South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
254 PM EDT Fri May 15 2026

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

Forecast Bulletin 15 May 2026 at 1900 UTC:

On Friday, an upper level ridge is centered over the Brasilian
Nordeste and the ridge axis extends into the western Amazon Basin,
weakening throughout the day. In the lower levels, a tropical wave
propagates over Venezuela, where expect moderate to locally heavy
precipitation totals on Friday. To the east, drier air is advected
from the Atlantic Ocean into the northeastern sector of South
America, favoring a decrease in total precipitation on Friday.
This pattern will be reinforced for the weekend due to the
convergent phase of the MJO over the forecast region. Embedded in
the drying air mass from the Atlantic, is a tropical wave that is
expected to the north of Maranhao-Brasil by the evening hours of
Friday, but the precipitation impacts of this wave are expected to
remain light to moderate for much of the Brasilian North Coast. On
Saturday, the decrease in precipitation continues over much of the
northern sectors of South America, with the decrease in available
moisture entering from the east. Expect one tropical wave to
propagate from Venezuela into Colombia with moderate precipitation
totals, while a second wave enters the Amazon Delta by the evening
hours, bringing moderate precipitation totals to the region. By
Sunday, the tropical wave has entered the Guianas to favor light
to moderate precipitation totals for the region, while the other
wave impacts the Andean regions of Colombia and Ecuador, favoring
moderate to heavy precipitation to the regions in the foothills of
the mountains.

In the central and southern portions of the continent, upper air
trough axes extend west of Peru, another over northern Chile, and
a third upper level trough entering the Sur region of Chile in the
afternoon hours of Friday. By early Saturday morning the trough
axis over northern Chile enters the Chaco region while the upper
trough over Sur Chile enters the Patagonia, enhancing the
Northwest Argentina Low in the low levels, and favoring the
development of the South American Low Level Jet (SALLJ) over
southern Bolivia. To the south, a frontal boundary is entering the
Patagonia region, while the warm, moist air enters northern
Argentina. These air mass differences will enhance instability and
produce troughing in the lower levels that could generate light
precipitation totals on Friday. By Saturday, the upper level
troughing over Bolivia interacts with the upper level ridge over
east Brasil, enhancing the upper jet stream in the central portion
of the continent, fueling the development of a frontal boundary
that is expected from south Bolivia, along the Paraguay/Brasil
border, and exciting by Rio Grande do Sul into the South Atlantic
Ocean. This frontal boundary will be interacting with the moist,
warm tropical air transported by the SALLJ, while in the mid to
upper levels, expect the passing of short wave troughs that will
enhance the instability in the region and support heavy
precipitation from east Paraguay through Mato Grosso do Sul and
Parana with a slight chance of severe weather. Similar conditions
are expected on Sunday as the upper jet over the central region of
the continent assists with the reorganization of the frontal
boundary and supports the advection of colder air masses in the
lower levels into south-central Bolivia by Sunday evening. Mato
Grosso do Sul and Parana can expect heavy precipitation on Sunday.

Castellanos...(WPC)