South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1200 PM EDT Thu Apr 09 2026

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

Forecast Bulletin 09 April 2026 at 1600 UTC:

In the tropics...

In the upper levels...

An upper ridge, otherwise known as the Bolivian High, is currently
centered across northern Bolivia. This ridge is expected to
broaden into central and northeastern Brasil through Saturday. By
Sunday evening, its structure will become disorganized, especially
across the western Amazon. This upper ridge will also be
interacting with transient upper troughs across the Chaco regions
and southeast Brasil, reinforcing upper diffluence and divergence.

Regionally, upper level diffluence will be present across the west
and northwest Amazon Basin through Sunday evening. Across the
southwest Amazon Basin, an upper level shortwave trough will be
embedded within the upper ridge and its exit region will be
sustaining upper divergence for Thursday and Friday. Meanwhile,
two upper level troughs will be propagating across southeastern
Brasil for the next three days that will also be interacting with
the Bolivian High. The first upper trough is expected to lose its
definition during the day on Thursday but upper level diffluence
will be present north of its base, which will support periods of
upper divergence across Serra do Mar. Thereafter, a jet streak max
will then develop in this region. Another mid-to-upper level
trough will be approaching the region on Friday and it will favor
upper diffluence starting late Friday night through Saturday. This
pattern will support upper divergence across Espirito Santo and
Serra do Mar.

In the surface-to-mid levels conditions and precipitation
impacts...

A mid-level ridge will be reflected across the tropics and this
feature will support persistent low-to-mid level easterly winds
(700mb to 500mb) across the Amazon Basin through Sunday. This
pattern will also support the transport of moisture across the
basin. During this time, there will be a series of low level
troughs propagating with the easterly trades. A notable low level
trough is currently moving across the eastern Amazon Basin and
will be approaching the central Amazon Basin by Thursday evening.
Expect an enhancement in moisture convergence and vertical ascent
with this feature. On Friday, the trough will be located across
the central and western Amazon Basin and will lose its structure
as it continues moving into the western Amazon Basin on Saturday.
These conditions, coupled with the upper level regime, will yield
moderate enhanced total precipitation maxima in the vicinity of
the propagating troughs for the next three days. Another region of
interest for precipitation impact is the northwest Amazon Basin,
where there will be ample moisture transport and upper divergence
present. Enhanced total precipitation maxima are expected for the
next three days for this region.

Across southeast Brasil, a surface cold front is currently across
the northern coast of Serra do Mar and will begin to move into
Espirito Santo overnight Thursday and into Friday. This front will
transition into a stationary front on Friday and will continue to
be over Espirito Santo through Saturday. After Saturday, the
stationary front will begin to move offshore.  A low level ridge
will also be present to the north and it will favor northwesterly
winds ahead of the front, which will lead to moisture pooling
along the front. Precipitable water values will be approaching
50mm for the next two days. On Saturday, there will be a decrease
in moisture pooling in the region due to the exiting of the
frontal boundary, though there will still be moisture air
advection into Espirito Santo and easterly low level winds. These
conditions will favor localized orographic enhancement and
moisture convergence. With respect to precipitation impacts,
expect moderate total precipitation maxima on Thursday across
Serra do Mar. On Friday, a mid-level trough will also be moving
across the region and it will also support mid-level divergence in
the region. Thus, expect enhanced total precipitation maxima
across Espirito Santo. On Sunday, moderate total precipitation
maxima are likely across Espirito Santo.

In the subtropics and mid-latitudes...

The axis of a mid-to-upper level trough will be moving east of the
Andes and into the Chaco region on Saturday morning. By Sunday
morning, the axis will be over Uruguay. A low level trough will be
developing during the day on Saturday across Paraguay and northern
Argentina. By Sunday morning, cyclogenesis will occur offshore, as
the system moves into the Atlantic. Meanwhile, the low level
trough will still be present across the aforementioned regions.
There will also be moisture transport from the deep tropics into
the region, leading to increasing precipitable water values. Thus,
expect enhanced total precipitation maxima across southeast
Paraguay, southeast Brasil, and Uruguay from Saturday morning
through Sunday morning with these conditions. There will also be a
risk for severe weather and the potential development of a
mesoscale convective system (MCS) due to the presence of upper
level divergence and an incoming mid-level trough.

In austral and southern Chile, a potent jet streak max will be
over the region for the next three days. At the surface, a series
of frontal boundaries will be moving into the continent during
this period. These fronts will be accompanied with a moisture
plume that will contain precipitable water values of up to 30mm.
Meanwhile, the low level wind speeds will be exceeding 40 knots as
the fronts move into the continent. Thus, expect enhanced upper
divergence and moisture convergence in the region, yielding
enhanced total precipitation maxima across austral Chile on
Thursday and enhanced maxima across the northern regions of
austral Chile on Friday. By Saturday, the region with the greatest
precipitation impact will be in southern Chile, though total
precipitation maxima values will be moderate.


Tinoco-Morales...(WPC)