South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
246 PM EDT Mon Jul 20 2026

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

Forecast Bulletin 20 July 2026 at 1845 UTC:

The next three days will feature an active pattern across South
America. The most significant threat will be over Rio de la
Plata, where a strengthening South America Low-Level Jet (SALLJ)
and a diffluent upper-level jet streak combine to support locally
heavy rain along with a risk of severe thunderstorms and hail
across Uruguay, northeastern Argentina, and southwestern Brazil.
At the same time, an upper-level trough and area of low pressure
moving in from the South Pacific will bring periods of rain and
mountain snow to the Chilean Andes, while steady tropical rains
continue across Colombia, Venezuela, and Ecuador as a tropical
disturbance or trough moves west over the area today (Monday)
and tonight.

For today and tonight, an upper trough and area of low pressure
approaching from the South Pacific will bring rain into central
Chile, with snow developing across the higher elevations of the
Andes. Some of this precipitation will fall over areas that
already picked up very heavy rain in recent days, though totals
this time should be relatively modest. Aloft, the jet streak tied
to this system strengthens to near 160 kt tonight as its core
noses onshore over central Chile.

The SALLJ is already established across Bolivia and
Paraguay, feeding a corridor of moist air southeastward into
northern Argentina and Uruguay. Precipitable water values are
running more than 2 standard deviations above normal in this
corridor. Expect scattered showers and thunderstorms this
afternoon and tonight across Uruguay, northeastern Argentina, and
southwestern Brazil, with daily totals generally in the 20-40 mm
range, locally higher up to 100 mm in the strongest storms, and a
few capable of turning strong to severe.

Across the tropics, persistent moisture will keep daily rain
going across Colombia, Venezuela, and Ecuador, with totals
generally 25-35 mm and locally higher amounts near the Andean
foothills. A low-level disturbance or trough near the Guianas
will begin drifting toward eastern Venezuela.

On Tuesday and Tuesday night, the upper jet relaxes over Chile as
its strongest core translates eastward off the coast. Even so, the
parent trough and low keep moving through with continued low-level
moisture transport into central Chile, so periods of rain persist
and the heaviest mountain snow of the period falls across the
central Andes through Tuesday night before activity begins to
taper as the system pulls east.

That eastward shift feeds the more serious threat over Rio de la
Plata. The SALLJ strengthens further, precipitable water anomalies
climb well above 2 standard deviations across Bolivia, Paraguay,
and northern Argentina, and an upper jet streak of around 120 kt
noses east of the Andes, placing increasingly diffluent flow over
the region. GR02T algorithm shows this as the peak window for
organized severe weather, with a slight to moderate risk, including
a couple of areas of elevated risk, for damaging hail and strong
thunderstorms centered over Uruguay, northeastern Argentina, and
southwestern Brazil Tuesday night into early Wednesday. Rainfall
through this window should run 30-60 mm with isolated higher
amounts up to 125 mm possible.

The tropics continue their steady daily convection across Colombia
and Ecuador as a disturbance moves into Venezuela.

By Wednesday and Wednesday night, the upper pattern stays active,
but the jet axis keeps shifting east and south, moving the bulk of
the divergence and lift out over Argentina and the adjacent
Atlantic. The GR02T severe threat becomes more scattered and mainly
marginal by Wednesday night, breaking into a couple of smaller
areas rather than one organized corridor, though an isolated
stronger storm cannot be ruled out near the Atlantic coast of
southern Brazil and Uruguay. Rain continues across Rio de la Plata,
generally 20-40 mm, and max up to 50 mm with less coverage than
Tuesday.

Over Chile, the parent trough and low pressure system have shifted
well south by this point, confining meaningful rain and mountain
snow mainly to southern Chile and Patagonia.

Across the tropics, the disturbance reaches into Colombia, keeping
heavy rain going there, while Venezuela and Ecuador continue their
typical daily convection.

Alamo...(WPC)