943  
FXSA20 KWBC 161625  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1225 PM EDT FRI APR 16 2021  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
FORECAST DISCUSSION FROM APR 16/16UTC: A PROGRESSIVE MID/UPPER  
LEVEL SHORT WAVE TROUGH ON THE SOUTHERN STREAM RACES ACROSS THE  
DRAKE PASSAGE/SOUTHERN PATAGONIA EARLY IN THE CYCLE. THIS IS TO  
THEN QUICKLY STREAM ACROSS THE SOUTHWEST ATLANTIC...INTENSIFYING  
AS IT NEARS THE SOUTH GEORGIA ISLANDS EARLY ON SATURDAY MORNING.  
AT LOW LEVELS THIS SUSTAINS A DEEPENING FRONTAL LOW THAT  
EXPERIENCES A RAPID RATE OF INTENSIFICATION AS IT MOVES ACROSS THE  
FALKLAND ISLANDS TO THE THE SOUTH GEORGIA ISLANDS EARLY ON  
SATURDAY MORNING. AS THE LOW DEEPENS...THE ASSOCIATED FRONT WILL  
PROPAGATE ACROSS PATAGONIA TO LA PAMPA IN ARGENTINA LATER THIS  
EVENING...WHERE IT IS FORECAST TO STALL. UNDER FAVORABLE JET  
DYNAMICS THERE IS A SLIGHT RISK OF SEVERE CONVECTION ACROSS LA  
PAMPA ON SATURDAY...WHILE SUSTAINING RAINFALL AMOUNTS OF 10-15MM.  
 
CONSISTENT WITH PREVIOUS FORECASTS...A HIGHER AMPLITUDE TROUGH IS  
TO FOLLOW. THE TROUGH AMPLIFIES NORTH BETWEEN 120W-80W TO 25S ON  
FRIDAY-SATURDAY. IN A SINUSOIDAL PATTERN...AS THE TROUGH DEEPENS  
TO THE WEST IT INDUCES THE SOUTHWARD AMPLIFICATION OF A LONG WAVE  
RIDGE ACROSS ARGENTINA TO THE DRAKE PASSAGE/ANTARCTIC PENINSULA.  
THE BUILDING RIDGE IS TO BRIEFLY BLOCK THE TROUGH TO THE WEST OF  
CHILE DURING THE WEEKEND. BUT LATER ON SUNDAY THE RIDGE WEAKENS  
ENOUGH TO ALLOW THE TROUGH TO START MOVING EAST ACROSS SOUTHERN  
CHILE TO SOUTHERN ARGENTINA. EARLY NEXT WEEK...UNDER THE  
FRICTIONAL EFFECTS OF THE CENTRAL ANDES OF CHILE...THE TROUGH IS  
TO SPLIT IN TWO. ITS SOUTHERN HALF IS TO THEN PROPAGATE ACROSS THE  
DRAKE PASSAGE/WEDDELL SEA WHILE ITS NORTHERN SEGMENT EVOLVES INTO  
A CLOSED LOW OFF THE CENTRAL COAST OF CHILE. ON TUESDAY-WEDNESDAY  
THE MODELS THEN DIVERGE ON HOW THE REMAINING NORTHERN STREAM  
SEGMENT OF THE TROUGH EVOLVES...WITH THE UKMET AND ECMWF  
FORECASTING THE LOW TO MEANDER OFF THE CENTRAL COAST OF CHILE  
THROUGH MID WEEK...WHILE THE GFS FORECASTS THIS LOW TO SPILL  
ACROSS THE CENTRAL ANDES TO CENTRAL ARGENTINA ON WEDNESDAY.  
ALTHOUGH A SLOWER EVOLUTION SEEMS MORE LIKELY...EXPECTING IT TO  
FALL SOMEWHERE IN BETWEEN THESE MODELS. AT LOW LEVELS...A BROAD  
TROUGH IS TO DOMINATE THE SOUTHEAST PACIFIC/SOUTHERN CONE THROUGH  
SUNDAY...MEANWHILE FOCUSING SHORT WAVE ENERGY AND SUBTROPICAL  
MOISTURE ACROSS THE SOUTHERN REGIONS OF CHILE. THROUGH SUNDAY  
CONVECTION IN THIS AREA INTENSIFIES...WITH MAXIMA INCREASING FROM  
20-35MM ON SATURDAY TO 20-45MM ON SUNDAY IN POTENTIALLY DEEP  
CONVECTION. ON MONDAY CONVECTION SPREADS NORTH ALONG THE COAST  
BETWEEN PUERTO MONTT-CONCEPCION...WITH MAXIMA PEAKING AT 20-40MM.  
OVER LA PAMPA/CENTRAL ARGENTINA EXPECTING MODERATE CONVECTION ON  
MONDAY TO RESULT IN RAINFALL AMOUNTS OF 20-35MM IN POTENTIALLY  
SEVERE CONVECTION. THIS IS TO INTENSIFY ON TUESDAY AS UNDER THE  
INFLUENCE OF A SOUTHERLY/PAMPERO LOW LEVEL JET THE FRONT SURGES  
NORTH ACROSS CENTRAL ARGENTINA...TO SUSTAIN MAXIMA OF 35-70MM.  
 
ON THE NORTHERN STREAM FLOW...IN A SPLIT FLOW PATTERN...A SLOW TO  
EVOLVE SHORT WAVE TROUGH LIFTS ACROSS SOUTHERN TO SOUTHEAST BRASIL  
DURING THE WEEKEND. THIS IS TO THEN MEANDER OVER THE ATLANTIC TO  
THE EAST-SOUTHEAST OF SAO PAULO WHILE EVOLVING INTO A CLOSED LOW  
NEAR 30S 42W EARLY NEXT WEEK. INITIALLY THIS IS TO SUSTAIN  
SCATTERED CONVECTION ACROSS SOUTHERN BRASIL-CORRIENTES/MISIONES IN  
ARGENTINA-SOUTHEAST PARAGUAY...WITH MAXIMA OF 15-20MM IN SEVERE  
CONVECTION...WHILE OVER SOUTHEAST BOLIVIA/CHACO PARAGUAYO THE  
MAXIMA PEAKS AT 30-60MM. ON SATURDAY FOCUS OF THE CONVECTION  
SHIFTS TO SOUTHEAST BRASIL WHERE IT IS TO SUSTAIN MAXIMA OF  
20-35MM. LATER DURING THE WEEKEND/EARLY NEXT WEEK...AS THE MID  
LEVEL LOW/TROUGH MEANDERS OVER ANOMALOUSLY WARM WATERS OFF THE  
SOUTHEAST COAST OF BRASIL...IT WILL SUSTAIN WARM CORE  
CYCLOGENESIS. DURING THIS PERIOD BOTH THE GFS AND ECMWF AGREE ON  
POTENTIAL FOR A SUBTROPICAL CYCLONE TO FORM...WITH THE GFS  
FAVORING A CONSIDERABLY DEEPER LOW THAN WHAT THE ECMWF SUGGESTS.  
AS THE SYSTEM BECOMES BETTER ORGANIZED...IT WILL ENHANCE MOISTURE  
TRANSPORT/CONVERGENCE ALONG THE COAST OF SAO PAULO. ON SUNDAY  
EXPECTING MAXIMA OF 35-70MM...WHILE ON MONDAY EXPECTING MAXIMA OF  
25-50MM BETWEEN RIO DE JANEIRO AND NORTHERN SAO PAULO. THROUGH  
MONDAY THIS IS TO WEAKEN AS THE SURFACE LOW MEANDERS FARTHER EAST  
INTO THE WESTERN SOUTH ATLANTIC.  
 
NORTH OVER THE CONTINENT...AT 250 HPA...A FLAT/EAST-TO-WEST CELL  
OF THE SUBEQUATORIAL RIDGE IS TO CONFINE TO EQUATORIAL SOUTH  
AMERICA. THIS IS TO VENT DIURNAL CONVECTION ACROSS  
AMAPA-PARA/RORAIMA AND AMAZONAS IN NORTHERN BRASIL. IN THIS AREA  
EXPECTING RAINFALL AMOUNTS OF 10-15MM/DAY WITH MAXIMA OF 20-40MM.  
LOCALLY HIGHER AMOUNTS ARE EXPECTED ACROSS AMAPA AND PARA IN  
NORTHERN BRASIL...WHERE EASTERLY WAVES PROPAGATING ACROSS NORTHERN  
SOUTH AMERICA ARE TO FAVORABLY INTERACT WITH THE NEAR EQUATORIAL  
TROUGH. OTHER CONVECTION IS EXPECTED TO THE WEST ACROSS  
ECUADOR/WESTERN COLOMBIA...WHERE PULSE OF THE MJO IS TO SUSTAIN A  
MOIST WESTERLY FLOW. IN THIS AREA CONVECTION RAPIDLY BECOMES  
BETTER ORGANIZED...INCREASING FRO 15-20MM EARLY IN THE CYCLE TO  
30-60MM ON SATURDAY-SUNDAY. THIS THEN WEAKENS EARLY NEXT WEEK.  
 
DAVISON...WPC (USA)  

 
 
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