804  
FXSA20 KWBC 291609  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1109 AM EST FRI DEC 29 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM DEC 29 AT 0000 UTC): THE  
DETERMINISTIC AND ENSEMBLE GLOBAL MEMBERS CONVERGE ON A SIMILAR  
SOLUTION ACROSS THE SOUTH AMERICAN DOMAIN THROUGH 96-108 HRS.  
LATER IN THE CYCLE THEY START TO DIVERGE ON THE AMPLITUDE AND/OR  
SPEED OF SHORT WAVE PERTURBATIONS ON THE NORTHERN STREAM  
PROPAGATING UNDER A BROAD RIDGE OVER THE EASTERN PACIFIC.  
DIFFERENCES AMONG THE MODELS RAPIDLY BECOME MORE PRONOUNCED BY  
132-144 HRS. LEADING TO A LOWER CONFIDENCE IN THE FORECAST DURING  
THE MEDIUM RANGE PERIOD.  
 
MODELS INITIALIZE A DIGGING MID/UPPER LEVEL TROUGH OVER THE  
SOUTHEAST PACIFIC...WITH MID LEVEL TROUGH TO MOVE ACROSS SOUTHERN  
CHILE TO PATAGONIA IN ARGENTINA EARLY IN THE CYCLE...WHERE IT IS  
TO INDUCE HEIGHT FALLS OF 75-125GPM. ON SATURDAY THE TROUGH  
MEANDERS EAST ACROSS 60W WHILE AMPLIFYING TO 30S...TO THEN STALL  
OVER THE SOUTHERN CONE THROUGH THE END OF THE CYCLE. AS IT  
MEANDERS OVER THE SOUTHERN CONE...A SHORT WAVE TROUGH IS TO ROUND  
THE BASE OF THE TROUGH. ON SUNDAY THE SHORT WAVE VORTEX MOVES  
ACROSS CENTRAL CHILE TO CUYO IN ARGENTINA...AND ON MONDAY IT  
CONTINUES ACROSS 60W WHILE AMPLIFYING TO 20N. THIS IS TO THEN  
SUSTAIN HEIGHT FALL OF 50-75GPM. AT LOW LEVELS THIS SUSTAINS A  
BROAD TROUGH THAT EXTENDS OVER THE DRAKE PASSAGE/SOUTHWEST  
ATLANTIC TO SOUTHERN CHILE/PATAGONIA IN ARGENTINA. POLAR FRONTS  
REVOLVE AROUND THIS AXIS...WITH ONE TO MOVE ACROSS NORTHERN  
PATAGONIA IN ARGENTINA-TEMUCO CHILE INTO THE EASTERN PACIFIC LATER  
TODAY. ON SATURDAY-SUNDAY THE FRONT MOVES NORTH ACROSS RIO DE LA  
PLATA/BUENOS AIRES PROVINCE-MENDOZA IN ARGENTINA WHILE TRAILING  
END OVER THE PACIFIC STARTS TO FRONTOLIZE. ON MONDAY-TUESDAY THE  
FRONT MOVES NORTH ACROSS SOUTHERN BRASIL-PARAGUAY TO NORTHERN  
ARGENTINA. EARLY IN THE CYCLE THIS IS TO FAVOR MODERATE CONVECTION  
OVER CHILE BETWEEN TEMUCO AND ISLA DE CHILOE...WITH MAXIMA OF  
20-30MM. ON SATURDAY IT IS TO THEN FAVOR MODERATE TO HEAVY  
CONVECTION ACROSS CENTRAL ARGENTINA...WITH MAXIMA OF 20-40MM.  
DURING THAT PERIOD THERE IS A SLIGHT RISK OF SEVERE CONVECTION. UN  
SUNDAY...AS THE FRONT SURGES ACROSS CENTRAL TO NORTHERN  
ARGENTINA...AN MCS IS EXPECTED ACROSS CORRIENTES-FORMOZA-MISIONES  
IN ARGENTINA-SOUTHEAST PARAGUAY AND URUGUAY...WITH MAXIMA IN THIS  
AREA PEAKING AROUND 75-125MM. ON MONDAY...UNDER INFLUENCE OF A  
SOUTHERLY LOW LEVEL JET ACROSS CHACO ARGENTINO/PARAGUAYO FOCUS OF  
THE HEAVY CONVECTION SHIFTS TO SOUTHERN BOLIVIA...WITH MAXIMA OF  
40-80MM. ON TUESDAY EXPECTING SIMILAR AMOUNTS ACROSS NORTHERN  
BOLIVIA-RONDONIA/ACRE IN BRASIL TO THE SOUTHERN/CENTRAL JUNGLE IN  
PERU. MEANWHILE...OVER SOUTHERN BRASIL THE MAXIMA DECREASES TO  
25-50MM.  
 
FARTHER NORTH...A MID/UPPER LEVEL SHORT WAVE TROUGH MOVES ACROSS  
THE MESOPOTAMIA VALLEY IN ARGENTINA-URUGUAY/SOUTHERN BRASIL LATER  
TODAY...TO THEN MEANDER OFF THE SOUTH COAST OF BRASIL THROUGH  
SUNDAY EVENING. EARLY NEXT WEEK...AS THE AFOREMENTIONED TROUGH  
DEEPENS ACROSS ARGENTINA...THE SHORT WAVE TROUGH WILL THEN  
ACCELERATE TO THE EAST-SOUTHEAST ACROSS THE SOUTH ATLANTIC. AT LOW  
LEVELS THIS IS TO INTERACT WITH A WANING SURFACE FRONT OVER  
SOUTHERN BRASIL-PARAGUAY TO TRIGGER MODERATE TO HEAVY  
CONVECTION...WITH MAXIMA OF 35-70MM. THROUGH SATURDAY THIS WILL  
INCREASE TO 40-80MM WHILE BUILDING NORTH ACROSS PARANA TO SAO  
PAULO-MINAS GERAIS. FURTHERMORE...MODELS CONTINUE TO FORECAST  
POSSIBILITY OF A SUBTROPICAL CYCLONE FORMING OFF THE SOUTHEAST  
COAST OF BRASIL ON SATURDAY-SUNDAY. IF A CYCLONE FORMS...THE  
BRAZILIAN NAVY WILL ISSUE THE OFFICIAL FORECAST FOR THIS FEATURE.  
 
AT 200 HPA...BROAD CELL OF THE SUBTROPICAL RIDGE HAS BECOME THE  
DOMINANT FEATURE OVER THE CONTINENT TO THE NORTH OF 30S. DURING  
THE NEXT FEW DAYS THIS IS TO ANCHOR ON A CLOSED HIGH THAT MEANDERS  
BETWEEN NORTHERN CHILE-CENTRAL BOLIVIA. EARLY NEXT WEEK...AS THE  
POLAR TROUGH DEEPENS ACROSS NORTHERN ARGENTINA TO PARAGUAY...THE  
RIDGE WILL SPLIT IN TWO CELLS. ONE IS TO CONFINE TO THE EASTERN  
PACIFIC...WHILE A WEAKER ONE REMAINS OVER NORTHERN SOUTH AMERICA.  
THE RIDGE ALOFT IS TO VENT DIURNAL CONVECTION ACROSS SOUTH  
AMERICA. ACROSS CENTRAL-NORTHWEST BRASIL INITIALLY EXPECTING  
MAXIMA OF 20-45MM. THROUGH SATURDAY THIS DECREASES TO 20-35MM IN  
SCATTERED CONVECTION. HIGHER AMOUNTS...HOWEVER...ARE EXPECTED  
ALONG THE NEAR EQUATORIAL TROUGH AS IT MEANDERS OVER NORTHERN  
AMAZONAS-PARA-RORAIMA IN BRASIL...WITH MAXIMA OF 20-45MM. ON THE  
CENTRAL-SOUTHERN JUNGLE OF PERU TO PORTIONS OF BOLIVIA EXPECTING  
MAXIMA OF 20-35MM EARLY IN THE CYCLE. THIS DECREASES TO 15-25MM ON  
SATURDAY-SUNDAY.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
RIVAS...SENAMHI (PERU)  
DAVISON...WPC (USA)  

 
 
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