FXUS20 KWBC 191915
PMDSA 

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
315 PM EDT Tue May 19 2026

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

Forecast Bulletin 19 May 2026 at 1915 UTC:

For the next three days, the regions of interest for precipitation
impacts will be across the subtropics and tropics. On Tuesday, a
broad mid-level ridge will be situated in the Caribbean Sea and
its southern periphery will extend across the Guianas, Venezuela,
and Colombia. Though its structure is expected to weaken after
Tuesday, mid-level easterly winds will dominate across the
northwest Amazon Basin and western Colombia for the rest of the
forecast period. A similar wind flow pattern will be present in
low levels, where increasingly southeasterly winds will prevail
after Tuesday. This wind pattern will favor the transport of moist
air into the region.

During this time, there will be multiple low level troughs and
tropical waves propagating through the region, leading to
increased chances for showers and deep convection. The first low
level trough is currently has an axis near 65W and will continue
to move into the northwest Amazon Basin on Wednesday. By Thursday,
the tropical wave will be crossing over the Andes and into
northwest Colombia. Thus, on Tuesday and Wednesday, enhanced total
precipitation maxima will be likely across the central and
northwest Amazon Basin. On Thursday, elevated maxima are likely
across the eastern slopes of the Andes in Colombia. Upper level
speed divergence will be dominating across northern Venezuela and
upper diffluence will be present across Colombia, which will
support the development of convective ventilation and support deep
convection.

Another tropical wave of interest is currently near 58W and it
will continue to move across Venezuela and Colombia on Wednesday
and Thursday. This feature will enhance moisture convergence and
low level cyclonic rotation, favoring the development of
thunderstorms in the region for the next three days. As the wave
moves into Venezuela, there will be an increase in upper level
support via speed divergence. Additionally, expect divergence
along the periphery of the aforementioned mid-level ridge. These
conditions will yield daily  moderate to elevated total
precipitation maxima across the region.

Meanwhile across the Pacific coast of Colombia and Ecuador,
starting Wednesday morning, anticipate the reemergence of the
Panamanian trough with the arrival of a low level trough. This
will lead to an increase in moisture and an enhancement in
cyclonic rotation across the region. The low level wind flow will
be orthogonal to the coastline and orography of the northern
Colombian Pacific coast. This will lead to enhanced moisture
convergence and orographic effects. As a result, elevated total
precipitation maxima will be likely across the north Pacific coast
of Colombia for Wednesday and moderate accumulations for Thursday.

Across the remainder of the tropical region, the Intertropical
Convergence Zone (ITCZ) will continue to support the transport of
moist air into the Amazon Delta and the Guianas for the next three
days. Embedded low level troughs within the ITCZ will further
boost rainfall chances. Elsewhere, daily diurnal showers and
thunderstorms are likely.

In the subtropics and mid-latitudes...

Currently, an upper level jet streak is over southeast Brasil.
This jet streak max will begin to move out of the continent on
Tuesday afternoon and an upper level shortwave trough will be
moving overhead. Expect an enhancement in upper divergence with
this pattern. At the surface, a pre-existing frontal boundary will
continue to meander along the Atlantic coast of Brasil and extend
into the southwest Amazon Basin. On Tuesday evening, the frontal
boundary will besituated across Rio de Janeiro. By Wednesday
evening, the frontal boundary will be across Espirito Santo and it
will meander in this region through Thursday while the segment
that extends into the southwest Amazon will begin to lose its
definition. The period with the greatest upper level support
across southeast Brasil will be on Tuesday and Wednesday,
thereafter, the upper level wind regime will become more zonal.
Light total precipitation maxima are likely with the presence of
the frontal boundary along the Atlantic coast of Brasil.

Another shortwave trough will be arriving into southeast Brasil on
Thursday and it will favor the development of a low level trough
across the Paraguay-Brasil border. In this region, expect moisture
pooling with the de-acceleration of wind speeds from the
northeast. Moderate total precipitation maxima is likely with
these conditions.

Across the rest of the region, expect seasonal conditions to
prevail.


Tinoco-Morales...(WPC)





$$