009  
FXSA20 KWBC 271800  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1259 PM EST WED DEC 27 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM DEC 27 AT 0000 UTC): MINOR  
CORRECTIONS TO PREVIOUS FORECAST...WITH GOOD CONTINUITY ACROSS THE  
CONTINENT-SOUTH ATLANTIC THROUGH DAY 06. AS A RESULT...OVERALL  
CONFIDENCE IN THE FORECAST IS HIGH.  
 
CONSISTENT WITH PREVIOUS FORECAST...A BROAD/LOW AMPLITUDE TROUGH  
ON THE SOUTHERN STREAM IS TO DOMINATES THE MID/UPPER LEVEL FLOW  
ACROSS THE SOUTHEAST PACIFIC-SOUTHERN CONE OF SOUTH AMERICA DURING  
THE NEXT COUPLE OF DAYS. THIS SUSTAINS A TIGHT BELT OF MID  
LATITUDE WESTERLIES ACROSS SOUTHERN CHILE AND OVER PATAGONIA IN  
ARGENTINA. LATER ON THURSDAY...AS A RIDGE BUILDS TO THE WEST ALONG  
110W...THE TROUGH IS TO AMPLIFY BETWEEN 105W-70W TO 30S. THE  
TROUGH IS TO THEN SLOWLY PUSH ACROSS SOUTHERN CHILE-PATAGONIA IN  
ARGENTINA ON FRIDAY-SATURDAY...MEANWHILE FAVORING HEIGHT FALLS OF  
75-125GPM OVER THE CONTINENT TO THE SOUTH OF 30S. AS IT  
AMPLIFIES...THE TROUGH WILL ESTABLISH A SUBTROPICAL CONNECTION...  
ADVECTING WARM/MOIST AIR ACROSS THE EASTERN PACIFIC TO SOUTHERN  
PORTIONS OF CHILE ON THURSDAY AND FRIDAY. DURING THE WEEKEND THE  
TROUGH IS TO THEN MOVE FARTHER INLAND ACROSS ARGENTINA...WHILE  
GRADUALLY DEEPENING TO ENVELOP CONTINENTAL AREA TO THE SOUTH OF  
30S. AT LOW LEVELS THIS SUSTAINS A BROAD TROUGH ACROSS THE  
SOUTHEAST PACIFIC TO EXTREME SOUTHERN PORTIONS OF CHILE/PATAGONIA.  
THIS IS TO SUSTAIN AN ELONGATED FRONT THAT IS TO STRETCH FROM THE  
SOUTH ATLANTIC TO THE SOUTH PACIFIC WHILE TRAVERSING  
PATAGONIA-SOUTHERN CHILE. THE FRONT IS TO PUSH NORTH ACROSS LA  
PAMPA ON FRIDAY...MOVING TO THE BUENOS AIRES PROVINCE-CENTRAL  
ARGENTINA ON SATURDAY. AS IT MEANDERS OVER SOUTHERN CHILE THIS IS  
TO FAVOR MODERATE RAINFALL AMOUNTS BETWEEN PUERTO MONTT AND ISLA  
DE CHILOE THROUGH THURSDAY MORNING...WITH ACCUMULATION OF 10-15MM.  
ON THURSDAY AND FRIDAY MAXIMA IN THIS AREA INCREASES TO  
20-40MM/DAY WHILE BUILDING NORTH ACROSS TEMUCO TO THE NORTH. IN  
THIS AREA EXPECTING STORM TOTAL AMOUNTS IN EXCESS OF 100MM. AS THE  
FRONT PUSHES NORTH ACROSS LA PAMPA ON SATURDAY IT IS TO SUSTAIN  
MODERATE CONVECTION WITH MAXIMA OF 20-30MM. LATER DURING THE  
WEEKEND...AS THE FRONT MEANDERS NORTH INTO MID SECTIONS OF SOUTH  
AMERICA...CONDITIONS WILL BECOME FAVORABLE FOR AN MCS TO  
DEVELOP...TO FAVOR HEAVY RAINFALL AMOUNTS.  
 
ANOTHER MID LEVEL TROUGH TO THE EAST IS TO EXTEND OVER THE SOUTH  
ATLANTIC BETWEEN 40W-10W TO 30S LATER THIS AFTERNOON. THROUGH THE  
END OF THE WEEK THE TROUGH IS TO SLOWLY DAMPEN WHILE PULLING TO  
THE EAST-SOUTHEAST. AT LOW LEVELS THE ASSOCIATED FRONT WILL TRAIL  
ACROSS THE SOUTH ATLANTIC TO THE SOUTHERN STATES OF  
BRASIL-NORTHERN ARGENTINA. THE FRONT WILL REMAIN NEARLY STATIONARY  
OVER NORTHERN ARGENTINA WHILE GRADUALLY FRONTOLIZING LATER ON  
THURSDAY...WHILE THE REMNANTS OVER THE ATLANTIC MEANDER NORTH  
ALONG THE COAST OF SAO PAULO TO RIO DE JANEIRO. THE FRONT IS TO  
THEN WEAKEN INTO A LOW/MID LEVEL TROUGH OVER SOUTHEAST BRASIL.  
OVER SOUTHEAST BRASIL THIS IS TO INITIALLY FAVOR MAXIMA OF  
20-30MM...INCREASING TO 20-40MM ON THURSDAY...AND 30-60MM ON  
FRIDAY. ON SATURDAY...UNDER INFLUENCE OF A SHORT WAVE TROUGH  
STREAMING ALONG THE SOUTHERN FRINGES OF THE SUBTROPICAL  
RIDGE...CONVECTION IS TO BECOME BETTER ORGANIZED...WITH MAXIMA  
INCREASING TO 40-80MM.  
 
AT 200 HPA...THE SUBTROPICAL RIDGE IS TO INITIALLY ANCHOR ON A  
CLOSED HIGH OVER MATO GROSSO-SANTA CRUZ IN EASTERN BOLIVIA. THIS  
IS TO EXTEND A RIDGE FROM PERU...ACROSS BOLIVIA TO SOUTHEAST  
BRASIL. THIS UNDERCUTS A CLOSED LOW TO THE NORTH OVER PARA IN  
BRASIL. THE LOW IS TO REMAIN CUTOFF FROM THE FLOW...AND AS SUCH IT  
IS FORECAST TO GRADUALLY FILL THROUGH FRIDAY AFTERNOON. AS THE  
TROUGH WEAKENS CONVECTION OVER NORTHERN SOUTH AMERICA IS TO ALSO  
WEAKEN. ACROSS PARA-AMAZONAS IN BRASIL THIS IS TO INITIALLY FAVOR  
RAINFALL AMOUNTS OF 30-60MM. THROUGH SATURDAY THIS DECREASES TO  
20-30MM. AS CONVECTION OVER THE CONTINENT WEAKENS THE UPPER LEVEL  
HIGH IS TO THEN RELOCATE ACROSS BOLIVIA TO NORTHERN CHILE...WHERE  
IT IS TO REMAIN WHILE THE RIDGE BUILDS ACROSS THE CONTINENT. THE  
RIDGE IS TO ALSO VENT CONVECTION ACROSS ECUADOR-NORTHERN PERU TO  
FAVOR MAXIMA OF 20-40MM EARLY IN THE CYCLE. THIS IS TO THEN SETTLE  
AT 15-20MM/DAY.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
RIVAS...SENAMHI (PERU)  
DAVISON...WPC (USA)  

 
 
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