South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
201 PM EDT Thu Oct 23 2025

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

South American Forecast Bulletin for 23 October 2025 at 1800 UTC

The main story continues to be the potential for severe
thunderstorms from around La Pampa, Buenos Aires and nearby areas
today, gradually moving north into Paraguay and Bolivia by this
weekend. This risk of severe weather, which may include heavy
rain, gusty winds, and hail, will be the product of the
combination of a frontal boundary moving north with moisture
convergence in the lower levels, along with favorable mid to upper
level dynamics. Each day, form today through Sunday morning, max
rainfall amounts could reach 100mm just ahead of the frontal
boundary, with lesser amounts nearby, but still observing showers
and thunderstorms.

In the upper levels, there is a broad trough extending from the
southeast Pacific, all the way up to around 20N. This trough is
tilted west, reaching Austral Chile by tonight, then the
northernmost base of the trough will reach Chile by Friday night.
The trough will move through central to southern South America
this weekend, while a secondary trough moves in to Austral South
America on Sunday, while another shortwave trough enter central
Chile. The upper levels also will have a strong jet that will be
particularly pronounced over northeastern Argentina late on Friday
into early Saturday, into Uruguay from Saturday into Sunday. This
jet will provide divergence aloft for the development of
thunderstorms over the area.

The mid levels and the upper levels seem to be vertically stacked,
and the same pattern of troughs are suggested in the mid levels by
the model guidance. One difference is that the mid level jet over
Argentina is just a bit further south than the upper levels, with
it being more over Northern Patagonia on Friday into Saturday,
though it also is expected to move north and a relative jet could
be over northeastern Argentina and Uruguay for Saturday into
Sunday. That said, there is also a very strong mid level jet being
suggested by the model guidance moving in across Austral South
America this weekend.

In the lower levels, there will be a series of frontal boundaries
moving into he southern half of South America. There will also be
low level jets across central South America from the north today
and Friday, but these will shift to southerly jets by Sunday.
These low level jets will bring plenty of moisture from the north
that will interact with the frontal boundaries and provide fuel
for the expected strong thunderstorms.

The situation across Tropical South America is a bit more benign.
The mid and upper levels are generally dominated by a high
pressure, providing some subsidence. There is also drier than
normal air moving in across the eastern portions of Brasil.
However, the available moisture will be above normal across
western Brasil into Colombia and northern Peru. The rainfall
pattern across Tropical South America will be mainly that of
diurnal and local effects combining with the available moisture,
with central to western Brasil observing much more rain than the
eastern sections. That said, there is some rain forecast across
the eastern coast of Brasil from around Espirito Santo to the
Brazilian Nordeste each day into the weekend.

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

Alamo...(WPC)