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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1502Z Jul 19, 2021)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1102 AM EDT MON JUL 19 2021

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

FORECAST DISCUSSION FROM JUL 19/15UTC: A DEEP LAYER MID/UPPER
LEVEL TROUGH LIES OVER THE WESTERN ATLANTIC WHILE CENTERING ON A
CLOSED LOW TO THE EAST-SOUTHEAST OF URUGUAY. AT LOW LEVELS...THE
TROUGH SUSTAINS AN ELONGATED FRONT THAT ORIGINATES ON A DEEP
OCCLUDED LOW OVER THE WESTERN SOUTH ATLANTIC...WITH BOUNDARY
EXTENDING TO THE WEST-NORTHWEST ACROSS ESPIRITO SANTO/MINAS GERAIS
TO AMAZONAS IN WESTERN BRASIL/CENTRAL PERU. THE MID/UPPER LEVEL
LOW IS TO INITIALLY MEANDER EAST...THEN ACCELERATE EARLY ON
TUESDAY AS A HIGHER AMPLITUDE TROUGH AMPLIFIES TO THE SOUTH.
LOSING ITS UPPER LEVEL SUPPORT...THE SURFACE FRONT IS FORECAST TO
FRONTOLIZE OVER BRASIL EARLY ON WEDNESDAY MORNING...WHILE
REMAINING PORTION OVER THE WESTERN ATLANTIC GRADUALLY DECAYS TO A
FRONTAL SHEAR LINE. MEANWHILE...THE WANING FRONT/SHEAR LINE WILL
ENHANCE MOISTURE CONVERGENCE ALONG THE COAST BETWEEN BAHIA AND
ESPIRITO SANTO. ON TUESDAY TO FRIDAY THIS WILL RESULT IN MODERATE
RAINFALL AMOUNTS WITH ACCUMULATION OF 15-25MM/DAY...WITH LOCALLY
HIGHER RAINFALL AMOUNTS POSSIBLE AS CONVECTIVE CELLS POTENTIALLY
TRAIN ALONG THE FRONTAL/PREFRONTAL SHEAR LINE. THE RISK OF ECHO
TRAINING IS CONSIDERABLY HIGHER ACROSS EASTERN BAHIA AS THE
PREFRONTAL SHEAR LINE COMBINES WITH A LOW LEVEL JET.

UPSTREAM OVER THE EASTERN PACIFIC...A PROGRESSIVE SHORT WAVE
TROUGH ON THE SOUTHERN STREAM RACES ACROSS THE DRAKE
PASSAGE/SOUTHERN CONE TO THE WESTERN ATLANTIC EARLY IN THE CYCLE.
THROUGH WEDNESDAY MORNING THIS EVOLVES INTO A BROAD TROUGH OVER
THE SOUTH ATLANTIC...WITH AXIS ENVELOPING AREA BETWEEN 60W-20W AND
TO THE SOUTH OF 30S. ON WEDNESDAY TO THURSDAY...UNDER THE
INFLUENCE OF A MID/UPPER LEVEL RIDGE BUILDING SOUTH OVER THE
CONTINENT...THE ENSUING SINUSOIDAL WAVE PATTERN WILL LEAD TO THE
NORTHWARD AMPLIFICATION OF THE TROUGH ALONG 30W TO 20S. AT LOW
LEVELS...THIS WILL SUSTAIN A PROGRESSIVE FRONT THAT RACES ACROSS
SOUTHERN CHILE TO THE WESTERN ATLANTIC EARLY IN THE CYCLE...THEN
WEAKEN AS IT MEANDERS NORTH ACROSS LA PAMPA TO THE BUENOS AIRES
PROVINCE ON TUESDAY. MOST ACTIVE CONVECTION WITH THIS
FRONT/FRONTAL TROUGH IS EXPECTED OVER CHILE BETWEEN TEMUCO AND
ISLA DE CHILOE...WHERE IT WILL SUSTAIN MODERATE RAINFALL AMOUNTS
WITH MAXIMA OF 15-30MM.

THIS WILL BE FOLLOWED BY A HIGHER AMPLITUDE TROUGH WITH AXIS NORTH
ALONG 100W EARLY ON WEDNESDAY MORNING. UNDER THE INFLUENCE OF THIS
TROUGH...A LONG WAVE RIDGE BUILDS TO THE EAST OVER THE CONTINENT.
AS THEY COLLIDE...THE TROUGH SLOW DOWN AS IT NEARS THE SOUTHERN
CONE ON THURSDAY. THE TROUGH IS TO THEN FOCUS ITS SHORT WAVE
ENERGY ACROSS THE SOUTHERN REGIONS OF CHILE TO PATAGONIA IN
ARGENTINA. FURTHERMORE...THE LONG WAVE TROUGH IS TO ESTABLISH A
MOIST ADVECTIVE PATTERN AS IT NEARS THE SOUTHERN CONE...WITH
DEEPER PLUME OF SUBTROPICAL MOISTURE ADVECTING ACROSS THE EASTERN
SOUTH PACIFIC. THIS WILL HELP SUSTAIN SEVERAL DAYS OF ORGANIZED
CONVECTION...STARTING ON WEDNESDAY WHEN THE BROAD POLAR TROUGH
NEARS THE SOUTHERN CONE...WITH MAXIMA OF 20-45MM. THROUGH FRIDAY
THIS WILL RESULT IN ADDITIONAL RAINFALL AMOUNTS OF
20-40MM/DAY...WITH STORM TOTAL AMOUNTS OF 60-120MM. ADDING TO THE
STORM TOTALS...ANOTHER TROUGH IS TO MAKE LANDFALL LATER IN THE
WEEK...TO ALSO TRIGGER MODERATE RAINFALL AMOUNTS AS IT ENTERS THE
SOUTHERN CONE.

AT 200 HPA...A MORE SEASONAL PATTERN IS EXPECTED ACROSS NORTHERN
SOUTH AMERICA...WITH AN ILL ORGANIZED RIDGE TO CONFINE TO
EQUATORIAL SOUTH AMERICA AS PROGRESSIVE POLAR PERTURBATIONS STREAM
TO THE SOUTH. OVER THE NEXT FEW DAYS...MOST ACTIVE CONVECTION IS
EXPECTED ACROSS EASTERN ECUADOR-SOUTHWEST COLOMBIA TO NORTHERN
JUNGLE OF PERU. IN THIS AREA INITIALLY EXPECTING RAINFALL MAXIMA
OF 30-60MM AS ENHANCED BY A DEEP LAYER EASTERLY FLOW. ON TUESDAY
EXPECTING ADDITIONAL RAINFALL AMOUNTS OF 25-50MM...SURGING AGAIN
TO 30-60MM ON WEDNESDAY UNDER INFLUENCE OF A LOW LEVEL TROUGH. ON
THURSDAY THIS WEAKENS TO 20-30MM AS THE WINDS SHIFT TO THE
SOUTHEAST AND BECOME MORE PARALLEL TO THE ANDES.

DAVISON...WPC (USA)