South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD


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

Forecast Bulletin 17 September 2026 at 1900 UTC:

Today, diurnal heating and high levels of evapotranspiration will
support high mid-level moisture availability from northern Mato
Grosso into the Amazon basin and portions of Venezuela. Low-level
wind convergence and moderate potential vorticity will develop
strong upward moisture transport to sustain this moisture
availability; while, in the upper levels, an area of wind
convergence present over the Central Amazon Basin will limit deep
convection. In the mid-levels, the synoptic eastward wind pattern
over the equatorial belt of theAmazon will advect and concentrate
high moisture availability into the western Amazon Basin, where
orographic forcing will help trigger thunderstorms over the
rainforest. Along the western Colombia coast and northern Ecuador,
these enhanced moisture levels will interact with strong low-level
moisture convergence associated with a tropical wave moving over
the Panama Bight to trigger convective storms during the afternoon
hours. Rainfall accumulation maxima of 15-35 mm are expected
today, with higher totals over the western Amazon basin and ChocĂ.
On Friday, the mid-to-low-level wind pattern over the Amazon will
reorganize, shifting slightly to the north and northwest over the
western and northwestern Amazon basin into northwestern Colombia,
where strong moisture convection will concentrate. This low-level
pattern will remain on Saturday while upper-level forcing
conditions will develop from Costa Rica to Venezuela, leading to
scattered thunderstorms along the western Colombian coast and more
isolated storms over northwestern Ecuador on both days. Over
Orinoquia and the Amazon Basin, a tropical wave will support storm
development, while low-level troughs and orographic forcing will
establish moderate to strong low-level moisture convergence to
trigger convective thunderstorms. Daily rainfall accumulation
totals of 15-25 mm with locally higher amounts are likely on
Friday, and 20-35 mm are forecast on Saturday along western
Colombia and portions of Orinoquia.

Further south, a mid-to-upper-level trough will sustain strong
north-to-northwesterly winds, transporting more moisture across
southern Peru into the Bolivian Altiplano, the Chaco, the Atacama
Desert, and northwest Argentina while increasing mid-level
potential vorticity across the region. This atmospheric setup will
persist for the next 48 hours as the trough moves across the
continent. A slight risk of severe weather will develop over
Paraguay late today into Friday. Rainfall accumulation maxima of
15-25 mm are expected across the Bolivian Altiplano, with mountain
snowfall accumulations of 4-8 cm likely today from southeastern
Bolivia to the Puna. On Friday, strong upper-level wind divergence
will develop over Paraguay, northeastern Argentina, Pantanal, and
Brazilâ€s Parana Basin, while high mid-level potential vorticity
will favor rapid upward motion and a deep layer of atmospheric
instability, promoting severe storm development. A dense plume of
low-level moisture will cross the region, providing the necessary
ingredients to feed this unstable setup. Rainfall totals ranging
from 15-50 mm are forecast over that region, with locally higher
accumulations possible. By Saturday, the upper troughâ€s axis
will pass over Paraguay, and unfavorable upper-level wind
convergence alongside mid-level divergence will position over
southern Peru into Paraguay. This will cut off the transport of
Amazon moisture into these regions while limiting the denser
moisture plume and more unstable dynamic conditions to
southeastern Paraguay and southeastern Brazil. A slight risk of
severe weather will remain over that area, with maximum rainfall
accumulations of 30â€60 mm forecast.

A mid-to-upper-level trough will support unstable conditions while
strong low-level winds favor the entry of a moisture plume into
Austral Chile today. A series of fronts and their interaction with
local terrain will provide the forcing to trigger rainfall and
mountain snowfall accumulations today and tomorrow (Friday). On
Saturday, a deeper mid-to-upper-level trough will develop offshore
Chile and is expected to enter the country during the afternoon
hours. In the lower levels, an extratropical cyclone will enhance
low-level moisture transport with strong northwesterly winds over
Central and Southern Chile. An increase in rainfall and mountain
snowfall totals and a larger extension further north is expected,
along with a marginal risk of severe weather. On the Argentinian
side, the saturated moisture plume and the eastern unstable flank
of the upper-level trough will reach the region late on Saturday.
This setup will favor maximum rainfall accumulations of 15-20 mm
and a slight risk of severe weather from western La Pampa to
southern Patagonia, expanding toward southern Cordoba and the Rio
de la Plata region by Sunday.


Sanchez-Perez...(WPC)
Castellanos...(WPC)