515  
FXSA20 KWBC 091823  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
123 PM EST FRI FEB 09 2018  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM FEB 09 AT 0000 UTC): FORECAST MODELS  
AND ENSEMBLE MEMBERS CONTINUE IN FAIRLY GOOD AGREEMENT ON FORECAST  
EVOLUTION THROUGH THE CYCLE. THE GFS CONTINUES TO MAKE ADJUSTMENTS  
OVER THE EASTERN PACIFIC SOUTH OF 45S AFTER 96 HRS...THUS...AFTER  
THIS PERIOD THE UKMET AND ECMWF SOLUTIONS ARE SLIGHTLY MORE  
RELIABLE. FORECAST CONFIDENCE IS HIGH THROUGH 96 FORECAST HOURS.  
 
FORECAST FOR SOUTH AND CENTRAL SOUTH AMERICA  
 
UPPER TROUGH IS PROPAGATING ACROSS CENTRAL ARGENTINA AND IS  
ENHANCING UPPER DIVERGENCE. THIS IS INTERACTING WITH A SURFACE  
FRONT...A MOISTURE POOL...AND PLENTIFUL INSTABILITY TO SUSTAIN  
SEVERE CONVECTION. THROUGH EARLY SATURDAY EXPECTING ACCUMULATIONS  
OF 25-50MM/DAY...WITH RISK FOR SEVERE WEATHER IN CENTRAL/NORTHERN  
ARGENTINA-URUGUAY-SOUTHERN RIO GRANDE DO SUL. BY SATURDAY-SUNDAY  
STRONG CONVECTION WILL FLARE UP IN THE CHACO TO PRODUCE MAXIMA OF  
50-100MM/DAY AND THE RISK FOR NOCTURNAL MCS FORMATION. DOWNSTREAM  
NORTH OF THE FRONT EXPECTING MAXIMA OF 20-35MM/DAY IN NORTHERN  
URUGUAY-CORRIENTES-RIO GRANDE DO SUL.  
 
THEN...MODELS ARE SUGGESTING THE FORMATION OF A SACZ-LIKE  
STRUCTURE...YET IT WILL ONLY LAST TWO DAYS THUS WILL BE ANALYZED  
AS A TROUGH. THE SYSTEM WILL START ORGANIZING ACROSS SOUTHERN  
BOLIVIA...PARAGUAY...MISIONES...SANTA CATARINA/PARANA ON  
SATURDAY...TO PRODUCE HEAVY RAIN. LARGEST AMOUNTS ARE EXPECTED IN  
THE PARAGUAYAN CHACO/SOUTHERN BOLIVIA WHERE EXPECTING MAXIMA OF  
75-150MM AND THE RISK OF MCS FORMATION. DOWNSTREAM INTO SOUTHERN  
BRASIL EXPECTING MAXIMA OF 25-50MM. DURING MONDAY...THE TROUGH  
WILL POSITION ACROSS PARANA...SOUTHERN MATO GROSSO...NORTHERN  
PARAGUAY WHETHER IT LASTS LONG ENOUGH TO BE CLASSIFIED AS A  
SACZ...THE OCCURRENCE OF LOCALLY HEAVY RAINFALL IS BECOMING MORE  
LIKELY. BY MONDAY...EXPECTING STRONG CONVECTION IN  
PARANA...SOUTHERN SAO PAULO...MATO GROSSO DO SUL...NORTHERN  
PARAGUAY AND EASTERN BOLIVIA/WESTERN MATO GROSSO TO PRODUCE  
MAXIMUM ACCUMULATIONS OF 40-80MM/DAY INCLUDING THE RISK OF MCS  
FORMATION. BY TUESDAY EXPECTING THE HEAVIEST PRECIPITATION  
OCCURRING IN MATO GROSSO DO AND WESTERN SAO PAULO...WHERE  
EXPECTING MAXIMA OF 15-25MM/DAY INCLUDING THE RISK OF MCS  
FORMATION. ISOLATED HIGHER AMOUNTS ARE EXPECTED IN THE SERRA DO  
MAR...WHERE OROGRAPHIC UPLIFT WILL LEAD TO MAXIMA OF 25-50MM/DAY  
 
FORECAST FOR THE TROPICAL ANDES AND PACIFIC COAST  
 
A REGION OF CONCERN IN THE TROPICAL ANDES IS THE ALTIPLANO AND  
THE ANDES OF SOUTH-CENTRAL PERU. CONVECTIVE INSTABILITY AND  
MOISTURE AVAILABILITY WILL REMAIN HIGH THROUGH THE CYCLE AND  
VENTILATION IN THE UPPER TROPOSPHERE WILL BECOME MORE FAVORABLE  
UNDER THE INFLUENCE OF ENHANCED VENTILATION IN THE WESTERN  
PERIPHERY OF THE STRENGTHENING SUBTROPICAL RIDGE.  
FURTHERMORE...THE MID-LEVEL WINDS...GENERALLY WEAK...WILL ALLOW  
THE MOISTURE TO MEANDER AND BUILD IN THE ALTIPLANO. EXPECTING  
MAXIMUM RAINFALL ACCUMULATIONS OF 20-35MM/DAY THROUGH LATE SUNDAY.  
THE AREAS OF MOST WIDESPREAD AND INTENSE THUNDERSTORMS WILL EXTEND  
OVER SOUTHERN PERU/BOLIVIAN ALTIPLANO ON FRIDAY. ON  
SATURDAY...ENHANCED NORTHERLIES ALONG THE EASTERN ALTIPLANO WILL  
BOOST AFTERNOON MOISTURE CONVERGENCE AND RELOCATE STORM FORMATION  
TO THE WESTERN CORDILLERA. THIS WILL BE FROM ANTOFAGASTA/SALTA  
NORTH TO AREQUIPA IN PERU AND AFFECT THE CENTRAL ALTIPLANO  
OVERNIGHT. DURING SUNDAY...STRONG CONVECTION WILL DEVELOP IN THE  
SOUTHEASTERN ALTIPLANO AND TARIJA/SUCRE/POTOSI REGIONS IN  
BOLIVIA...AS MID-LEVEL EASTERLIES ADVECT A SATURATED AIR MASS FROM  
THE CHACO...AND BOOST MOISTURE CONVERGENCE. ON MONDAY...LARGEST  
ACCUMULATIONS WILL REACH 15-25MM/DAY AND EXTEND FROM THE  
CHILE/ARGENTINA BORDER NORTH INTO SOUTHERN PERU.  
 
IN ECUADOR...A NEW INCREASE IN PRECIPITATION IS EXPECTED ON  
SATURDAY...AS THE PACIFIC ITCZ MEANDERS INTO THE ESMERALDAS REGION  
IN NORTHEASTERN ECUADOR. AN INCREASE IN THE GDI...PRECIPITABLE  
WAVER...AND MOISTURE CONVERGENCE WILL FAVOR AN INCREASE IN  
AFTERNOON/EVENING COASTAL CONVECTION...TO PRODUCE MAXIMA OF  
20-35MM/DAY. INSTABILITY AND WESTERLY WIND CONVERGENCE WILL BUILD  
FROM ECUADOR INTO THE SOUTHERN CHOCO IN COLOMBIA...TO SUSTAIN  
SIMILAR ACCUMULATIONS. ACCUMULATIONS IN THE ANDES WILL ALSO  
INCREASE IN SATURDAY TO SUSTAIN ISOLATED MAXIMA OF 15-20MM/DAY.  
 
AMAZON BASIN...CENTRAL AND EASTERN BRAZIL  
 
A TRANSITION IS OCCURRING IN TROPICAL SOUTH AMERICA AS THE  
SOUTH AMERICAN CONVERGENCE ZONE (SACZ) COLLAPSES AND THE UPPER  
CIRCULATION EVOLVES. THE DISINTEGRATION OF THE UPPER TROUGH  
ASSOCIATED WITH THE SACZ WILL GIVE WAY TO THE REFORMATION AND  
STRENGTHENING OF THE SUBTROPICAL RIDGE. AT MID-LEVELS...AN  
EASTERLY SURGE DEVELOPING IN THE ATLANTIC WILL BRING A DRY AND  
WARM AIR MASS INTO THE CONTINENT...WHICH WILL BE ACCOMPANIED BY A  
STRONG CAP. THIS WILL LEAD TO A SHARP DECREASE IN THE POTENTIAL  
FOR THUNDERSTORMS IN EASTERN BRASIL EXPANDING FROM EAST TO WEST.  
THE EDGE OF THE CAP AND DRY AIR WILL EXTEND ALONG 42W ON FRIDAY  
EVENING...TO 45W ON SATURDAY EVENING...TO 48W ON SUNDAY  
EVENING...TO 50W ON SUNDAY EVENING. AHEAD OF THE SURGE...ENHANCED  
MOISTURE CONVERGENCE WILL FAVOR STRONG CONVECTION. AS THE  
SUBTROPICAL RIDGE FORMS A HIGH OVER CENTRAL BRAZIL AND STRENGTHENS  
IN GENERAL STARTING FRIDAY...CONVECTION WILL SPREAD INTO THE  
NORTHWESTERN AMAZON AS THE CYCLE PROGRESSES. THIS WILL REPOSITION  
MOST ACTIVE CONVECTION TO WESTERN PARA...MOST OF THE STATE OF  
AMAZONAS...ACRE...RONDONIA AND EASTERN PERU BY MONDAY.  
ACCUMULATIONS WILL PERSIST IN THE 30-70MM/DAY RANGE WITH  
THUNDERSTORMS. ELSEWHERE ...EXPECTING MAXIMA OF 20-35MM/DAY IN  
THUNDERSTORMS.  
 
BALBINO...INMET (BRASIL)  
POMIER...SENAMHI (BOLIVIA)  
GALVEZ...WPC (USA)  

 
 
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