740  
FXSA20 KWBC 281732  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1231 PM EST TUE NOV 28 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM NOV 28 AT 0000 UTC): THE GFS AND  
UKMET MADE CORRECTIONS ON THE NORTHERN STREAM FLOW...NOW FAVORING  
A DEEPER AND SLOWER SHORT WAVE TROUGH TO MEANDER OFF THE SOUTH  
COAST OF BRASIL. THE GLOBAL ENSEMBLE MEMBERS ARE SLOWLY EMBRACING  
THIS SOLUTION. THUS...OUR CONFIDENCE IN THE FORECAST IS HIGH  
THROUGH 132-144 HRS.  
 
AT MID LEVELS...A SHORT WAVE TROUGH ON THE NORTHERN STREAM  
MEANDERS OVER THE EASTERN PACIFIC WHILE REMAINING JUST WEST OF  
CENTRAL CHILE. LATER TODAY/EARLY WEDNESDAY MORNING THE MID LEVEL  
VORTEX IS TO MEANDER EAST INTO CENTRAL REGIONS OF CHILE...TO SPILL  
ACROSS THE ANDES TO CENTRAL ARGENTINA LATER IN THE DAY. AS IT  
SPILLS ACROSS THE ANDES THIS IS TO SUSTAIN CYCLOGENESIS ALONG A  
SURFACE FRONT OVER CENTRAL ARGENTINA. THIS IS TO TRIGGER SCATTERED  
CONVECTION...WITH MAXIMA OF 20-30MM ON WEDNESDAY AND ON THURSDAY.  
 
CONSISTENT WITH PREVIOUS FORECAST...A PROGRESSIVE MID LEVEL TROUGH  
RACES ACROSS THE SOUTHEAST PACIFIC TO THE BELLINGSHAUSEN  
SEA-SOUTHERN CHILE LATER TODAY/EARLY ON WEDNESDAY MORNING. THE  
TROUGH IS TO THEN SPLIT AS IT PRESSES AGAINST A RIDGE TO THE  
EAST...WITH SOUTHERN SEGMENT ACCELERATING ACROSS THE SOUTH GEORGIA  
ISLANDS WHILE ITS NORTHERN HALF MEANDERS OVER PATAGONIA/SOUTHERN  
CHILE. AT LOW LEVELS THE TROUGH SUSTAINS AN ELONGATED FRONT ACROSS  
THE DRAKE PASSAGE-SOUTHERN CHILE/EASTERN PACIFIC LATER  
TODAY...THAT MOVES NORTH ACROSS PATAGONIA TO LA PAMPA IN ARGENTINA  
LATER ON WEDNESDAY/EARLY THURSDAY MORNING. OVER SOUTHERN CHILE  
THIS IS TO FAVOR LIGHT CONVECTION WITH ACCUMULATION OF  
05-10MM...WHILE OVER PATAGONIA ONLY EXPECTING TRACE RAINFALL  
AMOUNTS. AS IT RACES ACROSS LA PAMPA ON THURSDAY THIS IS TO  
SUSTAIN SCATTERED CONVECTION WITH ACCUMULATION OF 20-30MM.  
 
A SECONDARY MID LEVEL PERTURBATION IS TO FOLLOW...TO PULL ACROSS  
100W/105W LATER THIS AFTERNOON. ON WEDNESDAY IT IS TO THEN EXTEND  
ALONG 80W TO 30S. EARLY ON THURSDAY IT IS TO ENTER SOUTHERN  
REGIONS OF CHILE/WESTERN PATAGONIA IN ARGENTINA...TO MOVE OFF THE  
COAST TO THE WESTERN ATLANTIC LATER IN THE DAY. ON FRIDAY THE  
TROUGH SPLITS IN TWO...LEAVING BEHIND A SHORT WAVE TROUGH OFF THE  
COAST OF PATAGONIA. THIS IS TO THEN MEANDER TO THE EAST-SOUTHEAST.  
AT LOW LEVELS THIS IS TO SUSTAIN A PROGRESSIVE FRONT THAT ENTERS  
SOUTHERN CHILE LATER ON WEDNESDAY/EARLY ON THURSDAY MORNING. THIS  
IS TO ALSO FAVOR LIGHT CONVECTION AS IT ENTERS SOUTHERN CHILE.  
 
ALSO AT 500 HPA...A TROUGH IS TO EXTEND OVER THE EASTERN PACIFIC  
BETWEEN 50W-20W AND TO THE SOUTH OF 20S...WITH AXIS TO ANCHOR ON A  
CLOSED LOW NEAR 42S 33W LATER TODAY. SHORT WAVE TROUGH IS TO  
MEANDER OFF THE SOUTH COAST OF BRASIL THROUGH THE END OF THE WEEK  
WHILE SLOWLY WEAKENING AS IT SHEARS SHORT WAVE ENERGY TO THE  
EAST-SOUTHEAST. THE TROUGH SUSTAINS AN ELONGATED FRONT ACROSS THE  
SOUTH ATLANTIC TO RIO DE JANEIRO/SOUTHERN MINAS GERAIS TO MATO  
GROSSO DO SUL/PARAGUAY. THE BOUNDARY WILL BE THE FOCUS OF LOW  
LEVEL MOISTURE CONVERGENCE. ACROSS RIO DE JANEIRO-MINAS GERAIS  
EXPECTING MAXIMA OF 20-35MM OVER THE NEXT DAY OR SO...INCREASING  
TO 25-50MM ON THURSDAY. OVER PARAGUAY...SHORT WAVE TROUGH MOVING  
ACROSS CENTRAL/NORTHERN ARGENTINA ON WEDNESDAY IS TO INTERACT WITH  
THE SURFACE BOUNDARY... WITH CONDITIONS BECOMING HIGHLY FAVORABLE  
FOR AN MCS TO FORM DURING THE EARLY MORNING HOURS ON THURSDAY.  
THIS IS TO FAVOR HEAVY CONVECTION WITH MAXIMA OF 50-100MM. ON  
THURSDAY...AS ACTIVITY SPREADS ACROSS SOUTHEAST BOLIVIA-MATO  
GROSSO DO SUL...THE MAXIMA INCREASES TO 75-125MM.  
 
NORTH OVER THE CONTINENT...AT 200 HPA...A MEANDERING HIGH IS TO  
CENTER OVER NORTHERN BOLIVIA-RONDONIA IN BRASIL DURING THE NEXT  
FEW DAYS...MEANWHILE ANCHORING A RIDGE OVER NORTHERN SOUTH  
AMERICA. THE RIDGE UNDERCUTS A TROUGH THAT EXTENDS OVER THE  
NORTHEAST STATES OF BRASIL. THE TROUGH CENTERS ON A CLOSED LOW  
NEAR RECIFE. THIS WEAKENS TO AN OPEN TROUGH ON WEDNESDAY...LIFTING  
TO THE NORTH-NORTHEAST INTO THE TROPICAL ATLANTIC LATER IN THE  
WEEK. AS IT MOVES AWAY...THE RIDGE IS TO BECOME DOMINANT FEATURE  
OVER THE CONTINENT TO THE NORTH OF 20S. THE RIDGE ALOFT IS VENTING  
DEEP CONVECTION ACROSS NORTHERN SOUTH AMERICA. OVER MATO  
GROSSO-RONDONIA-NORTHERN BOLIVIA IT IS TO FAVOR MODERATE  
CONVECTION WITH ACCUMULATION OF 20-45MM THROUGH WEDNESDAY  
MORNING...AND 15-20MM/DAY THEREAFTER. HIGHER AMOUNTS ARE EXPECTED  
OVER ACRE IN BRASIL AND THE PERUVIAN JUNGLE...WHERE THE GDI SHOWS  
RISK OF ORGANIZED DEEP CONVECTION DURING THE NEXT 18-24 HRS...WITH  
MAXIMA OF 30-60MM. ON WEDNESDAY THIS DECREASES TO 15-25MM...WHILE  
ON THURSDAY THE MAXIMA PEAKS AT 10-15MM. ON THE SOUTHERN SIERRA OF  
PERU-ALTIPLANO OF BOLIVIA EXPECTING MAXIMA OF 15-25MM EARLY IN THE  
CYCLE.  
 
OVIEDO...SHN (ARGENTINA)  
VANNUCCI...SMN (ARGENTINA)  
RIVAS...SENAMHI (PERU)  
DAVISON...WPC (USA)  

 
 
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