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FXUS20 KWBC 031758
PMDSA

South America Forecast Discussion - International Desks
NWS Weather Prediction Center College Park MD
158 PM EDT Fri Oct 03 2025

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

South American Forecast Bulletin from  03 October 2025 at 1805 UTC

In the mid - latitudes...

In the upper levels, the axis of a positively tilted upper trough
will be moving over Chile on Friday and should be located east of
the Andes after Sunday morning. As the upper trough migrates
eastward into Argentina, expect strong upper level winds to favor
the sustenance of a potent upper jet streak. The interaction
between this trough and the subtropical upper ridge will continue
to intensify a pre-existing jet streak max and will favor the
maintenance of a diffluent upper level wind flow pattern to the
east across central South America from Friday afternoon through
the rest of the forecast cycle. These upper level features will
favor upper level divergence across the region.

In Chile, the aforementioned upper trough and affiliated upper jet
streak max will continue to promote upper divergence across
central Chile through Saturday. During this period, there will
also be a weakening moisture plume that will quickly move
northward and see a significant reduction in precipitable water.
Thus, expect the greatest precipitation impacts to occur from
Friday through Saturday. In the mid - levels, a trough axis will
also be moving into the continent and will be accompanied by
numerous shortwave troughs that contain vorticity maxima. The
additional vertical ascent may favor the development for
precipitation across central Chile. From Friday morning through
Saturday morning, expect a total precipitation maxima of 20 -
40mm. The area of greatest precipitation impact will shift
northward, with a decreasing trend in total precipitation maxima
over passing days. During this period, mountain snow is highly
likely.

Another region of interest is north - central Argentina, starting
late Saturday evening, a cold front will begin to develop across
Cuyo and La Pampa - Buenos Aires provinces. On Sunday evening, the
cold front will be located across northern Argentina, Paraguay,
and southern Brasil. Note that another cold front is trailing
behind, but the leading front will have the greatest precipitation
impact. At the low levels, there will be an increase in moist
southerly winds into the region, which is potentially attributed
to the aforementioned upper trough lowering surface pressures
across the region. Expect above normal precipitable water along
and ahead of this progressive cold front. Upper divergence,
vertical ascent associated with mid - level vorticity and the
progressive surface cold front, and increasing low level moisture
will assist in increasing total precipitation maxima and support
the development of deep convection. There will be a northward
shift in the region of greatest precipitation impact with passing
days, following the progression of the cold front. From Saturday
morning through Sunday morning, the area of greatest precipitation
impact will be La Pampa - Buenos Aires province, where a
generalized total precipitation maxima of 30 - 60mm is possible
and a slight risk for severe weather is likely. Note that a low
level jet will be present in the region and may help intensify
nocturnal convection across this region. After Sunday morning, the
region of greatest precipitation impact will be Uruguay, southern
Brasil, southeast Paraguay, and northern Argentina, where a total
precipitation maxima of 40 - 80mm is possible and a moderate risk
for severe weather is possible.

In the subtropics...

In the upper levels, two upper ridges, one in the west and the
other in the east regions of the basin, will continue to dominate
across the tropical basin. This will favor increasing subsidence,
and in turn also promote a stabilizing trend across the interior
of the basin. Diurnal tropical convection is still possible,
however, deep convection will be limited to regions where speed
and directional shear is minimal. In the west portion of the
basin, expect increasing mid - level winds, which may prohibit the
development of deep convection. Thus, areas of greatest
precipitation impact will be where surface to low level troughs
will be moving across. These low level troughs will favor a subtle
increase in moisture and moisture convergence. A generalized total
precipitation maxima surpassing 35mm is possible with these
features.

In Colombia, expect a moist environment for the next three days.
There will be periods of enhanced westerlies into the coast, which
will promote moisture convergence. Total precipitation maxima will
exceed 40mm for the next three days in this region. Some mid -
level shear may prohibit the sustenance of deep convection, but
the diurnal cycle, local effects, and orographic enhancement may
help produce higher total precipitation maxima in this sector.

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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