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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1553Z Jul 15, 2021)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1153 AM EDT THU JUL 15 2021

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

LAST NO AMENDS...NEXT BULLETIN ON MONDAY JULY 19

FORECAST DISCUSSION FROM JUL 15/16UTC: AT 500 HPA...A SHORT WAVE
TROUGH ON THE NORTHERN STREAM IS TO SLOWLY PULL ACROSS MID
SECTIONS OF SOUTH AMERICA EARLY IN THE CYCLE...WITH A SUBTROPICAL
AND A NORTHERN POLAR JETS TO BOUND THE NORTHERN FRINGES OF THIS
TROUGH AS IT MOVES ACROSS ARGENTINA TO BRASIL/RIO DE LA PLATA
BASIN. THE JET ALOFT WILL FAVOR TWO DISTINCT AREAS OF UPPER
DIVERGENCE...WITH ONE ENVELOPING THE SOUTHERN STATES OF
BRASIL-MISIONES IN ARGENTINA AND SOUTHEAST PARAGUAY...WHILE THE
OTHER IS TO EXTEND FARTHER NORTH ACROSS THE PERUVIAN JUNGLE TO
CENTRAL BOLIVIA. AT LOW LEVELS...THE TROUGH ALOFT SUSTAINS A
DEEPENING/RAPIDLY OCCLUDING LOW TO THE EAST OF URUGUAY. THIS
DRIVES A COLD FRONT NORTH ACROSS SOUTHERN BRASIL TO SAO
PAUL-NORTHERN BOLIVIA LATER THIS AFTERNOON...WHERE THE FRONT IS
EXPECTED TO STALL ON FRIDAY THROUGH SATURDAY. UNDER THE INFLUENCE
OF THIS BOUNDARY...AND FAVORABLE UPPER LEVEL JET
DYNAMICS...SCATTERED DEEP CONVECTION WILL BUILD FROM SOUTHERN SAO
PAULO/PARANA TO MATO GROSSO DO SUL AND SOUTHEAST PARAGUAY AS WELL
AS ACROSS CENTRAL BOLIVIA...WITH RAINFALL MAXIMA OF 20-35MM. ON
FRIDAY ACTIVITY ACROSS SAO PAULO-MATO GROSSO DO SUL SURGES TO
30-60MM...WHILE THE CONVECTION OVER BOLIVIA WEAKENS AS IT BUILDS
NORTH AND WEST ACROSS THE SOUTHERN TO CENTRAL JUNGLE OF
PERU...WHERE IT IS TO SUSTAIN MAXIMA OF 05-10MM.

SIMULTANEOUSLY...A MID/UPPER LEVEL TROUGH ON THE SOUTHERN STREAM
FLOW AMPLIFIES ACROSS THE SOUTHEAST PACIFIC TO THE SOUTHERN CONE
LATER TODAY. ON FRIDAY THE TROUGH IS FORECAST TO AMPLIFY FARTHER
NORTH ACROSS CENTRAL TO NORTHERN ARGENTINA...WITH AXIS REACHING
MAXIMUM AMPLITUDE OVER THE SOUTHERN STATES OF BRASIL EARLY ON
SATURDAY MORNING. UNDER FAVORABLE JET DYNAMICS THIS IS TO THEN
INTENSIFY...WITH A DEEPENING MID LEVEL LOW TO FORM ALONG THE BASE
OF TROUGH AS IT MEANDERS OFF THE COAST OF URUGUAY LATER IN THE
DAY. THIS IS TO THEN MEANDER EAST WHILE EXTENDING A SHORT WAVE
TROUGH TO SOUTHEAST BRASIL. AT LOW LEVELS THIS WILL SUSTAIN A
PROGRESSIVE FRONT THAT RACES ACROSS SOUTHERN CHILE TO PATAGONIA IN
ARGENTINA THIS AFTERNOON...WITH THE BOUNDARY TRIGGERING MODERATE
CONVECTION ACROSS THE SOUTHERN REGIONS OF CHILE WITH MAXIMA OF
20-35MM. ON FRIDAY THE FRONT SURGES ACROSS THE BUENOS AIRES
PROVINCE-MENDOZA...MOVING FARTHER NORTH ON SATURDAY AS THE
ASSOCIATED FRONTAL LOW INTENSIFIES OVER THE WESTERN ATLANTIC AND A
POLAR RIDGE BUILDS ACROSS ARGENTINA. IN A COLD ADVECTIVE PATTERN
THIS IS LIKELY TO TRIGGER RAIN/SNOW SHOWERS ACROSS THE SIERRA DE
LA VENTANA AND TANDIL IN SOUTHERN BUENOS AIRES PROVINCE...WITH
ACCUMULATION OF 05-10MM DURING THE WEEKEND. ON SUNDAY...DRIVEN BY
A SOUTHERLY LOW LEVEL JET...THE FRONT SURGES ACROSS SOUTHERN TO
SOUTHEAST BRASIL...WHERE IT IS TO REINFORCE THE AFOREMENTIONED
FRONT TO THE NORTH. AS THEY INTERACT...A PREFRONTAL SHEAR LINE IS
TO DEVELOP ACROSS NORTHWEST SOUTH AMERICA. OVER ACRE AND AMAZONAS
IN BRASIL TO THE CENTRAL/NORTHERN JUNGLE OF PERU THIS WILL SUSTAIN
MODERATE RAINFALL AMOUNTS WITH MAXIMA OF 20-35MM. HIGHER AMOUNTS
ARE EXPECTED ALONG THE LEE OF THE ANDES BETWEEN SOUTHWEST COLOMBIA
AND EASTERN ECUADOR...WITH MAXIMA OF 20-60MM ON SUNDAY AND 25-50MM
ON MONDAY...FOR STORM TOTAL AMOUNTS OF 50-100MM. NOTE THAT DUE TO
THE POSSIBILITY OF ECHO TRAINING CONDITIONS...LOCALLY HIGHER
RAINFALL AMOUNTS ARE POSSIBLE IN THIS AREA.

A HIGHER AMPLITUDE TROUGH IS TO FOLLOW...WITH THE MID LEVEL AXIS
RACING ACROSS THE EASTERN PACIFIC WHILE PRESSING AGAINST A RIDGE
TO THE EAST. INITIALLY...AS THE TROUGH AND THE RIDGE
COLLIDE...THIS WILL STEER THE BULK OF THE SHORT WAVE ENERGY
TOWARDS THE DRAKE PASSAGE/ANTARCTIC PENINSULA. BUT AS THE RIDGE
WEAKENS...INFLOW OF SHORT WAVE ENERGY SHIFTS TO TIERRA DEL
FUEGO/SOUTHERN CHILE LATER ON SUNDAY. THROUGH MONDAY AFTERNOON THE
BROAD TROUGH IS TO SLOWLY PULL ACROSS SOUTHERN CHILE/PATAGONIA IN
ARGENTINA TO THE WESTERN ATLANTIC. AT LOW LEVELS THIS WILL SUSTAIN
A BROAD TROUGH OFF THE SOUTH COAST OF CHILE/SOUTHEAST PACIFIC
LATER ON SATURDAY...THAT ENVELOPS THE SOUTHERN CONE LATER ON
SUNDAY. AS IT NEARS THE SOUTHERN CONE...THE BROAD TROUGH WILL
SUSTAIN A MOIST ADVECTIVE PATTERN. THIS WILL COMBINE WITH WARMER
THAN NORMAL SEA SURFACE TEMPERATURES OVER THE SOUTH PACIFIC TO
POTENTIALLY FAVOR MODERATE TO HEAVY RAINFALL AMOUNTS ACROSS THE
SOUTHERN REGIONS OF CHILE. ON SATURDAY THIS WILL START WITH MAXIMA
OF 20-40MM...WHILE ON SUNDAY IT PEAKS AT 25-50MM. EARLY NEXT
WEEK...IN A COLD ADVECTIVE PATTERN...HEAVY SNOW IS TO CLUSTER OVER
SOUTHERN CHILE TO THE SOUTH OF ISLA DE CHILOE...WITH MAXIMA OF
10-15CM.

NORTH OVER THE CONTINENT...AT 250 HPA...A BROAD/LOW AMPLITUDE CELL
OF THE SUBEQUATORIAL RIDGE IS TO ENVELOP CONTINENTAL AREA TO THE
NORTH OF 12S...WITH PATTERN TO HOLD NEARLY UNCHANGED DURING THE
NEXT THREE DAYS. LATER DURING THE WEEKEND...A SHORT WAVE TROUGH
STREAMING ACROSS BOLIVIA IS TO AMPLIFY ACROSS MATO GROSSO TO PARA
EARLY ON SUNDAY...SPLITTING THE SUBEQUATORIAL RIDGE IN TWO CELLS.
MEANWHILE...THE RIDGE ALOFT WILL VENT DIURNAL CONVECTION ACROSS
NORTHERN BRASIL TO NORTHERN PERU/EASTERN ECUADOR. IN THIS
AREA...ABOVE NORMAL MOISTURE CONTENT WILL COMBINE WITH WEAK LOW
LEVEL FORCING TO CONTINUE FAVORING ABOVE NORMAL RAINFALL AMOUNTS.
THIS IS TO INCREASE FROM 10-15MM EARLY IN THE CYCLE TO 15-30MM ON
FRIDAY TO SATURDAY. HIGHER AMOUNTS ARE EXPECTED LATER DURING THE
WEEKEND/EARLY NEXT WEEK AS THE AFOREMENTIONED FRONT/PREFRONTAL
SHEAR LINE STREAM ACROSS NORTHWEST SOUTH AMERICA.

DAVISON...WPC (USA)