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FXUS20 KWBC 181713
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
1212 PM EST Tue Nov 18 2025

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

South American Forecast Bulletin for 18 November 2025 at 1715 UTC

A mid-to-upper level shortwave trough will be moving across
southeastern Brasil on Tuesday and will be accompanied by a
frontal system at the surface. The cold front will be located
across Sao Paulo, Rio de Janeiro, and Minas Gerais Tuesday
evening. By Wednesday evening, this front will extend across Bahia
and will transition into a stationary front thereafter. The
presence of this frontal boundary in the region will help promote
low level moisture convergence and assist in the increase in
precipitation chances.

By Wednesday, the base of the aforementioned upper level shortwave
trough will extend across the area and will help increase upper
divergence across Espirito Santo and Bahia. The axis of this upper
trough will move into the Atlantic on Wednesday afternoon and
upper level support should diminish in the region. In the
mid-levels, troughing will persist through the forecast cycle and
will move northward with passing days. This mid-level troughing
will provide additional vertical ascent.

With respect to precipitation, on Tuesday, the area of greatest
impact will be Santa Catarina, Parana, and Sao Paulo. There will
also be a risk for severe weather due to the presence of the
aforementioned mid-level shortwave trough that will be cooling the
mid-level temperatures and increasing instability. A surface low
level pressure system will be located in this region, providing
surface convergence that will support the initiation of
convection. Precipitable water values will also be above normal.
The aforementioned conditions will favor a total precipitation
maxima of 20 - 40mm in Santa Catarina and Parana and a maxima of
20 - 35mm in Rio de Janeiro from Tuesday morning through Wednesday
morning. From Wednesday morning through Friday morning, the area
of greatest precipitation impact will extend from the Interior
Nordeste region of Brasil and Bahia and will be primarily
influenced by the presence of the aforementioned frontal system
and mid-level troughing.

Across the mid-latitudes, an upper level shortwave trough will
begin to move into the continent starting Wednesday and will be
east of the Andes by Thursday afternoon, when it will also have a
positively tilted axis. An upper level diffluent wind pattern will
develop over northern Argentina and Uruguay on Thursday afternoon,
positively contributing to upper level divergence. During this
period, expect frontogenesis across north-central Argentina and
Uruguay. Low level wind convergence will be enhanced along and
ahead of the cold front, which will also assist in increasing
precipitable water. These conditions will favor an increase in
precipitation chances starting Thursday afternoon and continuing
into Friday. Also note that the dynamic mid-to-upper levels will
support the development for deep convection.

Another region of interest in the mid-latitudes is the southern
half of Chile. Seasonal conditions will continue on Tuesday and
Thursday through most of the region. On Wednesday, a cold front
that is accompanied by a weak moisture tongue will begin to move
into the continent. This will increase moisture convergence across
southern Chile. A total precipitation maxima of up to 20mm is
possible in this region. Note that this cold front is being
supported by the aforementioned mid-to-upper level trough that
will be moving into the continent starting Wednesday and moving
east of the Andes by Thursday.

In tropical South America, mid-to-upper level ridging will prevail
through the Amazon Basin for the next three days. This pattern
will enable an increase in southerly low level winds across the
eastern regions of the Amazon Basin. The combination of increasing
low level moisture convergence, orographic enhancement, and the
diurnal cycle will favor higher total precipitation maxima across
this region. Elsewhere, a low level trough will be propagating
across the Guianas and Venezuela on Tuesday and Wednesday and will
favor daily total precipitation maxima of 15 - 30mm. Lastly, low
level westerlies will continue to affect the Pacific coasts of
Ecuador and Colombia through the forecast cycle. The low level
wind speeds will be weak, within the range of 5 to 10 knots, but
the moisture content will be plentiful. This will lead to higher
total precipitation maxima for the forecast cycle.

For the latest available charts please visit:
https://www.wpc.ncep.noaa.gov/international/sam_day1-3.shtml

Tinoco-Morales...(WPC)


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