South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
254 PM EDT Thu May 21 2026

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

Forecast Bulletin 21 May 2026 at 1900 UTC:

In the tropics...

In the upper levels, a broad region of upper diffluence will be
present across the north-central Amazon Basin on Thursday that
will be supported by the presence of two ridges off the northeast
Atlantic coast of the continent. This upper diffluence will
subside during the afternoon. Starting on Thursday evening into
Friday, an upper level ridge will be moving into the South
American continent from the Pacific, assisting in the development
of another diffluent pattern across the Orinoquia region. This
upper level ridging pattern will continue to migrate eastward into
the interior of the Amazon Basin with passing days. On Saturday,
the upper level wind speed will increase significantly across the
northwest and west Amazon Basin, which will support speed
divergence in the region. Meanwhile across the Guianas, upper
level diffluence and divergence will be overhead for the next
three days.

In the mid-levels, easterly low level winds will prevail north of
5S for the next three days. To the south, a broad anticyclonic
wind flow will prevail. In the low levels, a series of tropical
waves and low level troughs will be traversing the region. The
first tropical wave of interest is currently moving across
Colombia and is expected to move west of the Andes on Friday. With
its arrival into western Colombia, the Panamanian low level trough
will intensify and in turn increase cyclonic circulation along the
northern Pacific coast of Colombia. Thus expect enhanced total
precipitation maxima in the vicinity of this tropical wave for
Thursday and Friday. By Saturday, expect troughing to be present
off the Pacific coast of northern Colombia, favoring moderate
total precipitation maxima.

Another low level feature of interest is a wave that will be
moving into the Guianas early Friday morning. Precipitable water
values will exceed 60mm behind the trough axis and upper
divergence will be present in the region. The greatest
precipitation impact is poised for Friday, where enhanced
accumulations are likely. Thereafter, the tropical wave will
continue to propagate across Venezuela for Friday, where total
precipitation maxima will be elevated. The arrival of the wave
axis will lead to a change in the direction of the low level wind
flow, becoming more easterly. Before the wave arrives into
Venezuela, a southerly low level wind flow will be present across
the Guiana Highlands and the state of Amazonas in Venezuela. This
will lead to moisture pooling and convergence. These conditions
will favor enhanced total precipitation maxima for Thursday and
Friday.

In the subtropics and mid-latitudes...

A pre-existing frontal boundary will continue to influence the
precipitation patterns across the southeast Atlantic coast of
Colombia. On Thursday evening, the frontal boundary will be across
Espirito Santo. By Friday evening, it will be located across the
southern coast of Bahia and will remain in the region through
Saturday. At the same time, expect the development of a ridge
across southeast brasil, which will favor the increase in easterly
low level winds into the coast. This pattern will increase
moisture pooling and convergence in the region. Expect moderate
total precipitation maxima for the next three days across the
aforementioned regions.

Across southern Brasil, a mid-to-upper level trough will be moving
into the region on Saturday and it will support the development of
a surface to low level trough in the region. There will be
moisture pooling affiliated with this system and moisture
convergence will also increase. This will lead to an increased
chance in precipitation, where moderate accumulations are likely
to occur. At the same time, there will be mid-level divergence
that will support the development for thunderstorms.


Elsewhere in the mid-latitudes, expect seasonal conditions to
prevail.


Tinoco-Morales...(WPC)