256  
FXSA20 KWBC 291811  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
110 PM EST MON JAN 29 2018  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM JAN 29 AT 0000 UTC): MODELS GENERALLY  
AGREE ON PATTERN EVOLUTION THROUGH 72-84 HRS...THEN DIVERGE ON HOW  
UPSTREAM PATTERN OVER THE EASTERN PACIFIC IS GOING TO EVOLVE. BY  
96-120 HRS BOTH THE UKMET AND ECMWF FAVOR A PERSISTENT MID/UPPER  
LEVEL RIDGE TO EXTEND NORTH-TO-SOUTH ALONG THE COAST OF CHILE TO  
THE BELLINGSHAUSEN SEA. THE GFS...IN-CONTRAST...FORECASTS A TIGHT  
BELT OF MID LATITUDE WESTERLIES TO PERSIST THROUGH THE END OF THE  
CYCLE. THE DIFFERENCES AMONG THE DETERMINISTIC SOLUTION IS ALSO  
EVIDENT AMONG THE GLOBAL ENSEMBLE MEMBERS...WITH MODELS FAILING TO  
REACH A CONSENSUS ON HOW THE MEDIUM RANGE PATTERN IS GOING TO  
EVOLVE. AS A RESULT...EXPECTING MODELS TO MAKE MAJOR PATTERN  
CORRECTIONS DURING THE NEXT TWO-TO-THREE DAYS.  
 
A MID LEVEL TROUGH ENTERING THE SOUTHERN CONE OF SOUTH AMERICA  
BECOMES THE DOMINANT MESO-SYNOPTIC FEATURE OVER THE WESTERN  
ATLANTIC EARLY ON TUESDAY. THIS IS TO THEN EVOLVE INTO A BROAD  
TROUGH THAT IS TO MEANDER BETWEEN THE SOUTH ORKNEY AND THE SOUTH  
GEORGIA ISLANDS THROUGH THE END OF THE WEEK. AS IT MEANDERS OVER  
THE WESTERN ATLANTIC THE BROAD TROUGH IS TO THEN PHASE WITH A  
SHORT WAVE TROUGH TO THE NORTH. THE LATTER IS TO MEANDER OFF THE  
SOUTH/SOUTHEAST COAST OF BRASIL THROUGH THE MEDIUM RANGE PERIOD.  
AT LOW LEVELS...THE MID LEVEL TROUGH TO THE SOUTH WILL SUSTAIN A  
POLAR FRONT THAT SURGES ACROSS PATAGONIA-SOUTHERN CHILE LATER  
TODAY. ON TUESDAY-WEDNESDAY...A SURFACE LOW MOVING ACROSS THE  
FALKLAND ISLANDS/MALVINAS IS TO RAPIDLY INTENSIFY...WITH CENTRAL  
PRESSURE DROPPING 24-30HPA IN 24 HRS. THE METEOROLOGICAL BOMB  
REACHES MAXIMUM INTENSITY ON WEDNESDAY MORNING. THE DEEPENING LOW  
WILL THEN DRIVE THE FRONT NORTH ACROSS THE BUENOS AIRES  
PROVINCE-CENTRAL ARGENTINA LATER ON TUESDAY. THE FRONTAL BOUNDARY  
IS TO THEN STALL OVER RIO DE LA PLATA BASIN/SOUTHERN URUGUAY. AS  
IT ENTERS SOUTHERN CHILE THIS IS TO SUSTAIN MODERATE CONVECTION  
WITH ACCUMULATION OF 10-15MM. OVER MENDOZA-CORDOBA IT IS TO THEN  
TRIGGER SIMILAR AMOUNTS ON TUESDAY AND WEDNESDAY.  
 
A MEANDERING FRONT LIES TO THE NORTH...WITH BOUNDARY STRETCHING  
ACROSS THE SOUTH ATLANTIC TO PARANA/SANTA CATARINA IN SOUTHERN  
BRASIL-MISIONES IN ARGENTINA TO CORRIENTES/FORMOSA. THE MID LEVEL  
TROUGH OFF THE SOUTHEAST COAST OF BRASIL WILL INDUCE A FRONTAL LOW  
LATER TODAY. THIS WILL DRAW THE FRONT NORTH ACROSS SAO PAULO ON  
TUESDAY...WHERE IT IS TO MEANDER THROUGH THE END OF THE WEEK.  
ALOFT...BROAD CELL OF THE SUBTROPICAL RIDGE DOMINATES THE UPPER  
LEVELS...WITH AXIS ANCHORING ON A CLOSED HIGH THAT MEANDERS  
BETWEEN PARAGUAY AND SOUTHERN BOLIVIA DURING THE NEXT FEW DAYS.  
THE RIDGE ALOFT WILL FAVOR A DIVERGENT PATTERN ACROSS  
SOUTHEAST-CENTRAL BRASIL-BOLIVIA. FURTHERMORE...THE CLIMATE  
FORECAST SYSTEM SHOWS MJO PATTERN BECOMING BETTER ORGANIZED AND  
CONDUCIVE TO ORGANIZED DEEP CONVECTION THROUGH THE MIDDLE OF THE  
WEEK. THIS...IN-TURN...WILL ALLOW THE SOUTH AMERICA CONVERGENCE  
ZONE (SACZ) TO DEVELOP BETWEEN SAO PAULO/SOUTHERN MINAS GERAIS IN  
BRASIL TO SANTA CRUZ IN EASTERN BOLIVIA. THIS BOUNDARY WILL BE THE  
FOCUS OF LOW LEVEL MOISTURE CONVERGENCE. OVER SAO PAULO-RIO DE  
JANEIRO INITIALLY EXPECTING RAINFALL AMOUNTS OF 30-60MM. ON  
TUESDAY THIS INCREASES TO 50-100MM. ON WEDNESDAY TO THURSDAY THE  
FOCUS OF THE HEAVY CONVECTION SHIFTS TO ESPIRITO SANTO-MINAS  
GERAIS-RIO DE JANEIRO...WITH MAXIMA OF 75-150MM IN HIGH RISK OF AN  
MCS FORMING. NOTE THAT DURING THIS PERIOD...DUE TO STRONG  
TOPOGRAPHICAL FORCING...SOME OF THE MODELS SHOW LOCALIZED MAXIMA  
IN EXCESS OF 200MM. OTHER CONVECTION IS EXPECTED ACROSS MATO  
GROSSO/MATO GROSSO DO SUL-RONDONIA IN BRASIL AND BOLIVIA. IN THIS  
AREA INITIALLY EXPECTING MAXIMA OF 20-30MM...INCREASING TO 25-50MM  
ON TUESDAY. ON WEDNESDAY TO THURSDAY THIS INCREASES TO 35-70MM  
WHILE BUILDING NORTH ACROSS PARA TO EASTERN AMAZONAS IN BRASIL.  
OTHER CONVECTION IS TO CLUSTER OVER SOUTHERN-SOUTHEAST BOLIVIA AND  
THE CHACO PARAGUAYO...WITH MAXIMA OF 20-40MM EARLY IN THE CYCLE  
INCREASING TO 50-100MM ON TUESDAY-WEDNESDAY AS AN MCS FORMS ALONG  
THE TRAILING END OF THE SACZ OVER BOLIVIA. ON WEDNESDAY TO  
THURSDAY HEAVY RAINS ARE TO PERSIST WITH MAXIMA OF 40-80MM.  
 
ALSO AT 200 HPA...TWO TROUGHS BOUND THE NORTHERN FRINGES OF THE  
SUBTROPICAL RIDGE. ONE TO THE WEST EXTENDS ALONG 20S 90W TO  
CENTRAL PERU...WHERE IT IS TO REMAIN DURING THE NEXT FEW DAYS. AS  
IT PRESSES AGAINST THE SUBTROPICAL RIDGE TO THE EAST THE GRADIENT  
IS TO TIGHTEN...WITH A SUBEQUATORIAL JET TO FORM BETWEEN THE  
TROUGH AND THE RIDGE ON WEDNESDAY. THE JET IS TO ENHANCE  
DIVERGENCE ALOFT ALONG THE SIERRA OF PERU...TO FAVOR LIGHT TO  
MODERATE CONVECTION WITH ACCUMULATION OF 05-10MM/DAY. HIGHER  
AMOUNTS ARE EXPECTED ACROSS THE ALTIPLANO OF BOLIVIA AND SOUTHERN  
PERU...WITH MAXIMA IN THIS AREA TO PEAK AT 15-25MM.  
 
ANOTHER UPPER LEVEL TROUGH BOUNDS THE EASTERN FRINGES OF THE RIDGE  
AS IT EXTENDS OVER THE TROPICAL ATLANTIC TO THE NORTHEAST STATES  
OF BRASIL. THIS IS TO ALSO PERSIST DURING THE NEXT FEW DAYS...WITH  
UPPER LEVEL FLOW DIFFLUING BETWEEN THIS TROUGH AND THE RIDGE TO  
THE SOUTH. THIS WILL VENT CONVECTION ALONG THE NEAR EQUATORIAL  
TROUGH OVER NORTHERN BRASIL. OVER PARA-AMAZONAS EXPECTING  
ACCUMULATION OF 20-40MM/DAY...WHILE ALONG THE NORTHEAST COAST  
MAXIMA WILL PEAK AT 20-30MM EARLY IN THE CYCLE. THIS WILL  
GRADUALLY DECREASES TO 15-20MM/DAY ON TUESDAY-THURSDAY.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
DAVISON...WPC (USA)  

 
 
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