107
FXUS20 KWBC 281717
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
116 PM EDT Tue Jul 28 2026

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

Forecast Bulletin 28 July 2026 at 1720 UTC:

In the mid-latitudes and subtropics...
The first synoptic system of interest is an upper level trough
that is currently crossing the Chilean Andes and its axis is
expected to be east of the Andes by this afternoon. Along its exit
region, a jet streak max is being sustained, though it is expected
to separate into two distinct jet streak maxes this evening. At
this point, a diffluent upper level pattern will be developing
over south Brasil and north Uruguay. The mid-level regime will
also be dynamic, sustaining two shortwave troughs that will be
propagating across the region through Wednesday morning. The upper
trough axis will be over Uruguay and south Brasil by Wednesday
morning, when a surface cold front and warm front are expected to
gain definition across Mesopotamia (Argentina) and south Brasil.
The upper trough axis will move into the Atlantic by Wednesday
afternoon, though, the surface frontal boundary will still be
propagating northward along the Brasilian Atlantic coast.

Also on Tuesday and Wednesday, an ongoing broad low level ridging
pattern and the arrival of the upper trough will continue to
maintain the South American Low Level Jet (SALLJ). In turn, expect
above normal precipitable water values across south Brasil and
Uruguay for the next two days. A broad cyclonic circulation in the
low level wind flow will develop on Tuesday evening across Uruguay
and Paraguay, which will reinforce ascent in the region. Overall,
elevated total precipitation maxima is likely across south Brasil
for Tuesday and decreasing for Wednesday.

Meanwhile across Pantanal (Brasil) and Paraguay, a shortwave upper
level trough is expected to be moving across this region on
Wednesday afternoon, where there will be a subtle uptick in upper
diffluence for Wednesday afternoon/evening. In the mid-levels,
also anticipate the passage of a shortwave trough for Wednesday.
The environment will already be primed for unstable conditions and
moisture advection, which will favor the development of
thunderstorms in the region for Wednesday into Thursday morning.
Rainfall accumulations will be moderate.

The second synoptic is a highly amplified upper trough that will
be gaining definition after Wednesday afternoon across 100W. Its
exit region will arrive into the continent by Thursday and its
axis will move into the continent by Friday. This upper level
system will sustain a frontal boundary at the surface, which will
also be accompanied by a long fetch moisture plume. The
accompanying warm front will begin to converge into southern Chile
on Wednesday evening while the cold front will converge into
central Chile during the day on Thursday. The occluded front will
straddle the coastline and will remain off the coast through the
end of the work week. Precipitable water values will remain above
average in the region, where north-northwesterly low level wind
speeds will be reaching 25 knots from Thursday into Friday.
Precipitation chances are expected to increase by early Thursday
morning and continue into the day. The combination of a moist
environment and dynamic mid-to-upper level pattern will yield
elevated precipitation (water equivalent) accumulations from
Thursday morning through Friday morning.

The dynamic upper level pattern across the region will favor the
development of multiple frontal boundaries across Patagonia
(Argentina) for the forecast cycle. A low level feature of
interest is a ridge that will be present across Uruguay and
Argentina, which will be promoting northerly low level winds into
Patagonia. This will lead to the resurgence in precipitable water
values. Expect daily showers and snow showers across Patagonia in
the vicinity of the development frontal boundaries and low level
troughs in the region.

In the tropics...
For the next three days, there will be continuous easterly low to
mid level winds that will pool moisture across the northwest
Amazon Basin and Venezuela. However, dry air advection will begin
to prevail after Thursday. During this period, a tropical wave
will be propagating across the extreme northern regions of South
America. In the vicinity of this wave, anticipate the increase in
moisture advection and thus convection. Moderate rainfall
accumulations are likely with the passage of this wave. Otherwise,
seasonal conditions will persist for the rest of the tropical
domain.



Tinoco-Morales...(WPC)





$$