South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
252 PM EDT Thu Mar 12 2026

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

Forecast Bulletin 12 March 2026 at 1855 UTC:

Across the mid-latitudes...

The axis of a mid-to-upper level shortwave trough will be
propagating across austral Chile during the day on Thursday.
Meanwhile, a surface frontal system will be moving into the
continent on Thursday afternoon and will be accompanied by an
increase of strong northwesterly low level winds, which will
exceed 50 knots. Also anticipate an increase in precipitable water
values. As the frontal system moves east of the Andes overnight
Thursday into Friday, it will enhance moisture convergence across
Tierra de Fuego. Thus, expect enhanced total precipitation maxima
across austral Chile and Tierra de Fuego through Friday morning.
Precipitation conditions will be seasonal for Friday.

By Saturday, expect a jet streak max to be present across southern
Chile. Simultaneously, a mid-to-upper level trough will be gaining
amplitude off the coast of Chile. This will favor a divergent
mid-to-upper level pattern and the sustenance of a frontal system
at the surface. The frontal system will be accompanied by an
atmospheric river that will contain precipitable water values of
up to 38mm and it will primarily be impacting southern Chile. The
most intense moisture convergence is also anticipated in this
region due to the low level wind flow being from the west.
Meanwhile in austral Chile, there will be strong
north-northwesterly low level winds present in the region prior to
the arrival of the surface occluded front. The occluded front will
move into the continent early Sunday morning. Precipitable water
values will be less in this region in comparison to southern
Chile, only reaching 20mm. Thus, the most intense daily total
precipitation accumulations will be in southern Chile and moderate
accumulations are likely across austral Chile from Saturday
morning through Sunday morning. Note that this frontal system and
accompanying atmospheric river will continue moving northward into
central Chile from Sunday through the early part of the next work
week, producing above normal total precipitation accumulations.

Meanwhile in north-central Argentina, expect the traversal of
mid-to-upper level troughs for the next three days across the
region. These conditions will favor the daily development of
surface to low level troughs across the Cuyo region and the
development of frontal systems off the Atlantic coast.
Precipitable water values will be limited for Thursday and Friday,
leading to moderate total precipitation maxima. By Saturday, there
will be an increase in moisture due to the intensification of the
South American Low Level Jet across Bolivia, Paraguay, and
northern Argentina. During the morning on Saturday, a short-lived
cold front will develop across Cordoba and Santa Fe. This cold
front will quickly propagate northward, being located across the
far northern regions of Argentina by Saturday evening. This front
will help enhance moisture convergence. These conditions will
yield enhanced total precipitation maxima across Cordoba, Santa
Fe, and the Chaco region. Note that there is a daily potential for
severe weather across central Argentina due to the presence of
cold mid-level temperatures and warm surface temperatures.

Across the subtropics...

The Bolivian High, a subtropical upper level ridge, will continue
to influence the upper level regime on Thursday and Friday before
starting to lose definition on Saturday. This system will interact
with a zonal upper level flow across Paraguay, center-west and
southeast Brasil, which will favor an upper level diffluent
pattern across Bolivia. Within this zonal flow, there will be
multiple regions of speed divergence and jet streak maxes. This
pattern will support the sustenance of divergence across Bolivia
and Brasil through Saturday. A low level trough will be present
across the region for this period as well, enhancing low level
cyclonic circulation. Thus, enhanced total precipitation maxima
are likely for Thursday and Friday across the aforementioned
regions. By Saturday, the South America low level jet (SALLJ) will
intensify and will cut-off the moisture transport to Brasil from
the Amazon. Thus, there will be a subtle decrease in total
precipitation maxima for Saturday.

Precipitation patterns across Peru and Ecuador will also be
influenced by the presence of the Bolivian High for the next three
days. There will be upper level diffluence present that will
support divergence. East of the Andes, there will be enhanced
northeasterly low level winds across the east Amazon Basin that
will assist in moisture pooling along the foothills of the Andes
in Ecuador and Peru. Thus, anticipate enhanced total precipitation
maxima across the region. West of the Andes, sea surface
temperature positive anomalies will be present in the region of El
Nino 1+2. Expect low level westerlies to dominate, which will
support the development and sustenance of precipitation across
Ecuador and across the interior north coast of Peru. Across the
Sierra Central, the presence of upper divergence and moist
mid-levels will favor snowfall.

Meanwhile across the northeast corner of the continent, there will
be upper divergence across the Guianas and central Brasil. This
divergence is being sustained by the presence of speed divergence
and diffluence. With respect to moisture content, central and west
Brasil will see above normal precipitable water values. This
region will also exhibit the most intense total precipitation
maxima for the next three days. Also expect enhanced total
precipitation maxima in the vicinity of low level troughs.

Otherwise, seasonal conditions will prevail for the rest of the
continent. Daily diurnal showers and thunderstorms are likely.


Tinoco-Morales...(WPC)
Garay-Marzano...(SPM/DMA/SENAMHI)