South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
207 PM EDT Wed Jul 15 2026

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

Forecast Bulletin 15 July 2026 at 1810 UTC:

In the mid-latitudes...
An upper level trough is currently moving eastward and will
continue to favor upper divergence across central Chile for the
next three days. This upper divergence is being reinforced by the
arrival of an eastward propagating jet streak max that will be
perpendicular to the orography of northern and central Chile. This
dynamic upper level regime will support the sustenance of surface
frontal boundaries that will be a driving force for the initiation
of precipitation across the country. On Wednesday, expect the
arrival of another frontal boundary into the region for the
afternoon and an increase in low level north-northwesterly
windspeeds. There will also be an increase in precipitable water
during this period, leading to enhanced moisture convergence
across the region. Thereafter, the accompanying cold front will be
converging into central Chile while its northernmost extension is
anticipated to continue propagating across Coquimbo and Atacama
after Wednesday.

With respect to precipitation impacts, expect elevated total
precipitation maxima across south-central Chile for Wednesday and
Thursday. Meanwhile across north-central Chile, precipitation
impacts will increase on Friday. Note that precipitable water
values will decrease as the atmospheric river loses intensity with
its northward propagation. Regardless, expect above normal
precipitation accumulations across Coquimbo, Atacama, and
Valparaiso for the rest of the forecast cycle.

The upper level jet streak max will begin moving into the
continent on Thursday and is expected to move east of the northern
Chilean/Argentinian Andes by Thursday evening. Prior to the
arrival of strong upper level windspeeds, expect the development
of a shortwave mid-to-upper level trough across north-central
Argentina. This feature will support the development of a frontal
boundary that will be gaining definition during the day on Friday
across north-central Argentina and will extend into Uruguay and
northern-Argentina by Saturday morning. At the same time, a
low-to-mid level ridge will be situated across southeast Brasil,
which will support the enhancement of northerly low level winds
into the mid-latitudes of South America. This pattern will
reinforce the strength of the South America Low Level Jet (SALLJ).
The SALLJ will help transport moist air into the region, and with
the development of the upper trough, also expect an increase in
moisture pooling. With respect to precipitation, expect an
increase in precipitation coverage starting on Thursday. For
Friday, upper diffluence will continue to dominate in the region
and will continue to support the development of deep convection
across Uruguay and southern Brasil. Severe thunderstorms are
likely for Thursday and Friday with moderate daily total
precipitation maxima.

In the tropics...
Seasonal conditions will persist for the next three days.
Anticipate the westward propagation of tropical waves across the
extreme north regions of South America, primarily influencing the
precipitation regime in Venezuela. A low to mid level ridging
pattern will favor the enhanced easterly low level winds across
the northern half of the Amazon and an anticyclonic turn in the
wind flow across the western Amazon. This will limit the transport
of moisture into the higher terrain in Colombia, Ecuador, and
Peru. Slow low level wind speeds will be present across Colombia,
Ecuador, and Peru and will yield the development of isolated
showers and thunderstorms across the basin. Otherwise, expect the
development of diurnal showers and thunderstorms.


Tinoco-Morales...(WPC)