197  
FXSA20 KWBC 201618  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1217 PM EDT FRI APR 20 2018  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM APR 20 AT 0000 UTC): MODELS CONTINUE  
IN GENERALLY GOOD AGREEMENT THROUGH THE CYCLE. FORECAST CONFIDENCE  
DECAYS SLIGHTLY AFTER 84-96 HRS TO THE SOUTH OF 45S...ESPECIALLY  
OVER THE PACIFIC AND ATLANTIC. AFTER THIS PERIOD...THE CONFIDENCE  
DECAYS VERY GRADUALLY THROUGH 120 HRS.  
 
UNDER PRESENCE OF AN EXTENSIVE LONG WAVE TROUGH APPROACHING THE  
SOUTHERN CONE...ACTIVE WEATHER PATTERN IS EXPECTED IN SOUTHERN  
CHILE THROUGH THE CYCLE. THROUGH EARLY SATURDAY...EXPECTING A MIX  
OF RAIN AND SNOW TO AFFECT AREAS TO THE SOUTH OF THE TAITAO  
PENINSULA ACCUMULATING MAXIMA OF 10-15MM/DAY. TO THE  
NORTH...EXPECT SIMILAR AMOUNTS IN RAINFALL IN AREAS FROM  
TEMUCO/VALDIVIA TO THE SOUTH. A PEAK IN ACCUMULATIONS IS EXPECTED  
ON SATURDAY-SUNDAY FROM VALDIVIA INTO NORTHERN AYSEN...AS A  
SUBTROPICAL MOISTURE CONNECTION INTERACTS WITH UPPER DYNAMICS TO  
SUSTAIN MAXIMA OF 15-25MM/DAY.  
 
ACTIVE PATTERN OD CONVECTION IS TO CONTINUE IN THE RIO DE LA PLATA  
BASIN AS MOISTURE AND INSTABILITY REMAIN IN PLACE...AND A COUPLE  
OF SHORT WAVE TROUGHS ACCOMPANIED BY UPPER JET DYNAMICS CROSS THE  
REGION THROUGH THE CYCLE. ON FRIDAY...A SHORT WAVE TROUGH IS  
CROSSING INTO THE ATLANTIC ALONG 56-58W. THIS WILL CONTINUE  
SUSTAINING STRONG CONVECTION IN URUGUAY/RIO GRANDE DO SUL THROUGH  
EARLY SATURDAY. THIS WILL SUSTAIN MAXIMA OF 20-40MM/DAY WITH THE  
RISK OF SEVERE WEATHER/MCS...INCLUDING THE POSSIBILITY OF HAIL. BY  
SATURDAY-SUNDAY...REMAINING INSTABILITY IN NORTHERN URUGUAY/ENTRE  
RIOS/CORRIENTES AND RIO GRANDE DO SUL WILL SUSTAIN THE DEVELOPMENT  
OF A FEW THUNDERSTORMS TO PRODUCE ACCUMULATIONS OF 10-15MM/DAY.  
VERY ISOLATED MAXIMA NEAR 20MM IS POSSIBLE. A MORE SIGNIFICANT  
WEATHER EVENT IS EXPECTED ON SUNDAY. MODELS CONTINUE AGREEING ON A  
PEAK IN SEVERE CONVECTION...AS ENHANCED LOW-LEVEL MOISTURE  
CONVERGENCE IN THE EXIT OF THE SOUTH AMERICA LOW-LEVEL JET  
INTERACTS WITH AN UPPER TROUGH AND THE TROPICAL ENTRANCE OF AN  
UPPER JET. LIFTED INDEX VALUES HAVE DECREASED SLIGHTLY FROM  
PREVIOUS RUNS...YET THEY REMAIN CLOSE TO -8. THE FORECAST  
CONTINUES TO SHOW MAXIMA OF 40-80MM/DAY WITH THE RISK FOR SEVERE  
WEATHER AND MCS FORMATION. UPPER DYNAMIC FORCING WILL GRADUALLY  
DECREASE AFTER...YET INSTABILITY AND LOW-LEVEL MOISTURE  
CONVERGENCE WILL REMAIN ENHANCED THROUGH WEDNESDAY. BY  
MONDAY-TUESDAY EXPECTING MAXIMA OF 30-60MM/DAY IN SOUTHERN SANTA  
FE-ENTRE RIOS INTO URUGUAY...WHILE EXPECTING MAXIMA OF 20-40MM/DAY  
IN SANTIAGO DEL ESTERO/NORTHERN CORDOBA. BY TUESDAY-WEDNESDAY  
EXPECTING A DECREASE TO MAXIMA OF 20-40MM/DAY IN CORRIENTES/ENTRE  
RIOS/URUGUAY...AND MAXIMA OF 15-30MM/DAY UPSTREAM.  
 
SEASONALLY ACTIVE PATTERN IS TO ALSO CONTINUE ALONG THE EASTERN  
COAST OF BRASIL...UNDER THE INFLUENCE OF A SURFACE FRONT...SHEAR  
LINE...AND DEEPENING MID-LEVEL TROUGH. INITIALLY...FRONTAL  
CONVERGENCE WILL SUSTAIN A FEW HEAVY THUNDERSTORMS BETWEEN  
NORTHERN BAHIA AND ALAGOAS TO PRODUCE MAXIMA OF 30-60MM/DAY THOUGH  
EARLY SATURDAY. NORTH...WITH THE SHEAR LINE...EXPECTING MAXIMA OF  
20-30MM/DAY CLUSTERING BETWEEN SERGIPE AND RIO GRANDE DO NORTE. BY  
SATURDAY-MONDAY...EXPECTING MAXIMA OF 20-40MM/DAY BETWEEN NORTHERN  
BAHIA AND PERNAMBUCO...WHILE FRONT AND SHEAR LINE GRADUALLY  
WEAKEN. SIMILAR AMOUNTS ARE EXPECTED BY MONDAY-TUESDAY ACROSS  
CENTRAL BAHIA-ALAGOAS...AIDED BY TRADE WIND CONVERGENCE. AFTER  
THIS PERIOD...UNDER INFLUENCE OF STRENGTHENING UPPER  
TROUGH...EXPECTING AN INCREASE IN SOUTHERN BAHIA/ESPIRITO SANTO TO  
MAXIMA OF 20-35MM/DAY. NORTH ACROSS THE NORDESTE EXPECTING A  
DECREASE TO MAXIMA OF 15-20MM/DAY ALONG COASTAL LOCATIONS.  
 
ALSO IN TROPICAL SOUTH AMERICA...ACTIVITY WILL REMAIN ENHANCED  
ALONG THE NET. A SIGNIFICANT FEATURE...EARLY IN THE CYCLE...WILL  
BE AN UPPER TROUGH TO EXTEND ACROSS NORTHERN PERU ON FRIDAY  
THROUGH SUNDAY. THIS TROUGH IS ENHANCING UPPER DIVERGENCE IN THE  
AMAZON BASIN OF NORTHERN PERU AND WESTERN BRASIL...WHERE IT WILL  
SUSTAIN MAXIMA OF 30-60MM/DAY ON FRIDAY-SATURDAY...DECREASING  
AFTER. IN ECUADOR EXPECTING MAXIMA OF 20-30MM/DAY INITIALLY...AS  
UPPER CONVERGENT PATTERN LIMITS THE POTENTIAL FOR EXCESSIVE  
AMOUNTS. THIS WILL PEAK ON SATURDAY-SUNDAY WHEN EXPECTING A PEAK  
IN OROGRAPHIC MOISTURE CONVERGENCE. ACTIVE CONVECTION ALONG THE  
NET WILL SUSTAIN MAXIMA IN THE ORDER OF 25-50MM/DAY THROUGH THE  
CYCLE.  
 
BALBINO...INMET (BRASIL)  
DIAZ...INUMET (URUGUAY)  
GALVEZ...WPC (USA)  

 
 
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