South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
326 PM EDT Tue Oct 28 2025

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

South American Forecast Bulletin for 28 October 2025 at 1930 UTC

A broad upper level trough pattern dominates the southern portion
of South America, with an active subtropical jet extending across
central Chile into southern Brazil. This pattern favors strong
upper level divergence and deep layer ascent across northern
Argentina, Paraguay, and southern Brazil during midweek. At low
levels, a moist and unstable air mass prevails across tropical
South America, while a cold front advances northward from
Argentina into southern Brazil. The combined effects will result
in widespread convection across central and southeastern South
America, including episodes of severe weather, while heavy
rainfall continues across the western Amazon and the eastern
slopes of the Andes. Moderate precipitation will also affect
southern Chile as frontal systems move inland.

At upper levels, model guidance shows a progressive pattern across
the southern portion of the continent. On Tuesday and Wednesday, a
broad trough dominates the southern stream while embedded short
wave perturbations propagate eastward along 40 to 50S. Farther
north, a subtropical jet maxima extends across Chile into northern
Argentina and southern Brazil. Through Wednesday evening, this jet
strengthens and shifts northeastward in association with an
amplifying trough that digs from Chile into central Argentina.
Upper level divergence ahead of this trough and strong low level
moisture convergence will promote organized deep convection across
Paraguay, northern Argentina, and southern Brazil. Through early
Thursday, expect widespread convection producing rainfall maxima
of 40 to 80 mm, with embedded severe storms possible given
favorable upper divergence and 850 hPa wind convergence. By
Thursday into early Friday, the system moves eastward across
southern Brazil and into the western South Atlantic while
convection gradually weakens.

At mid levels, the trough pattern follows the upper level
evolution. The short wave trough entering central Argentina on
Wednesday induces cyclogenesis over the western South Atlantic by
Thursday. As the associated cold front advances northward,
convection redevelops along and ahead of the boundary from Mato
Grosso do Sul into Parana and Sao Paulo. Moisture convergence and
dynamic forcing from the upper jet will sustain additional maxima
of 25 to 50 mm, with isolated higher amounts in thunderstorms.

Over tropical South America, abundant precipitable water above 60
mm across much of Amazonia and northern Brazil supports daily
rounds of moderate to heavy convection. Through the period,
diurnal convection will produce maxima of 25 to 50 mm per day
across central and western Amazonia, with locally higher amounts
near the eastern slopes of the Andes in Peru, Bolivia, and
southern Colombia where low level westerly flow enhances moisture
convergence. Lightning activity is forecast to remain high across
these regions through the period.

Over southeastern Brazil, convection will increase on Thursday
into Friday as moisture from the tropics interacts with the
approaching frontal zone. Expect scattered thunderstorms with
localized maxima near 35 to 70 mm, decreasing thereafter as the
front weakens.

Across the Altiplano, conditions remain generally dry under
subsident flow aloft, while moderate convection continues along
the eastern Andes from central Peru southward into northern
Bolivia and northwest Argentina, favored by upslope flow and upper
level divergence on the poleward flank of the Bolivian High.
In the southern cone, the next frontal system reaches southern
Chile by late Thursday into Friday, producing rainfall maxima of
15 to 30 mm along coastal and Andean areas between Los Lagos and
Aysen. Farther south, post frontal cold advection supports
scattered showers and snow showers over southern Patagonia and
Tierra del Fuego.

A strong upper trough crossing central Argentina and southern
Brazil midweek will generate widespread thunderstorms, with a risk
of locally severe convection and heavy rainfall between northern
Argentina, Paraguay, and southern Brazil. Farther north,
persistent convection across the Amazon Basin will maintain a high
potential for locally heavy rain and frequent lightning through
the period. Frontal passages over southern Chile will also bring
moderate rain and gusty winds to the coastal and Andean regions,
while snow showers are possible over southern Patagonia and Tierra
del Fuego late in the week.

For the latest available charts please visit:
https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml

Alamo...(WPC)