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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1521Z Jul 20, 2021)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1121 AM EDT TUE JUL 20 2021

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

FORECAST DISCUSSION FROM JUL 20/15UTC: CONSISTENT WITH PREVIOUS
GUIDANCE...A DEEP LAYER MID/UPPER LEVEL TROUGH IS INITIALIZED OVER
THE WESTERN ATLANTIC...WITH BASE OF THE DEEP TROUGH BOTTOMING OUT
OVER SAO PAULO/RIO DE JANEIRO. THROUGH THE DAY THIS IS TO WEAKEN
WHILE RAPIDLY SHEARING UNDER THE INFLUENCE OF A DEEPENING TROUGH
TO TO THE SOUTH. AT LOW LEVELS...THE TROUGH ALOFT SUSTAINS A FRONT
THAT TRAILS ACROSS THE SOUTH ATLANTIC TO ESPIRITO SANTO/MINAS
GERAIS...WITH A PREFRONTAL SHEAR LINE TO THE NORTH EXTENDING
ACROSS BAHIA TO NORTHEAST BRASIL. THE FRONT IS EXPECTED TO
GRADUALLY WEAKEN THROUGH THURSDAY EVENING...WHILE THE SHEAR LINE
DISSIPATES LATER ON FRIDAY. MEANWHILE...THESE WILL ENHANCE COASTAL
CONVERGENCE BETWEEN ESPIRITO SANTO AND EASTERN BAHIA...WITH
RAINFALL MAXIMA GRADUALLY INCREASING FROM 15-25MM EARLY IN THE
CYCLE TO 20-35MM/DAY ON WEDNESDAY TO FRIDAY. LOCALLY HIGHER
RAINFALL AMOUNTS ARE POSSIBLE...AS CONVECTIVE CELLS ALIGN ALONG
THE CONVERGENT ASYMPTOTE IN AN ECHO TRAINING PATTERN. THE RISK OF
ECHO TRAINING IS CONSIDERABLY HIGHER ACROSS EASTERN BAHIA AS THE
PREFRONTAL SHEAR LINE COMBINES WITH A LOW LEVEL JET.

A HIGHER AMPLITUDE TROUGH FOLLOWS...WITH LONG WAVE AXIS TO EXTEND
NORTH BETWEEN 110W-90W TO 20S LATER TODAY. IN RESPONSE TO THE LONG
WAVE TROUGH TO THE WEST...IN A SINUSOIDAL PATTERN...A LONG WAVE
RIDGE BUILDS TO THE EAST OVER CHILE/ARGENTINA EARLY ON WEDNESDAY.
AS THE TROUGH PRESSES AGAINST THIS RIDGE...IT WILL START TO SHEAR
ITS SHORT WAVE ENERGY ACROSS THE SOUTHERN REGIONS OF CHILE TO
PATAGONIA IN ARGENTINA. THROUGH FRIDAY THE TROUGH IS TO WEAKEN AND
THE RIDGE FLATTENS WHILE GRADUALLY RELOCATING TO THE WESTERN
ATLANTIC. AT LOW LEVELS...THIS WILL SUSTAIN A BROAD TROUGH OVER
THE SOUTHEAST PACIFIC LATER ON WEDNESDAY...WITH THIS AXIS DRAWING
A DEEP PLUME OF SUBTROPICAL MOISTURE ACROSS SOUTHERN CHILE. ON
WEDNESDAY AND THURSDAY THIS WILL RESULT IN MODERATE RAINFALL
AMOUNTS BETWEEN TEMUCO AND ISLA DE CHILOE IN SOUTHERN CHILE...WITH
RAINFALL MAXIMA OF 20-45MM/DAY. ON FRIDAY THE FRONTAL TROUGH
FINALLY PUSHES EAST ACROSS THE SOUTHERN CONE TO THE WESTERN
ATLANTIC...DRAWING THE FRONT NORTH ACROSS LA PAMPA TO CENTRAL
ARGENTINA/RIO DE LA PLATA BASIN. THE FRONT MOVES FARTHER NORTH ON
SATURDAY...WHEN IT WILL STRETCH ACROSS URUGUAY/SOUTHERN BRASIL TO
NORTHERN ARGENTINA. AS THE FRONT MOVES NORTH...IT WILL CLASH WITH
MOIST TROPICAL AIR ADVECTING FROM BRASIL ACROSS PARAGUAY TO
NORTHERN ARGENTINA. THIS WILL FEED ORGANIZED DEEP CONVECTION ON
SATURDAY WITH MAXIMA OF 25-50MM CLUSTERING BETWEEN ENTRE RIOS
ARGENTINA AND URUGUAY. UNDER THE INFLUENCE OF COUPLED
JETS...STRONG/POTENTIALLY SEVERE CONVECTION IS HIGHLY LIKELY.

AT 250 HPA...NORTH OVER THE CONTINENT...AN ILL ORGANIZED RIDGE
WILL CONFINE TO EQUATORIAL SOUTH AMERICA AS PROGRESSIVE POLAR
PERTURBATIONS STREAM TO THE SOUTH. OVER THE NEXT SEVERAL DAYS THE
MOST ACTIVE CONVECTION IS EXPECTED ACROSS EASTERN
ECUADOR-SOUTHWEST COLOMBIA TO NORTHERN JUNGLE OF PERU. IN THIS
AREA INITIALLY EXPECTING RAINFALL MAXIMA OF 25-50MM AS ENHANCED BY
A DEEP LAYER EASTERLY FLOW. ON WEDNESDAY THIS INCREASES TO MAXIMA
OF 30-60MM WITH MOST INTENSE BETWEEN ECUADOR AND SOUTHWEST
COLOMBIA AS ENHANCED BY A LOW LEVEL TROUGH. THIS WEAKENS ON
THURSDAY TO 20-35MM/DAY AS THE WINDS SHIFT TO THE SOUTHEAST AND
BECOME MORE PARALLEL TO THE ANDES.

DAVISON...WPC (USA)