region, moisture pooling will be
amplified.

The greatest precipitation impacts are expected across south
Brasil, particularly during the morning and day on Wednesday.
Total precipitation maxima of up to 100mm is likely in the region
through Thursday morning. A risk for severe weather will also be
present for this time period. Thereafter, the intensity of the
rainfall accumulations will decrease as the connection to the
tropical moist air is expected to break. Regardless, showers and
thunderstorms are still expected to develop across southeast
Brasil for Thursday, where rainfall accumulations will reach 40mm
with isolated higher amounts.

Also by Thursday, the aforementioned upper ridge will become
broader, extending across much of north-central Brasil and will
push the aforementioned subtropical jet streak max farther south.
However, on Friday, a secondary upper level trough will be moving
into north-central Argentina from Chile and will encourage the
development of a subtropical trough as well. Note that this trough
is also being reflected into the mid-levels and being accompanied
with significant vorticity advection. This upper level trough will
be traversing the south/austral Chilean Andes on Wednesday and its
axis is anticipated to extend into Argentina on Wednesday. The
system will become elongated on Friday, where its axis will be
extending from south Chile and off the coast of south Argentina.

With the arrival of the upper trough axis, expect an increase in
precipitation chances. In Chile for Wednesday, elevated total
precipitation maxima are likely as a weakening surface frontal
boundary moves inland. A moisture plume will also accompany this
feature. On Thursday, a cold front will be developing across
Patagonia. Moisture will be limited and thus, precipitation
accumulations will be on the moderate end, reaching 20mm from
Thursday morning through Friday morning. From Friday morning
through Saturday morning, total precipitation maxima upwards of
25mm is likely across north-central Argentina, where the surface
convergence trigger will be the cold front. Expect precipitation
chances to increase with the arrival of this front. As the frontal
system moves into north-central Argentina, also anticipate an
increase in the risk for severe weather.

Across the Amazon, the presence of the broad upper ridge will also
be reflected in the low-to-mid levels. Easterly winds will be
dominating at these levels, leading to the transport of moisture
into the foothills of the Peruvian, Ecuadorian, and Colombian
Andes. This will lead to moisture pooling and orographic effects.
Additionally, the presence of any low-to-mid level troughs will
help initiate and sustain deep convection in the region. Across
central/northeast Brasil (Para, Maranhao, Tocantins, Bahia), the
periphery of an upper level trough will be overhead, helping
destabilize the environment for the next three days. This system
will support the daily development of scattered showers and
thunderstorms in the region.

Along the Pacific coasts of Colombia and Ecuador, expect the
greatest precipitation impacts to occur today, Wednesday. A low
level trough is present along the coast of Ecuador and is helping
promote cyclonic circulation.  Mid-level wind speeds will also be
elevated for Wednesday, which will favor the increase in vorticity
across west Ecuador. These conditions will yield total
precipitation maxima upwards of 50mm. A northerly low level wind
regime will become dominant thereafter and the mid-level wind
speeds will decrease slightly. Convection will continue to develop
but will be more isolated in nature. Along the Colombian coast,
convection will be driven by local and orographic effects.


Tinoco-Morales...(WPC)



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