360  
FXSA20 KWBC 111810  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
110 PM EST MON DEC 11 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM DEC 11 AT 0000 UTC): GLOBAL MODELS  
AND ENSEMBLE MEMBERS AGREE QUITE WELL ON FORECAST EVOLUTION AS  
THEY SHOW THE REINFORCEMENT OF A LONG-WAVE PATTERN IN SOUTHERN  
SOUTH AMERICA. FORECAST CONFIDENCE IS HIGH THROUGH 108-120 HRS AND  
STARTS DECREASING GRADUALLY AFTER.  
 
A TROUGH IN THE EASTERN PACIFIC OFF THE COAST OF CENTRAL CHILE IS  
FORECAST TO SHEAR A SHORT WAVE AXIS THAT WILL CROSS THE ANDES OF  
CENTRAL CHILE ON TUESDAY/EARLY WEDNESDAY. AS THE SHORT WAVE  
CROSSES THE ANDES...IT WILL INTERACT WITH AN UNSTABLE AIR MASS  
BREWING IN INTERIOR PORTIONS OF NORTHERN PATAGONIA. DUE TO THE  
PRESENCE OF THE SOUTH ATLANTIC CONVERGENCE ZONE (SACZ) IN TROPICAL  
PORTIONS OF THE CONTINENT...MOISTURE IN ARGENTINA WILL BE LIMITED.  
THIS WILL LIMIT THE POTENTIAL FOR ACCUMULATIONS...YET WILL ALLOW  
FOR SCATTERED THUNDERSTORM DEVELOPMENT. ON TUESDAY-EARLY  
WEDNESDAY...EXPECTING A PEAK IN ACTIVITY LATE ON TUESDAY INTO  
EARLY HOURS OF WEDNESDAY...WHERE MAXIMA WILL REACH 15-20MM/DAY. BY  
WEDNESDAY INTO EARLY THURSDAY...REMAINING CONVECTION WILL LEAD TO  
MAXIMA OF 10-15MM/DAY IN THE SOUTHERN BUENOS AIRES PROVINCE...AND  
MAXIMA OF 05-10MM OTHERWISE. THE NEW ROUND OF CONVECTION IN  
ARGENTINA IS EXPECTED DURING THE WEEKEND...WHEN MOISTURE IS  
FORECAST TO START RECOVERING AS THE SACZ COLLAPSES.  
 
ALSO IN SOUTHERN SOUTH AMERICA...A SUBTROPICAL MOISTURE  
CONNECTION/PLUME OR ATMOSPHERIC RIVER WILL ARRIVE INTO SOUTHERN  
CHILE ON WEDNESDAY. THE CONNECTION WILL BE ZONAL ACCOMPANIED BY  
850 WINDS EXCEEDING 35KT. THIS WILL LEAD TO ACCUMULATIONS OF  
20-30MM/DAY BETWEEN LOS LAGOS AND AYSEN ON WEDNESDAY-EARLY  
THURSDAY. A NEW SUBTROPICAL MOISTURE CONNECTION ARRIVES DURING  
FRIDAY...ACCOMPANIED BY A BETTER STRUCTURED UPPER TROUGH.  
EXPECTING A NEW INCREASE IN ACCUMULATIONS TO PRODUCE MAXIMA OF  
15-20MM BY FRIDAY-EARLY SATURDAY.  
 
A SYSTEM OF INTEREST IN TROPICAL SOUTH AMERICA IS THE PRESENCE OF  
THE SACZ. THIS SYSTEM IS FORECAST TO REMAIN ORGANIZED THROUGH  
WEDNESDAY...TO THEN START LOSING DEFINITION. WHILE THE SACZ  
REMAINS IN PLACE...IT WILL STIMULATE CONVECTION TO PRODUCE  
ACCUMULATIONS OF 40-80MM/DAY AND THE RISK OF MCS FORMATION IN  
EASTERN BRASIL ON MONDAY THROUGH WEDNESDAY. AMOUNTS WILL START TO  
DECREASE GRADUALLY AFTER WEDNESDAY. SIMILAR AMOUNTS ARE EXPECTED  
IN CENTRAL PERU ON MONDAY. BY TUESDAY...THE FOCUS FOR STRONG  
CONVECTION IN THE WESTERN AMAZON WILL CONCENTRATE BETWEEN NORTHERN  
PERU AND ECUADOR WHERE EXPECTING MAXIMA OF 25-50MM. AMOUNTS WILL  
DECREASE AFTER.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
RIVAS...SENAMHI (PERU)  
DAVISON/GALVEZ...WPC (USA)  

 
 
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