South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
118 PM EDT Thu Mar 20 2025

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

South American Forecast Bulletin from 20 March 2025 at 17 UTC

The analysis of large scale conditions shows a Tropospheric Kelvin
wave propagating across the central Pacific. This wave will likely
enhance tropical convection during early and mid-next week.

In western Tropical South America, the subtropical upper ridge
(Bolivian High) centers over southern Peru, where it is forecast
to persist through the forecast cycle. This will continue to
ventilate convection in areas located to the north of the ridge,
in combination with seasonally high values of precipitable water
or in the 55-60mm range present in the central Amazon Basin. The
areas prone to the heaviest precipitation in the next three days
will cluster in the western Amazon, particularly in southern
Colombia and in the oriente Ecuatoriano, where southeasterly
low-level trades will stimulate low-level moisture convergence.
This will trigger maxima in the 25-70 mm/Day range on a daily
bases in areas east of the Andes. In the Pacific basin, heavy
convection is expected to continue in coastal Ecuador given warm
SSTs, precipitable water exceeding 60mm and weak onshore flow.
Expect maxima of 40-80mm in the Gulf of Guayaquil region on
Thursday. This decreases to 20-35 on Friday, and increases to
maxima of 35-70mm on Saturday. Elsewhere, expect seasonal
convection in the northern Altiplano and Peruvian Andes where
generally conservative daily totals are expected.

In eastern Tropical South America, seasonally moderate amounts are
expected on Thursday and Friday in inland portions of the Amazon
Delta, in Para and Maranhao. This in response to active Atlantic
ITCZ convection and an easterly wave-like perturbation propagating
across the region. Expect maxima of 35-70mm on Thursday,
decreasing to maxima of 25-50mm on Friday, decreasing thereafter.
In coastal areas expect generally maxima of 20-40mm on a daily
basis.

In southeast Brasil, a weak front and prefrontal trough structure
are highlighting moisture convergence in combination with an upper
trough. This is favoring seasonal accumulation. On Thursday expect
maxima of 15-30mm in Espirito Santo and maxima of 15-20mm in areas
south and west. On Friday expect maxima of 20-35mm from Minas
Gerais into Goias/Tocantins. on Saturday expect maxima of 20-45mm
in Minas Gerais, eastern Rio de Janeiro and southern Espirito
Santo.

Generally dry conditions are present in extratropical South
America with exception of a waning moist plume located in Austral
Chile. However, the arrival of several short wave mid-upper
troughs from the Pacific will continue to lower the
pressures/geopotentials in Argentina favoring moisture return east
of the Andes. This will sustain an increasing trend in
accumulation and isolated areas with the risk of severe
convection. The risk for accumulation increases on Friday when
expect potential maxima of 30 - 60mm in the Chaco, including a
risk for MCS formation and a marginal risk for severity.
Convection along a surface front in San Luis-La Pampa-southern
Buenos Aires Province will favor maxima of 15 - 25mm including a
slight risk for severity, especially in the Buenos Aires province.
on Saturday, expect the development of strong thunderstorms with a
slight risk of severity in central Paraguay and Formosa, where
expect maxima of 40-80mm. Similar conditions are expected in
northern Cuyo. In northern Paragonia, widespread easterly flow
will sustain an unusually widespread wet spells. Expect amounts in
the 05-10mm/day Range and isolated maxima of 15-25mm.

For associated QPF graphical information:
https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml

Galvez...(WPC)
Alamo...(WPC)