991
FXUS20 KWBC 171819
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
218 PM EDT Fri Jul 17 2026

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

Forecast Bulletin 17 July 2026 at 1825 UTC:

An active upper level pattern will continue to support the
development and sustenance of surface frontal boundaries across
Chile. The first frontal boundary will be moving into the
continent today during the day. Expect strong low level
north-northwesterly wind speeds across central Chile and
precipitable water values reaching 25mm. This cold front will move
into the continent and lose its structure across south-central
Chile. However, its northernmost extension will continue to
meander near Coquimbo for Saturday. There will still be above
normal precipitable water values in this region through Saturday,
but by the evening, expect the frontal boundary and moisture plume
to weaken. Another surface frontal system will begin to influence
the precipitation regime and low level pattern across
north-central Chile starting late Saturday night. There will be an
increase in moisture transport into the region and overall
moisture convergence. Thus, expect an uptick in precipitation
activity on Sunday across Valparaiso, Coquimbo, and the
southernmost regions of Atacama. The mid-to-upper level regime
will continue to favor enhanced upper divergence.

With respect to precipitation, expect the highest total
precipitation maxima for Friday with the majority of the
precipitation activity occurring during the day. As the frontal
boundary moves into the continent, activity is expected to
diminish into the evening hours across south-central Chile.
Thereafter, expect lingering showers across central Chile for
Saturday. There will be a resurgence in precipitation activity
starting on Sunday across Coquimbo, Atacama, and Valparaiso and it
will persist through Monday morning.

Meanwhile across Rio de la Plata, the presence of the potent jet
streak max across the Pacific will begin to project east of the
Andes and will support development of mid-to-upper level troughs
and upper diffluence across the region for the next three days.
Expect the development of weak frontal boundaries and low level
troughs in the region. Currently, a frontal boundary is developing
across northern Argentina and will be extending into Rio de la
Plata. At the same time, expect the South America Low Level Jet
(SALLJ) to be present in the region. The SALLJ is being reinforced
by the presence of a low to mid level ridge that is favoring the
increase in northwesterly low level winds into the region. Thus,
expect the transport of moist air into Rio de la Plata.
Furthermore, moisture pooling will be prominent along the frontal
system, leading to high precipitable water. The dynamic
mid-to-upper level regime will favor the development for deep
convection across the region for the next three days as there will
be minimal change in the atmospheric conditions. There will also
be a risk for severe weather and hail in the region through the
weekend.

Another region of interest is the tropics. For the next three
days, there will be a series of low level troughs and tropical
waves moving across the region. Though, the highest precipitation
accumulations relating to the tropical waves will stay confined to
the far northern extremes of the South American domain. A tropical
wave of interest is currently moving across Venezuela and will
continue to move westward into the Andes on Saturday. By Sunday,
the tropical wave will be exiting the South American continent. At
this time, there will be a reactivation of the Panamanian low
level trough. This will lead to the increase in moisture and
onshore flow into the Pacific coasts of Colombia. Note that an
upper level trough across northeast Brasil will begin to interact
with the southern periphery of a ridge in the central Atlantic
that will support the increase in upper level winds across the
Amazon Delta. There will be a subtle increase in moisture and
instability in this region during the weekend as well, leading to
isolated thunderstorms. Otherwise, expect relatively calm and
seasonal conditions across the rest of the tropical basin.


Tinoco-Morales...(WPC)





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