537  
FXSA20 KWBC 161721  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1221 PM EST FRI FEB 16 2018  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM FEB 16 AT 0000 UTC): CONFIDENCE IN  
THE FORECAST CONTINUES DETERIORATING AFTER 84-96 HRS HRS TO THE  
SOUTH OF 30S. MODELS AGREE ON THE EVOLUTION OF THE LARGE SCALE  
PATTERN THROUGH 120 HRS...YET THEY STRUGGLE WITH THE SHORT WAVES  
IN THE ATLANTIC AND AROUND A LARGE LOW IN THE PACIFIC EARLIER.  
 
INTERMITTENT MOISTURE PLUMES ARE ARRIVING INTO SOUTHERN CHILE. THE  
NEXT PLUME WILL ARRIVE ON FRIDAY EVENING TO SUSTAIN A BRIEF PERIOD  
OF MODERATE RAINFALL. THIS WILL SUSTAIN ACCUMULATIONS OF  
15-20MM/DAY DURING FRIDAY-EARLY SATURDAY IN AYSEN AND NORTHERN  
MAGALLANES. AFTER THIS PERIODS...ACTIVITY WILL CONSTRAIN TO COLD  
AIR CUMULUS. THIS WILL SUSTAIN MAXIMA OF 10-15MM/DAY BY  
SATURDAY-SUNDAY...DECREASING AFTER. A NEW INCREASE IS EXPECTED ON  
TUESDAY AS A COLD AIR MASS ACCOMPANIED BY A COLD AIR CUMULUS  
PATTERN APPROACHES FROM THE WEST...YET DETAILS ON THE EXACT  
ARRIVAL TIME ARE STILL UNCERTAIN.  
 
TO THE EAST ACROSS ARGENTINA...MOISTURE WILL GRADUALLY RETURN FROM  
THE CYCLE AS A NORTHERLY LOW-LEVEL JET PERSISTS FROM NORTHERN  
ARGENTINA INTO PATAGONIA. A WEAK SHORT WAVE CROSSES THE ANDES ON  
FRIDAY. AS A RESPONSE...ISOLATED THUNDERSTORMS WILL INCREASE IN  
COVERAGE THROUGH FRIDAY. BY FRIDAY-SATURDAY EXPECTING MAXIMA  
PEAKING AT 15-25MM/DAY IN INTERIOR PORTIONS OF CENTRAL AND  
NORTHERN ARGENTINA...WHILE EXPECTING MAXIMA OF 10-15MM/DAY IN LA  
PAMPA/NORTHERN PATAGONIA. A NEW BURST IN ACTIVITY IS EXPECTED BY  
SUNDAY-MONDAY IN ASSOCIATION WITH A COLD FRONT FORECAST TO EXTEND  
ACROSS NORTHERN PATAGONIA-CUYO BY SUNDAY EVENING. THIS WILL  
SUSTAIN MAXIMA OF 15-25MM/DAY...WITH THE RISK OF ISOLATED SEVERE  
CONVECTION. ACCUMULATIONS WILL DECREASE TO 10-15MM/DAY BY  
MONDAY-TUESDAY IN CENTRAL ARGENTINA...IN AREAS TO THE NORTH OF THE  
SURFACE FRONT.  
 
IN CENTRAL SOUTH AMERICA...DEEP-LAYER MOISTURE WILL BUILD THROUGH  
THE CYCLE. MODELS HAVE GAINED CONFIDENCE RESOLVING AN UPPER TROUGH  
ARRIVING FROM THE SOUTHWEST AND DEEPENING FROM FRIDAY THROUGH  
SATURDAY. INITIALLY...THIS WILL SUSTAIN MAXIMA OF 20-40MM/DAY IN  
EASTERN BOLIVIA INTO THE NORTHERN CHACO. THIS WILL INCREASE TO  
30-60MM/DAY BY SATURDAY-SUNDAY WITH THE RISK OF MCS FORMATION IN  
THE CHACO. THE EVENT WILL PEAK ON SUNDAY-MONDAY WHEN EXPECTING  
NOCTURNAL MCS FORMATION IN PARAGUAY AND MAXIMA OF 50-100MM/DAY. BY  
MONDAY-TUESDAY EXPECTING ENHANCED CONVECTION STRETCHING FROM  
EASTERN PARAGUAY INTO NORTHERN URUGUAY AND SOUTHERN BRASIL...TO  
PRODUCE SCATTERED MAXIMA OF 30-60MM/DAY.  
 
IN TROPICAL SOUTH AMERICA...THE SUBTROPICAL UPPER RIDGE/BOLIVIAN  
HIGH IS FORECAST TO MEANDER OVER SOUTHERN PERU/PACIFIC OCEAN EARLY  
IN THE CYCLE TO THEN WEAKEN AND LOSE STRUCTURE. BY SATURDAY  
EVENING...MODELS SUGGEST THE RIDGE CENTERING IN SOUTHWESTERN  
PERU...WHILE A DISORGANIZED PATTERN EXTENDS ACROSS THE AMAZON  
BASIN. THIS SHOULD LEAD TO A DECREASE IN ACCUMULATIONS IN THE  
CENTRAL/SOUTHERN AMAZON BY THE LATE WEEKEND...AND A MEANDERING OF  
STRONGEST CONVECTION INTO THE NORTHERN AMAZON. MODELS SUGGEST THE  
RIDGE REFORMING IN MATO GROSSO-SOUTHEASTERN BRASIL DURING EARLY  
NEXT WEEK. ALSO AT UPPER LEVELS...FAVORABLE PATTERN FOR UPPER  
DIVERGENCE WILL CONTINUE...FIRST AIDED BY THE INTERACTION OF A  
NORTHERN HEMISPHERE RIDGE AND ONE IN THE SOUTHERN HEMISPHERE...AND  
THEN BY THE SUBTROPICAL RIDGE BUI9LDING FROM THE WEST-SOUTHWEST.  
AT LOW-LEVELS...ENHANCED MOISTURE CONVERGENCE DUE TO AN ACTIVE  
ATLANTIC ITCZ WILL STIMULATE CONVECTION THROUGH THE CYCLE TO  
PRODUCE MAXIMA OF 20-60MM/DAY THROUGH EARLY SUNDAY. THIS WILL  
DECREASE TO 20-40MM/DAY AFTER WHILE ACTIVITY RELOCATES FROM CEARA  
TO THE WEST AND AREAS TO THE EAST OF 40W DRY OUT. WEST ACROSS THE  
CONTINENT...ACTIVITY WILL MEANDER NORTH EARLY IN THE CYCLE TO  
PRODUCE MAXIMA OF 20-40MM/DAY FROM PARA INTO EASTERN ECUADOR IN  
BOTH FRIDAY AND SATURDAY. A STRENGTHENING OF THE LOW-LEVEL JET IN  
COLOMBIA WILL LEAD TO A DRYING TREND AFTER. AFTER SATURDAY...THE  
FOCUS FOR STRONGEST CONVECTION WILL CONCENTRATE IN THE SOUTHERN  
AND CENTRAL AMAZON...AND REACH 20-35MM/DAY.  
 
IN THE ALTIPLANO...EXPECTING A GRADUAL DECREASE IN ACCUMULATIONS  
AS THE SUBTROPICAL UPPER RIDGE RELOCATES AND FAVORS THE  
DEVELOPMENT OF MID-UPPER WESTERLIES MEANDERING NORTHWARD FROM  
SOUTHERN BOLIVIA ON WEDNESDAY...TO THE LAKE TITICACA REGION BY  
SUNDAY. DAILY ACCUMULATIONS WILL BE IN THE 10-15MM/DAY RANGE WHERE  
CONVECTION OCCURS. LOCALLY HIGHER AMOUNTS ARE EXPECTED IN SOUTHERN  
PERU ON THURSDAY...AND THE EASTERN CORDILLERA OF SOUTHERN PERU AND  
BOLIVIA ON FRIDAY...WHERE MAXIMA WILL REACH 15-25MM/DAY. LARGER  
AMOUNTS ARE EXPECTED IN NORTHERN PERU WHERE MAXIMA WILL BE IN THE  
15-30MM/DAY RANGE ON A DAILY BASIS.  
 
IN ECUADOR...PLENTIFUL MOISTURE WILL ALLOW FOR MODERATE RAINFALL  
AMOUNTS THROUGH THE CYCLE...EVEN WHEN AN ELEVATED CAP IS FORECAST  
TO PERSIST. ACTIVITY WILL PEAK ON SATURDAY TO PRODUCE MAXIMA OF  
20-50MM/DAY. OTHERWISE...EXPECTING MAXIMA GENERALLY UNDER 30MM/DAY.  
 
BALBINO...INMET (BRASIL)  
POMIER...SENAMHI (BOLIVIA)  
GALVEZ...WPC (USA)  

 
 
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