174
FXUS20 KWBC 301923
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
323 PM EDT Thu Jul 30 2026

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

Forecast Bulletin 30 July 2026 at 1925 UTC:
**CORRECTED**

In the mid-latitudes...
The exit region of a potent and highly amplified upper trough will
continue to influence the upper level regime across Chile through
the day today. At the surface, this upper trough is being
accompanied by a frontal boundary. The cold front will be
extending into central Chile for today where a moisture plume will
also be converging into the region. The upper level dynamic regime
will be most favorable for upper divergence through Friday
afternoon, when the axis of the upper trough is anticipated to
move into the continent. Though, as the cold front converges into
the continent, the moisture plume and low level wind speeds will
be decaying. This will lead to a decrease in moisture convergence
across the area on Friday. Thus, expect the most intense
precipitation impacts for today, where total precipitation maxima
of 35 - 70mm is likely in south-central Chile. By Friday, total
precipitation maxima will decrease with the decay of the frontal
boundary and moisture plume.

Prior to the arrival of the upper trough, northerly low level
winds will intensify, which is associated with the South American
Low Level Jet (SALLJ). This will lead to a significant increase in
moisture into Argentina, extending as far south as Patagonia.
Thus, expect above average precipitable water values. On Thursday,
mid-level shortwave troughs and developing surface frontal/low
level troughs will help with the development of snow showers
across Patagonia. Above average precipitable water values will
remain for Friday, when snow showers will still be likely in the
region.

By Friday, the upper trough axis will begin to move into the
continent. As the exit region moves east of the Andes, expect the
development of a strong surface frontal boundary. Moisture pooling
will be significant across the frontal boundary as it develops.
Note that at this time, upper level winds will be reaching 100
knots and speed shear will be significant across the vertical
column. These dynamic conditions will favor the development of
severe weather across the Rio de la Plata (Argentina/Uruguay),
Buenos Aires/La Pampa, and northern Argentina.

The most significant rainfall accumulations are expected for Rio
de la Plata where total precipitation maxima will be 50 - 100mm
from Friday morning through Saturday morning. The upper trough is
expected to decay as it continues to move eastward into the
Atlantic while the surface frontal boundary will continue to
progress northward into Bolivia/Paraguay/south Brasil. At this
point, there will be a discontinue with the moisture flow from the
tropics and the SALLJ will reorient itself to a more
north-to-south component. Thus, expect a subtle decrease in
precipitation accumulations across the region for Saturday.
Moderate precipitation accumulations are still likely.

On Saturday, another upper level shortwave trough will be
approaching south Chile that will again support a surface frontal
boundary. This system will be accompanied by above average
precipitable water values and the mid-to-upper level dynamic will
be favorable for the sustenance of precipitation in the region.
Thus, expect an increase in precipitation accumulations in the
region.

In the tropics...
Seasonal conditions will prevail through most of the region. The
main drivers for precipitation will be the propagation of tropical
waves across the northern extremes of the continent and low level
troughs. There will be brief periods of moisture convergence
across the region that may help reinforce diurnal convection
across the Amazon Basin. On Saturday, there will be a resurgence
in more north-northeasterly low level winds into Peru as an upper
level trough is expected to move into the region. Overall, expect
light to moderate precipitation accumulations in the region for
Saturday.


Tinoco-Morales...(WPC)





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