561
FXUS20 KWBC 011937
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
337 PM EDT Fri May 01 2026

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

Forecast Bulletin 01 May 2026 at 1940 UTC:

UPDATED AND CORRECTED VERSION

In central South America...

In the upper levels, the axis of an upper level trough is
currently moving across the Andes in southern Chile/Patagonia and
is expected to be east of the Andes by Friday afternoon. The
trough will amplify during the day on Friday and its base will
extend into north-central Argentina and Uruguay. Also on Friday,
there will be another jet streak max present across south Brasil,
southeast Paraguay and the Mesopotamia region of Argentina. These
two features will interact with each other and lead to coupled jet
streak dynamics across the aforementioned region. Thus, expect
enhanced upper divergence for Friday. After Saturday morning, the
axis of the upper trough will be moving into the Atlantic.

On Friday, a mid-to-upper level shortwave trough will be moving
across south Brasil and southeastern Uruguay early in the morning
and it will be projected into the surface and low levels as a
trough. Moisture convergence will be amplified as well as upper
divergence. Ahead of the arrival of the potent upper trough, there
will be an influx of moist tropical air into central South America
via the South American Low Level Jet (SALLJ), yielding
precipitable water values of up to 44mm in the region. This region
will see an enhancement in low level cyclonic circulation as it
will sit in the left region of the SALLJ.

Meanwhile, the upper trough will also sustain a surface cold front
that will be developing over La Pampa/Buenos Aires and the Cuyo
region during the day on Friday. The cold front will arrive into
Uruguay, southern Brasil, and Paraguay late Friday evening and
into early Saturday morning. With its arrival, expect an increase
in moisture convergence and surface lift. At the same time,
coupled jet dynamics will be favoring enhanced divergence in this
region. The active surface-to-upper level pattern throughout
Friday in the aforementioned regions will yield elevated total
precipitation maxima with a risk for severe weather.

The cold front will continue propagating northward during the day
on Saturday and it will be across Brasil, Paraguay, and extending
into south-central Bolivia. On Saturday evening, the cold front
will be located near Santa Catarina/Parana, Paraguay, and across
the Chiquitania region of Bolivia. Though, the most significant
moisture convergence will be located along the Atlantic coast of
Brasil and northern Paraguay. Additionally, across northern
Paraguay, a mid-level shortwave trough will be traversing the area
on Saturday and it will amplify vertical ascent. Thus, expect
moderate total precipitation maxima from Saturday morning through
Sunday morning across the Brasil coast and enhanced total
precipitation maxima across northern Paraguay.

On Sunday, the cold front will still be propagating across the
Atlantic coast of Brasil during the day. It is expected to be
located near Espirito Santo by Sunday evening. Meanwhile, the cold
front across Paraguay and Bolivia will lose its definition during
the afternoon/evening. Along the coasts of Espirito Santo, low
level winds will become more easterly/southeasterly during the
evening on Sunday and into Monday, which will further enhance
moisture convergence and orographic effects. Expect moderate total
precipitation maxima from Sunday morning through Monday morning in
this region. Also on Sunday evening and into Monday morning, a
mid-to-upper level shortwave trough will be moving across southern
Brasil, which will favor the development of a low level trough in
the region. Expect moderate total precipitation maxima with these
conditions.

In Chile...

A potent mid-to-upper level trough will be impacting southern and
austral Chile starting on Sunday and it will begin to support
upper divergence across the region starting on Saturday evening.
Low level wind speeds will increase on Saturday afternoon/evening
with its approach. Wind speeds will be exceeding 30 knots. The
upper level pattern will also support the sustenance of a surface
frontal boundary, which will begin to move into the continent
starting Saturday afternoon/evening. A weak moisture plume will
accompany the cold front, where precipitable water values will
reach 30mm. Meanwhile, precipitable water values of up to 20mm
will accompany the occluded front. Thus, expect moderate total
precipitation maxima with the low-to-upper level pattern for
Sunday across southern and northern austral Chile.

Peru, Ecuador, Colombia, and the Amazon...

For the northern coast sectors between Ecuador and Colombia, a low
level trough will be present and it will support instability and
moisture influx, primarily for Sunday. Subsequently, low level
winds will change direction (shifting toward the northwest),
reducing moisture flow and the occurrence of rainfall.
In the Amazon region, mainly between Peru, Colombia, and Brazil,
the continuous influx of moisture and the increasing amplitude of
a low-level trough will increase rainfall. Higher accumulations
are expected toward the northern sector due to low level wind flow
directions being influenced by the rotation produced by the
trough`s ridge.

For the north and northeast coast of Brazil, the arrival and
movement of low-level troughs are anticipated. These, combined
with the continuous injection of moisture, will favor an increase
in precipitation throughout the three-day forecast period. On
Friday, a low level trough of interest currently has an axis near
40W and it will be moving across the northeast coast of Brasil. It
will favor moisture convergence, yielding elevated total
precipitation maxima from Friday morning through Saturday morning.
This low level trough will continue moving along the north coast
of Brasil for the next three days and support daily enhanced total
precipitation maxima for the weekend.

Additionally, the continuous presence of the ITCZ (Intertropical
Convergence Zone) is observed over the Amazon Delta, which will
support the generation of rain in that sector. An upper level
ridge will be favoring diffluence in this region on Friday, which
will also promote divergence. Also, a mid-level trough will be
propagating across the Guianas. Thus, expect enhanced total
precipitation maxima for Friday.


Tinoco-Morales...(WPC)
Garay-Marzano...(SENAMHI-PERU)



$$