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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1801Z Sep 05, 2018)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
201 PM EDT WED SEP 05 2018

GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/

MODEL DISCUSSION (VALID FROM SEP 05 AT 0000 UTC): THE GLOBAL
MODELS CONTINUE TO AGREE ON LONG WAVE PATTERN EVOLUTION THROUGH
DAY 06. THE DETERMINISTIC SOLUTION IS WELL SUPPORTED BY THE MEAN
OF THE GLOBAL ENSEMBLE MEMBERS...LEADING TO A HIGH CONFIDENCE IN
THE FORECAST.

ON THE NORTHERN STREAM FLOW...A SLOW TO EVOLVE TROUGH EXTENDS FROM
THE WESTERN ATLANTIC TO THE SOUTHEAST STATES OF BRASIL...WITH
MID/UPPER LEVEL TROUGH TO MEANDER OFF THE COAST OF BRASIL LATER ON
THURSDAY....TO MEANDER EAST ACROSS 20W LATER ON SATURDAY. AT LOW
LEVELS...THE TROUGH ALOFT SUSTAINS A SURFACE FRONT WITH AXIS
NORTHWEST ACROSS ESPIRITO SANTO-MINAS GERAIS IN BRASIL. PORTION
OVER THE CONTINENT IS FORECAST TO FRONTOLIZE LATER TODAY...WHILE
SEGMENT OVER THE ATLANTIC MEANDERS OVER THE NORTHEAST COAST OF
BRASIL. AS IT MEANDERS NORTH...AND UNDER THE INFLUENCE OF A RIDGE
TO THE SOUTH...A PREFRONTAL SHEAR LINE FORMS ACROSS BAHIA LATER ON
THURSDAY. THIS SHEAR LINE WILL ENHANCE COASTAL CONVERGENCE
STARTING LATER ON WEDNESDAY THROUGH FRIDAY. ON WEDNESDAY FRONTAL
CONVECTION ACROSS ESPIRITO SANTO AND BAHIA WILL SUSTAIN RAINFALL
MOUNTS OF 15-20MM. ON THURSDAY...AS THE 850 HPA WINDS INCREASE TO
20-25KT AND PWAT CONTENT PEAK AROUND 40MM...THIS IS TO SUSTAIN
SCATTERED CONVECTION WITH MAXIMA OF 20-35MM. ON FRIDAY-SATURDAY
EXPECTING ADDITIONAL RAINFALL AMOUNTS OF 15-25MM/DAY.

A DEEP SHORT WAVE TROUGH ON THE NORTHERN STREAM PULLS ACROSS
SOUTHERN CHILE TO PATAGONIA IN ARGENTINA LATER TODAY/EARLY ON
THURSDAY MORNING. THE DEEP MID LEVEL LOW CLEARS THE COAST OF
PATAGONIA LATER IN THE DAY. THROUGH SATURDAY THE LOW/TROUGH IS TO
MEANDER EAST ACROSS 40W. ALTHOUGH THE GLOBAL MODELS GENERALLY
AGREE ON THE TIMING OF THIS PERTURBATION... BOTH EUROPEAN MODELS
TEND TO FAVOR A DEEPER TROUGH THAN WHAT THE GFS SUGGESTS...WITH
THE ECMWF AND UKMET SHOWING ASSOCIATED LOW LASTING BEYOND 96/108
HRS. AS THE DEEP LOW/TROUGH MEANDERS EAST...THIS WILL DRIVE A
FRONT NORTH ACROSS PATAGONIA-LA PAMPA-CENTRAL CHILE LATER TODAY.
ON THURSDAY THE FRONT WILL MOVE NORTH ACROSS THE BUENOS AIRES
PROVINCE TO MENDOZA IN ARGENTINA...WHERE IT IS TO STALL. THE DEEP
LOW...AS IT STREAMS ACROSS THE SOUTHERN CONE...WILL SUSTAIN THE
ADVECTION OF COLD MOIST AIR ACROSS SOUTHERN PATAGONIA. THIS WILL
FEED CONVECTIVE DEVELOPMENT...WITH POTENTIAL MODERATE RAINFALL
AMOUNTS AND THUNDERSTORMS OVER SOUTHERN CHILE TO TRIGGER RAINFALL
MAXIMA OF 20-35MM. ON THURSDAY EXPECTING ADDITIONAL RAINFALL
AMOUNTS OF 05-10MM. ON THE SOUTHERN ANDES EXPECTING MODERATE
SNOWFALL AMOUNTS WITH ACCUMULATION OF 10-15CM...WHILE OVER
SOUTHERN PATAGONIA SCATTERED RAIN SHOWERS/THUNDERSTORMS ARE TO
SUSTAIN ACCUMULATION OF 20-30MM. AS THE FRONT STALLS OVER THE
BUENOS AIRES PROVINCE/LA PAMPA ON FRIDAY-SATURDAY...THIS IS TO
THEN ENHANCE MOISTURE CONVERGENCE OVER CENTRAL ARGENTINA TO
TRIGGER MAXIMA OF 15-20MM.

CONSISTENT WITH PREVIOUS FORECAST...GLOBAL MODELS CONTINUE TO
FORECAST THE NORTHWARD AMPLIFICATION OF A HIGH AMPLITUDE LONG WAVE
TROUGH BETWEEN 130W-85W TO 20S LATER ON SATURDAY. THE TROUGH IS TO
MEANDER EAST TOWARDS THE SOUTHERN REGIONS OF CHILE/DRAKE PASSAGE
LATER ON SUNDAY. MEANWHILE...AS THE TROUGH DEEPENS...THIS IS TO
INDUCE THE SOUTHWARD AMPLIFICATION OF A LONG WAVE RIDGE ACROSS
CHILE/ARGENTINA. THE BUILDING RIDGE IS TO THEN SUSTAIN A STRONG
SUBSIDENCE INVERSION. THIS EVOLUTION IS WELL SUPPORTED BY THE MEAN
OF THE GLOBAL ENSEMBLE MEMBERS.

NORTH OVER THE CONTINENT...AT 200 HPA...LOW AMPLITUDE RIDGE
CONFINES TO NORTHERN PARA-AMAZONAS IN BRASIL-PARAGUAY. SOUTH OF
THIS RIDGE...A LONG WAVE TROUGH IS STREAMING ACROSS THE SOUTHEAST
STATES OF BRASIL. AS THE TROUGH PULLS AWAY....THE RIDGE IS TO
BUILD SOUTH ACROSS PERU TO BOLIVIA...WITH AXIS TO GRADUALLY BECOME
WELL ENTRENCHED ACROSS NORTHERN SOUTH AMERICA DURING THE WEEKEND.
EARLY IN THE CYCLE THE LONG WAVE TROUGH WILL ENHANCE DIURNAL
CONVECTION ACROSS NORTHERN BRASIL WITH ACCUMULATION OF 05-10MM. ON
THURSDAY-FRIDAY THIS INCREASES TO 15-25MM...WITH MOST INTENSE OVER
NORTHERN PARA-ILHA DE MARAJO. THIS IS TO GRADUALLY BUILD WEST
ACROSS EQUATORIAL SOUTH AMERICA THROUGH SATURDAY. FURTHERMORE...AS
THE RIDGE BUILDS...IT WILL THEN STEER A SHORT WAVE TROUGH ACROSS
SOUTHERN PERU/NORTHERN CHILE TO BOLIVIA DURING THE WEEKEND. THIS
WILL LEAD TO A GRADUAL INCREASE IN DIURNAL CONVECTION ON THE
SOUTHERN SIERRA OF PERU AND THE ALTIPLANO OF BOLIVIA...WITH MAXIMA
OF 05-10MM ON SUNDAY.

SANCHEZ...SMN (ARGENTINA)
SALAZAR...SENAMHI (BOLIVIA)
DAVISON...WPC (USA)