018  
FXSA20 KWBC 281658  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1157 AM EST THU DEC 28 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM DEC 28 AT 0000 UTC): OVER THE FAR  
EASTERN PACIFIC-SOUTH AMERICA AND MOST OF THE ATLANTIC THE GLOBAL  
MODELS MAINTAIN GOOD CONTINUITY WITH PREVIOUS FORECAST.  
ENVIRONMENTAL CONDITIONS REMAIN FAVORABLE FOR A HEAVY RAINFALL  
PRODUCING MCS TO FORM ACROSS MID SECTIONS OF SOUTH AMERICA ON  
SUNDAY-MONDAY...WHILE SIMULTANEOUSLY A SUBTROPICAL CYCLONE COULD  
FORM OFF THE SOUTH COAST OF BRASIL.  
 
BUILDING MID/UPPER LEVEL RIDGE IS TO SUSTAIN THE NORTHWARD  
AMPLIFICATION OF A TROUGH ON THE SOUTHERN STREAM OVER THE  
SOUTHEAST PACIFIC EARLY IN THE PERIOD...WITH AXIS TO EXTEND  
BETWEEN 105W-70W AND TO THE SOUTH 30S LATER TODAY. THE TROUGH  
SUSTAINS A SUBTROPICAL CONNECTION...ENHANCING TRANSPORT OF  
WARM/MOIST AIR INTO SOUTHERN REGIONS OF CHILE. ON FRIDAY-SATURDAY  
THE TROUGH IS TO THEN SLOWLY PUSH EAST ACROSS SOUTHERN CHILE TO  
PATAGONIA IN ARGENTINA...MEANWHILE FAVORING HEIGHT FALLS OF  
75-125GPM OVER THE CONTINENT TO THE SOUTH OF 30S. ON SUNDAY TO  
MONDAY THE MODELS FORESEE FURTHER AMPLIFICATION OF THIS TROUGH  
OVER THE CONTINENT TO JUST SOUTH OF 20S LATER ON MONDAY. AS IT  
AMPLIFIES ACROSS ARGENTINA...A JET MAXIMA WILL LIE TO THE  
NORTH...FAVORING AN UPPER DIVERGENT PATTERN THAT IS TO VENT  
ORGANIZED DEEP CONVECTION. AT LOW LEVELS THIS SUSTAINS A BROAD  
TROUGH ACROSS THE DRAKE PASSAGE TO THE SOUTHERN CONE OF SOUTH  
AMERICA...WITH AN ELONGATED FRONT THAT IS TO STRETCH FROM THE  
SOUTH ATLANTIC TO THE SOUTH PACIFIC WHILE TRAVERSING  
PATAGONIA-SOUTHERN REGIONS OF CHILE. ON FRIDAY THE FRONT MOVES  
NORTH ACROSS LA PAMPA IN ARGENTINA... CROSSING RIO DE LA  
PLATA-CENTRAL ARGENTINA ON SATURDAY...THEN NORTH ACROSS SOUTHERN  
BRASIL-PARAGUAY ON SUNDAY-MONDAY. AS IT MEANDERS OVER SOUTHERN  
CHILE THIS IS TO FAVOR MODERATE TO HEAVY RAINFALL AMOUNTS BETWEEN  
PUERTO MONTT AND ISLA DE CHILOE EARLY IN THE CYCLE...TO TRIGGER  
MAXIMA OF 30-60MM. ON FRIDAY THIS DECREASES TO 20-30MM. OVER  
TIERRA DEL FUEGO/EXTREME SOUTHERN CHILE THIS WILL SUSTAIN  
ACCUMULATION OF 15-20MM. AS THE FRONT PUSHES NORTH ACROSS LA PAMPA  
ON SATURDAY IT IS TO SUSTAIN MODERATE CONVECTION WITH MAXIMA OF  
20-30MM. LATER DURING THE WEEKEND...AS THE FRONT MEANDERS NORTH  
INTO MID SECTIONS OF SOUTH AMERICA...CONDITIONS WILL BECOME  
FAVORABLE FOR AN MCS TO DEVELOP OVER FORMOSA-CORRIENTES-MISIONES  
IN ARGENTINA AND PARAGUAY...WITH RAINFALL MAXIMA OF 75-125MM.  
UNDER INFLUENCE OF A SOUTHERLY LOW LEVEL JET...FOCUS OF THE HEAVY  
CONVECTION SHIFTS TO SANTA CRUZ/NORTHERN BOLIVIA ON MONDAY...WITH  
MAXIMA OF 30-60MM.  
 
SHORT WAVE TROUGH ON THE NORTHERN STREAM IS TO PULL ACROSS  
URUGUAY/SOUTHERN BRASIL LATER THIS EVENING...TO THEN MEANDER OFF  
THE SOUTHEAST COAST OF BRASIL THROUGH SUNDAY AFTERNOON. AS IT  
MOVES ACROSS MID SECTIONS OF SOUTH AMERICA THIS IS TO INTERACT  
WITH A SURFACE FRONT OVER SOUTHEAST BRASIL-PARAGUAY. EARLY IN THE  
CYCLE THIS IS TO SUSTAIN MODERATE TO HEAVY RAINFALL AMOUNTS.  
BETWEEN PARAGUAY AND MATO GROSSO DO SUL THIS WILL FAVOR MAXIMA OF  
20-45M...WHILE OVER RIO DE JANEIRO/SOUTHERN MINAS GERAIS THE  
MAXIMA WILL PEAK AT 30-60MM. ON SATURDAY...UNDER INFLUENCE OF THE  
COLD CORE TROUGH ALOFT... A WARM CORE LOW WILL FORM OFF THE  
SOUTHEAST COAST OF BRASIL. THE LOW IS TO GRADUALLY INTENSIFY WHILE  
DRIFTING TO THE SOUTH-SOUTHEAST. OVER NORTHERN SAO PAULO-SOUTHERN  
RIO DE JANEIRO AND MINAS GERAIS THE DAILY MAXIMA WILL GRADUALLY  
INCREASE TO 40-80MM LATER ON SATURDAY/SUNDAY MORNING.  
 
AT 200 HPA...THE SUBTROPICAL RIDGE EXTENDS WEST-TO-EAST OVER THE  
CONTINENT ALONG 20S...WHILE CENTERING ON A CLOSED HIGH OVER  
CENTRAL BOLIVIA AND ANOTHER OVER MATO GROSSO DO SUL IN BRASIL. ON  
SUNDAY THESE ARE TO COMBINE INTO A SINGLE HIGH OVER BOLIVIA. THE  
RIDGE ALOFT...MEANWHILE...WILL VENT DEEP CONVECTION OVER THE  
CONTINENT TO THE NORTH OF 20S. MOST ACTIVE IS EXPECTED BETWEEN  
PARA-AMAZONAS IN BRASIL-SOUTHERN COLOMBIA EARLY IN THE  
CYCLE...WITH MAXIMA OF 20-45MM. THIS PERSIST THROUGH SATURDAY  
EVENING. LATER ON SUNDAY THE FOCUS OF THE CONVECTION SHIFTS TO  
NORTHERN AMAZONAS IN BRASIL TO SOUTHERN COLOMBIA/ VENEZUELA...WITH  
MAXIMA TO PEAK AT 20-45MM. OVER THE SOUTHERN SIERRA OF  
PERU-ALTIPLANO OF BOLIVIA DIURNAL CONVECTION IS TO INITIALLY  
TRIGGER MAXIMA OF 20-30MM. THIS IS TO DECREASES TO 10-15MM/DAY  
LATER ON WEDNESDAY.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
RIVAS...SENAMHI (PERU)  
DAVISON...WPC (USA)  

 
 
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