479  
FXSA20 KWBC 211558  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1157 AM EDT FRI APR 21 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM APRIL 21 AT 0000 UTC): THERE IS POOR  
CYCLE-TO-CYCLE CONTINUITY AMONG THE GLOBAL MODEL GUIDANCE...WITH  
MAJOR CHANGES TO FORECAST PATTERN AT 60 HRS AND ONWARD. MODELS ARE  
MAKING CORRECTIONS TO A SHORT WAVE TROUGH ENTERING THE SOUTHERN  
CONE ON SUNDAY...NOW PROJECTING A DEEPER TROUGH THAN WHAT THEY  
PREVIOUSLY SUGGESTED. WE BELIEVE THE MODELS HAVE  
OVERCORRECTED...AND EXPECT FUTURE RUNS TO GRADUALLY DAMPEN THIS  
FEATURE.  
 
GLOBAL MODELS AGREE ON LONG WAVE TROUGH TO DEEPEN ALONG THE COAST  
OF ARGENTINA TO THE SOUTHERN STATES OF BRASIL LATER TODAY. THE  
DEEPENING TROUGH IS TO THEN INTERACT WITH A MID LEVEL VORTEX TO  
THE NORTH. THIS WILL DRAW THE MID LEVEL TROUGH ACROSS THE SOUTHERN  
STATES OF BRASIL LATER TODAY...AND THROUGH SATURDAY EVENING IT IS  
TO THEN MERGE INTO THE LONG WAVE PATTERN. AT LOW LEVELS...POLAR  
RIDGE BUILDS ACROSS ARGENTINA...DISPLACING A WANING FRONT NORTH  
ACROSS URUGUAY-NORTHERN ARGENTINA LATER TODAY...WHILE ANOTHER  
FRONT MEANDERS TO THE NORTH ACROSS SOUTHEAST BRASIL-MATO GROSSO TO  
BOLIVIA. ON SATURDAY TO SUNDAY THE LATTER IS TO THEN MEANDER OVER  
ESPIRITO SANTO-MINAS GERAIS-MATO GROSSO-SOUTHEAST BOLIVIA...WHERE  
IT IS TO REMAIN WHILE GRADUALLY WEAKENING. ACROSS SOUTHEAST BRASIL  
THIS IS TO FAVOR SCATTERED CONVECTION WITH ACCUMULATION OF 20-35MM  
THROUGH SATURDAY MORNING...AND 10-15MM ON SATURDAY AND SUNDAY. AS  
IT TRAILS ACROSS CENTRAL/NORTHERN BOLIVIA-RONDONIA-SOUTHERN JUNGLE  
OF PERU THE FRONT WILL FAVOR SCATTERED CONVECTION WITH  
ACCUMULATION OF 20-45MM THROUGH SATURDAY MORNING...AND 20-35MM ON  
SATURDAY AND SUNDAY.  
 
PROGRESSIVE MID LEVEL TROUGH QUICKLY PULLS ACROSS THE SOUTHEAST  
PACIFIC ON SUNDAY. AS IT ENTERS THE SOUTHERN CONE A BUILDING RIDGE  
TO THE WEST IS TO THEN INDUCE THE NORTHWARD AMPLIFICATION OF THIS  
TROUGH ACROSS CHILE/ARGENTINA...WITH AXIS REACHING MAXIMUM  
AMPLITUDE OVER THE CONTINENT ON MONDAY-TUESDAY. THE HIGH AMPLITUDE  
LONG WAVE TROUGH WILL FOCUS SHORT WAVE ENERGY ACROSS  
PATAGONIA-CENTRAL PROVINCES IN ARGENTINA...WHERE IT IS TO SUSTAIN  
HEIGHT FALLS OF 75-100GPM. AT LOW LEVELS THIS SUSTAINS AN  
OCCLUDING LOW OVER THE SOUTHERN CONE LATER ON SUNDAY. AS IT  
MEANDERS ACROSS PATAGONIA TO THE WESTERN ATLANTIC ON MONDAY...THE  
SYSTEM IS TO ENCOUNTER ABOVE NORMAL SEA SURFACE TEMPERATURES. THIS  
WILL FUEL THE GRADUAL INTENSIFICATION OF THIS TROUGH ON  
MONDAY-TUESDAY. AS THE SURFACE TROUGH DEEPENS...THE ASSOCIATED  
FRONT WILL SURGE ACROSS PATAGONIA TO LA PAMPA IN ARGENTINA EARLY  
ON MONDAY...AND ACROSS RIO DE LA PLATA BASIN-CORDOBA EARLY ON  
TUESDAY. THE SURGING BOUNDARY WILL THEN REINFORCE A MEANDERING  
FRONT TO THE NORTH. OVER SOUTHERN CHILE THE DEEPENING LOW WILL  
SUSTAIN MODERATE RAINFALL AMOUNTS WITH ACCUMULATION OF 10-15MM ON  
SUNDAY-MONDAY...WHILE OVER SOUTHERN PATAGONIA/TIERRA DEL FUEGO  
EXPECTING ACCUMULATION OF 05-10MM. ACROSS PARAGUAY-NORTHERN  
PROVINCES IN ARGENTINA IT IS TO THEN TRIGGER MODERATE TO HEAVY  
CONVECTION LATER ON MONDAY/EARLY TUESDAY MORNING...WITH MAXIMA OF  
25-50MM...INCREASING TO 35-70MM LATER IN THE DAY.  
 
AT 200 HPA THE FLOW OVER NORTHERN SOUTH AMERICA SPLITS BETWEEN THE  
SUBTROPICAL RIDGE OVER BRASIL AND THE SUBEQUATORIAL RIDGE TO THE  
NORTH. THIS PATTERN IS TO HOLD DURING THE NEXT FEW  
DAYS...RESULTING IN AREAS OF ENHANCED UPPER DIVERGENCE ACROSS  
EQUATORIAL SOUTH AMERICA. THIS WILL PROVIDE THE VENTILATION ALOFT  
TO SUSTAIN SCATTERED DEEP CONVECTION...WITH MOST ACTIVE CLUSTERING  
ALONG THE NEAR EQUATORIAL TROUGH OVER NORTHERN BRASIL-SOUTHERN  
GUIANAS/AMAZONIA IN VENEZUELA. IN THIS AREA EXPECTING RAINFALL  
AMOUNTS OF 15-20MM/DAY...WITH MAXIMA TO PEAK AROUND 25-50MM/DAY.  
LOCALLY HIGHER AMOUNTS ARE HIGHLY LIKELY. CONVECTION IS TO ALSO  
CLUSTER ACROSS SOUTHERN COLOMBIA-NORTHERN JUNGLE OF PERU/EASTERN  
ECUADOR...WITH MAXIMA OF 15-20MM. ON SATURDAY...AS A DEEP LAYER  
EASTERLY FLOW ESTABLISHES ACROSS BRASIL...THIS WILL INCREASE TO  
20-45MM...BUT THEN RAPIDLY WEAKENS ON SUNDAY.  
 
ABDEMUR...FAA (ARGENTINA)  
ESPINOLA...DNM (PARAGUAY)  
DAVISON...WPC (USA)  

 
 
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