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South American Forecast Discussion
 
(Latest Discussion - Issued 1522Z Jul 26, 2021)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1122 AM EDT MON JUL 26 2021

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

FORECAST DISCUSSION FROM JUL 26/16UTC: CONSISTENT WITH PREVIOUS
FORECASTS...A HIGH AMPLITUDE MID/UPPER LEVEL TROUGH EXTENDS NORTH
OVER THE WESTERN ATLANTIC TO MID SECTIONS OF SOUTH AMERICA...WHILE
TO THE WEST A BROAD RIDGE DOMINATES THE FLOW OVER THE EASTERN
PACIFIC. THE HIGHLY AMPLIFIED SINUSOIDAL PATTERN WILL LEAD TO A
SLOW EVOLUTION...WITH THE RIDGE TO SLOWLY RELOCATE TO THE SOUTHERN
CONE BY THE MIDDLE OF THE WEEK. YIELDING TO THE RIDGE...THE TROUGH
WILL THEN DRIFT TOWARDS THE CENTRAL ATLANTIC. MEANWHILE...THE
TROUGH WILL PRESS AGAINST A BROAD AND UNSEASONABLY STRONG CELL OF
THE SUBTROPICAL RIDGE OVER NORTHERN SOUTH AMERICA. THE ENSUING
GRADIENT WILL FAVOR A STRONG BELT OF LOWER LATITUDE WESTERLIES
ACROSS NORTHERN CHILE/ARGENTINA-SOUTHERN STATES OF BRASIL.
INITIALLY THIS WILL FAVOR AN UPPER DIVERGENT PATTERN THAT IS TO
VENT ORGANIZED DEEP CONVECTION ACROSS SOUTHERN AND SOUTHEAST
STATES OF BRASIL...AND IT WILL THEN CONTRIBUTE TO GENERATION OF
MOUNTAIN WAVE TURBULENCE ACROSS NORTHERN ARGENTINA/SOUTHERN
BOLIVIA AND PARAGUAY.

AT LOW LEVELS...A BUILDING POLAR RIDGE DOMINATES ARGENTINA WHILE
TO THE EAST...A BROAD/DEEP OCCLUDED FRONTAL TROUGH EXTENDS OVER
THE WESTERN ATLANTIC. THIS FAVORS A SOUTHERLY JET AND COLD
ADVECTIVE PATTERN ACROSS ARGENTINA. THIS...IN-TURN...DRIVES THE
SURFACE FRONT NORTH ACROSS SOUTHERN BRASIL-PARAGUAY TO SOUTHEAST
BOLIVIA THIS AFTERNOON...WITH THE BOUNDARY SURGING ACROSS SAO
PAULO-MATO GROSSO TO RONDONIA/NORTHERN BOLIVIA ON TUESDAY. AS THE
POLAR RIDGE EXPANDS ACROSS PARAGUAY TO BOLIVIA/SOUTHEAST BRASIL ON
WEDNESDAY...THIS DRIVES THE FRONT DEEPER INTO TROPICAL SOUTH
AMERICA...WHILE SIMULTANEOUSLY FAVORING THE GENERATION OF A
PREFRONTAL SHEAR LINE ACROSS AMAZONAS IN BRASIL. THE FRONTAL
TROUGH...AS IT MEANDERS OVER THE WESTERN ATLANTIC EARLY IN THE
CYCLE...IS TO TRIGGER SCATTERED CONVECTION ACROSS THE BUENOS AIRES
PROVINCE AND URUGUAY...TO SUSTAIN RAINFALL AMOUNTS OF 05-10MM/DAY
IN A MIX OF RAIN/SNOW SHOWERS. ACROSS SOUTHERN BRASIL TO PARAGUAY
THIS WILL RESULT IN RAINFALL MAXIMA OF 20-40MM...WITH SIMILAR
AMOUNTS ACROSS SAO PAULO ON TUESDAY. STARTING LATER
TODAY...PREFRONTAL CONVECTION IS TO CLUSTER ACROSS CENTRAL
BOLIVIA...TO SUSTAIN RAINFALL MAXIMA OF 15-20MM. ON TUESDAY THIS
INCREASES TO 20-40MM WHILE BUILDING NORTH AND WEST ACROSS SOUTHERN
PERU. ACTIVITY IS TO THEN BUILD NORTH ALONG THE LEE OF THE ANDES
ON WEDNESDAY-FRIDAY...WITH ACTIVITY SURGING ACROSS NORTHERN
PERU/EASTERN ECUADOR-SOUTHERN COLOMBIA.

AS THE MID LEVEL RIDGE RELOCATES FROM THE EASTERN PACIFIC TO THE
SOUTHERN CONE/WESTERN ATLANTIC ON WEDNESDAY...ANOTHER HIGHLY
AMPLIFIED MID/UPPER TROUGH ON THE SOUTHERN STREAM IS TO ENTER THE
SOUTHEAST PACIFIC. THE LONG WAVE AXIS IS TO THEN FOCUS DEEP LAYER
SUBTROPICAL MOISTURE ACROSS THE SOUTHERN REGIONS OF CHILE...WHILE
AT LOW LEVELS IT SUPPORTS A BROAD TROUGH AND AN ELONGATED FRONT.
THE FRONT REACHES TIERRA DEL FUEGO LATER ON WEDNESDAY...AND ON
THURSDAY IT SURGES ACROSS SOUTHERN CHILE TO PATAGONIA IN
ARGENTINA. BUT AS THE UPPER LEVEL TROUGH SHEARS UNDER THE RIDGE TO
THE EAST...LOSING UPPER LEVEL SUPPORT...THE FRONT IS EXPECTED TO
STALL OVER RIO DE LA PLATA BASIN/CENTRAL ARGENTINA ON LATER ON
FRIDAY TO SATURDAY. MEANWHILE...THIS WILL SUSTAIN SEVERAL DAYS OF
MODERATE TO HEAVY RAINFALL AMOUNTS OVER SOUTHERN CHILE. STARTING
ON WEDNESDAY EXPECTING ACCUMULATION OF 15-30MM...WHILE ON THURSDAY
IT SURGES TO 25-50MM AS IT BUILD NORTH ACROSS SOUTHERN CHILE TO
PUERTO MONTT. ON FRIDAY TO SATURDAY EXPECTING ADDITIONAL RAINFALL
AMOUNTS OF 05-10MM/DAY WITH ISOLATED MAXIMA OF 15MM...WHILE ACROSS
PATAGONIA TO LA PAMPA IN ARGENTINA FAIR WEATHER CONDITIONS WILL
PREVAIL.

NORTH OVER THE CONTINENT...AT 250 HPA...A BROAD AND UNSEASONABLY
STRONG CELL OF THE SUBTROPICAL RIDGE PREVAILS OVER THE CONTINENT
TO THE NORTH OF 15S. THE RIDGE ALOFT IS VENTING DIURNAL CONVECTION
ACROSS EQUATORIAL SOUTH AMERICA...WITH RAINFALL AMOUNTS OF
10-15MM/DAY AND MAXIMA OF 20MM EXPECTED OVER THE NEXT COUPLE OF
DAYS. UNDER THE INFLUENCE OF THE SURFACE FRONT SURGING ACROSS
BRASIL...THIS CONVECTION TENDS TO INTENSIFY/BECOME BETTER
ORGANIZED ON THURSDAY-FRIDAY...WITH MAXIMA OF 15-30MM EXPECTED
ACROSS PARA AND AMAPA...WHILE TO THE NORTHWEST HIGHER AMOUNTS ARE
TO FALL ACROSS AMAZONAS...NORTHERN PERU/EASTERN ECUADOR AND
SOUTHERN COLOMBIA.

DAVISON...WPC (USA)