South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
206 PM EDT Fri Jun 26 2026

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

Forecast Bulletin 26 June 2026 at 1810 UTC:

Across the mid-latitudes and subtropics...

Starting on Saturday, expect an increase in precipitation chances
and daily rainfall accumulations for the regions of southern
Paraguay, the Mesopotamia region of Argentina, southern Brasil,
and northern Uruguay. A mid-to-upper level trough, which currently
has a positively tilted axis near 95W, will be moving into the
continent on Saturday morning and is expected to gain amplitude.
This upper trough will be completely east of the Andes and moving
into north-central Argentina by late Saturday evening and early
Sunday morning. The upper trough is also expected to trigger
frontogenesis and cyclogenesis in the central regions of the
continent. Prior to Sunday morning, a pre-existing surface frontal
boundary will still be meandering across south and southeastern
Brasil and Paraguay for Friday and Saturday and eventually be
influenced by the incoming upper trough. Starting on Sunday
morning, expect the advancement of a cold front across northern
Argentina, Paraguay, and southern Brasil. This cold front will
extend into south-central Bolivia by Monday morning.

Precipitable water values will increase significantly starting on
Saturday as well as the atmosphere reacts to the lowering of the
pressure values being influenced by the presence of the upper
trough. The South American Low Level Jet (SALLJ) will intensify
after Saturday afternoon. The aforementioned conditions will favor
an increase in total precipitation maxima, where enhanced rainfall
accumulations are likely, from Saturday morning through Sunday
morning across Paraguay, Argentina, south Brasil, and northern
Uruguay. Though, precipitation spatial coverage and intensity will
increase on Saturday evening. By Sunday, elevated total
precipitation maxima is anticipated across south Brasil and
southeast Paraguay. During this period, expect a risk for severe
weather as well as there will be ample moisture, vertical speed
shear, and upper divergence to support the development of deep
convection along the frontal boundary.

Another region of interest is southern and austral Chile where
rain and snow showers will be persistent during the weekend. For
the next three days, also expect a series of surface frontal
boundaries to be moving into the continent. The first is a
decaying occluded front that will be converging into Aysen and Los
Lagos. Precipitable water values will be decreasing as the system
decays and only light total precipitation maxima is expected in
the region for Friday. By Saturday, a warm front will be moving
into austral Chile and it will be accompanied by strong low level
winds (reaching 40 knots) and high precipitable water values
(reaching 20mm) that will support moderate total precipitation
maxima in the region. On Sunday, another frontal system will be
moving into Tierra de Fuego and it will be accompanied by above
normal precipitable water values and strong low level wind speeds
as well (reaching 40 knots), yielding light total precipitation
maxima in the region.

In the tropics...

For the next three days, expect the greatest chance for
precipitation to be centralized across the west and northwest
Amazon basin. A diffluent upper level pattern will dominate in the
region through the weekend, which will support the enhancement in
upper divergence. At the same time, expect a zonal wind flow
pattern from the east in the low to mid levels for the entire
forecast cycle. This will support the transport of moist tropical
air from the tropical Atlantic ocean into the interior of the
continent. The west and northwest Amazon basin will remain highly
unstable as well. Thus, expect daily moderate to enhanced total
precipitation maxima in this region for the next three days.

Meanwhile across the Atlantic coastline, a tropical wave will be
approaching the Guianas late Friday and into Saturday. This
feature will continue moving across the region for Sunday and it
will support the increase in total precipitation maxima with its
passage. Otherwise, isolated diurnal showers and thunderstorms
across the rest of the tropical region.



Tinoco-Morales...(WPC)