303  
FXSA20 KWBC 151752  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1252 PM EST FRI DEC 15 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM DEC 15 AT 0000 UTC): THE GFS AND  
ECMWF MAINTAIN GOOD CONTINUITY WITH THE PREVIOUS FORECAST THROUGH  
72-84 HRS. THEY THEN MAKE PATTERN CORRECTIONS TO A DEEP TROUGH  
REACHING THE SOUTHEAST PACIFIC LATER IN THE CYCLE. IN THIS RUN  
THESE MODELS NOW FAVOR A DEEPER/HIGHER AMPLITUDE TROUGH TO ENTER  
SOUTHERN CHILE BY 108-120 HRS. THE UKMET IS SLOWLY SHIFTING TO A  
MORE AMPLIFIED PATTERN AS INDICATED BY THE OTHER MODELS... THUS  
LEADING TO A HIGHER CONFIDENCE IN THE FORECAST THROUGH THE MEDIUM  
RANGE PERIOD.  
 
DEEPENING MID LEVEL TROUGH REACHES SOUTHERN/CENTRAL REGIONS OF  
CHILE LATER THIS EVENING...MEANWHILE FOCUSING ITS SHORT WAVE  
ENERGY OVER THE CONTINENT TO THE SOUTH OF 34S. ON SATURDAY THE  
TROUGH MOVES ACROSS PATAGONIA TO THE CENTRAL PROVINCES IN  
ARGENTINA...FOCUSING ITS SHORT WAVE ENERGY ACROSS CENTRAL-NORTHERN  
ARGENTINA/RIO DE LA PLATA BASIN. ON SUNDAY THE TROUGH IS TO  
CONTINUE ACROSS 40W...AND ACCELERATES ACROSS 15W ON MONDAY. AT LOW  
LEVELS THIS WILL SUSTAIN A DEEPENING TROUGH ACROSS PATAGONIA IN  
ARGENTINA/SOUTHERN CHILE LATER TODAY...WITH A SURFACE FRONT ACROSS  
PATAGONIA IN ARGENTINA-SOUTHERN REGIONS OF CHILE. AS THE TROUGH  
DEEPENS...AND A POLAR RIDGE BUILDS ACROSS PATAGONIA IN  
ARGENTINA...IT IS TO FAVOR A SOUTHERLY PAMPERO LOW LEVEL JET. THIS  
DISPLACES THE FRONT NORTH ACROSS THE BUENOS AIRES  
PROVINCE-CORDOBA/MENDOZA IN ARGENTINA LATER ON  
SATURDAY...MEANWHILE TRIGGERING SQUALLY WEATHER/SEVERE CONVECTION.  
IN THIS PERIOD FORECAST HELICITY VALUES SHOW POTENTIAL FOR TORNADO  
INDUCING SUPER CELL DEVELOPMENT ACROSS THE BUENOS AIRES  
PROVINCE-ENTRE RIOS...WITH LARGE HAIL HIGHLY PROBABLE. IN THIS  
AREA THE RAINFALL MAXIMA WILL PEAK AT 40-80MM...WITH LOCALLY  
HIGHER HIGHLY POSSIBLE. ON SUNDAY THE SOUTHERLY JET DRIVES THE  
FRONT NORTH TO SOUTHERN BRASIL-PARAGUAY. THIS DAY THE  
ENVIRONMENTAL CONDITIONS ARE FAVORABLE FOR AN MCS TO DEVELOP OVER  
CHACO ARGENTINO/PARAGUAYO TO SOUTHERN BOLIVIA...WITH MAXIMA IN  
THIS AREA TO PEAK AT 35-70MM. ON MONDAY TO TUESDAY...AS THE FRONT  
LOSES ITS UPPER LEVEL SUPPORT...IT IS TO MEANDER SOUTH TO NORTHERN  
ARGENTINA.  
 
THE MODELS THEN FORECAST A SHORT WAVE TROUGH ON THE NORTHERN  
STREAM TO MOVE ACROSS THE EASTERN PACIFIC TO CENTRAL REGIONS OF  
CHILE LATER ON SUNDAY. THE MID LEVEL PERTURBATION WILL GRADUALLY  
SPILL ACROSS THE CENTRAL ANDES OF CHILE WHILE SHEARING SHORT WAVE  
ENERGY ACROSS CENTRAL PROVINCES IN ARGENTINA. SOME INTERACTION IS  
POSSIBLE BETWEEN THIS FEATURE AND THE AFOREMENTIONED FRONT  
MEANDERING OVER SOUTHERN BRASIL-NORTHERN ARGENTINA. ON  
MONDAY-TUESDAY THIS IS LIKELY TO TRIGGER ORGANIZED CONVECTION  
ACROSS THE MESOPOTAMIA VALLEY IN ARGENTINA-NORTHERN URUGUAY TO RIO  
GRANDE DO SUL...WITH RAINFALL MAXIMA OF 30-60MM.  
 
LATER IN THE CYCLE...AS PREVIOUSLY INDICATED...PROGRESSIVE MID  
LEVEL TROUGH IS TO AMPLIFY BETWEEN 120W-90W TO 35S LATER ON  
MONDAY...WITH AXIS PULLING ACROSS 85W/90W TO 30S ON TUESDAY. THE  
TROUGH IS TO FOCUS ITS SHORT WAVE ENERGY OVER CHILE/ARGENTINA TO  
THE SOUTH OF CONCEPCION...WHILE ALSO SUSTAINING A SUBTROPICAL  
CONNECTION AS IT APPROACHES THE SOUTHERN CONE. AT LOW LEVELS THIS  
IS TO SUSTAIN A DEEP OCCLUDED LOW OVER THE BELLINGSHAUSEN  
SEA/SOUTHEAST PACIFIC ON MONDAY-TUESDAY...WHILE THE ASSOCIATED  
FRONT REACHES SOUTHERN CHILE LATER ON MONDAY/EARLY TUESDAY  
MORNING. THIS IS TO TRIGGER MODERATE CONVECTION ACROSS SOUTHERN  
REGIONS OF CHILE WITH MAXIMA OF 20-30MM.  
 
AT 200 HPA...A CLOSED HIGH OVER NORTHERN BOLIVIA ANCHORS A CELL OF  
THE SUBTROPICAL RIDGE ACROSS NORTHWEST SOUTH AMERICA. MODELS  
CONTINUE TO FORECAST THE GRADUAL INTENSIFICATION OF THIS RIDGE ON  
SUNDAY-MONDAY...WHEN IT BUILDS ACROSS BOLIVIA-MATO GROSSO/MATO  
GROSSO DO SUL TO THE SOUTHERN STATES OF BRASIL. EARLY IN THE CYCLE  
THE UPPER LEVEL RIDGE IS TO INDUCE THE NORTHWARD AMPLIFICATION OF  
A TROUGH ACROSS THE SOUTHEAST STATES OF BRASIL. BUT AS THE RIDGE  
BUILDS TO THE SOUTH-SOUTHEAST LATER DURING THE WEEKEND...THE  
TROUGH IS TO THEN AMPLIFY ACROSS NORTHEAST BRASIL TO PARA/AMAPA  
LATER ON MONDAY-TUESDAY. AS THE UPPER PATTERN EVOLVES...FOCUS OF  
THE ORGANIZED CONVECTION SHIFTS FROM NORTHEAST TO CENTRAL BRASIL  
ON SATURDAY-SUNDAY...AND TO NORTHWEST BRASIL-PERUVIAN  
JUNGLE/SOUTHERN COLOMBIA ON MONDAY-TUESDAY. OVER  
TOCANTINS-GOIAS-PARA IN BRASIL INITIALLY EXPECTING MAXIMA OF  
35-70MM. THROUGH SUNDAY THIS DECREASES TO 20-30MM...WHILE ACROSS  
PARA AND AMAZONAS IT INCREASES FROM 20-35MM EARLY IN THE CYCLE TO  
40-80MM ON SATURDAY...RANGING BETWEEN 30-60MM ON SUNDAY TO MONDAY.  
ON TUESDAY THIS WANES...DECREASING TO 20-35MM. ACROSS THE  
CENTRAL/SOUTHERN JUNGLE OF PERU...A SURGE IN CONVECTION IS  
EXPECTED ON TUESDAY...WITH MAXIMA OF 30-60MM.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
RIVAS...SENAMHI (PERU)  
DAVISON/GALVEZ...WPC (USA)  

 
 
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