113  
FXSA20 KWBC 271731  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1230 PM EST MON NOV 27 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM NOV 27 AT 0000 UTC): GOOD AGREEMENT  
AMONG THE GLOBAL MODEL GUIDANCE THROUGH 84-96 HRS. THEY THEN  
DEVELOP SHORT WAVE PATTERN DIFFERENCES DURING MEDIUM RANGE  
PERIOD...WITH THE ECMWF FAVORING A DEEPER TROUGH TO MEANDER OFF  
THE SOUTHEAST COAST OF BRASIL BY 96-120 HRS. THE MEAN OF THE  
GLOBAL ENSEMBLES...HOWEVER... TEND TO FAVOR A FASTER EVOLUTION AS  
SUGGESTED BY THE GFS AND THE UKMET.  
 
PROGRESSIVE MID LEVEL TROUGH ON THE SOUTHERN STREAM IS TO MOVE  
ACROSS 105W TO 30S LATER TODAY...CROSSING 80W TO THE DRAKE  
PASSAGE-SOUTHERN CHILE EARLY ON WEDNESDAY MORNING. THE TROUGH IS  
TO THEN DAMPEN AS IT PRESSES AGAINST A SHORT WAVE RIDGE TO THE  
EAST. AT LOW LEVELS THIS IS TO INTERACT WITH AN ELONGATED FRONT  
EXTENDING ACROSS PATAGONIA-SOUTHERN CHILE TO THE EASTERN PACIFIC.  
THE MID LEVEL TROUGH INDUCES A FRONTAL WAVE ALONG THIS BOUNDARY  
LATER TODAY...THAT IS TO DEEPEN/OCCLUDE AS IT CROSSES THE DRAKE  
PASSAGE ON TUESDAY. THE DEEP OCCLUDED LOW MOVES TO THE WEDDELL SEA  
ON WEDNESDAY EVENING...WHILE THE ASSOCIATED FRONT MEANDERS NORTH  
ACROSS PATAGONIA TO LA PAMPA IN ARGENTINA-SOUTHERN REGIONS OF  
CHILE. ACROSS SOUTHERN CHILE/TIERRA DEL FUEGO THIS IS TO FAVOR  
LIGHT CONVECTION WITH ACCUMULATION OF 05-10MM/DAY OVER THE NEXT  
COUPLE OF DAYS...DECREASING TO 00-05MM/DAY LATER ON WEDNESDAY.  
HIGHER AMOUNTS ARE EXPECTED OVER CENTRAL ARGENTINA ON  
WEDNESDAY...WHERE IN PREFRONTAL CONVECTION EXPECTING MAXIMA OF  
20-30MM.  
 
A SECONDARY MID LEVEL PERTURBATION IS TO FOLLOW...TO PULL ACROSS  
100W/105W ON TUESDAY...AND ALONG 80W TO 30S ON WEDNESDAY. EARLY ON  
THURSDAY IT IS TO ENTER SOUTHERN REGIONS OF CHILE/WESTERN  
PATAGONIA IN ARGENTINA...TO MOVE OFF THE COAST TOT EH WESTERN  
ATLANTIC LATER IN THE DAY. AT LOW LEVELS THIS IS TO SUSTAIN A  
PROGRESSIVE FRONT THAT ENTERS SOUTHERN CHILE LATER ON  
WEDNESDAY/EARLY ON THURSDAY MORNING. THIS IS TO ALSO FAVOR LIGHT  
CONVECTION AS IT ENTERS SOUTHERN CHILE LATER IN THE WEEK.  
 
ALSO AT 500 HPA...A LONG WAVE TROUGH EXTENDS OVER THE WESTERN  
ATLANTIC TO CENTRAL PROVINCES IN ARGENTINA. THIS IS PHASING WITH A  
SHORT WAVE TROUGH THAT LIES TO THE EAST-NORTHEAST. AS THEY  
INTERACT...THE TROUGH IS TO SPLIT IN TWO....WITH ITS SOUTHERN HALF  
TO CONTINUE ACROSS THE SOUTH ATLANTIC WHILE THE NORTHERN HALF  
MEANDERS OFF THE SOUTHERN/SOUTHEASTERN COAST OF BRASIL. THE LATTER  
EVOLVES INTO A CLOSED LOW ON TUESDAY...WHEN IT IS TO CENTER NEAR  
40S 35W. THIS IS TO REMAIN NEARLY STATIONARY THROUGH THE END OF  
THE WEEK. AT LOW LEVELS...AN ELONGATED FRONT EXTENDS ACROSS THE  
SOUTH ATLANTIC TO SOUTHEAST BRASIL-CHACO PARAGUAYO...WHERE IT IS  
TO REMAIN NEARLY STATIONARY DURING THE NEXT FEW DAYS. A SECONDARY  
BOUNDARY LIES TO THE SOUTH...EXTENDING OVER THE SOUTH ATLANTIC TO  
SOUTHERN BRASIL-URUGUAY-CENTRAL ARGENTINA. ON TUESDAY THIS SURGES  
ACROSS SOUTHERN BRASIL-PARAGUAY...REINFORCING THE OLD BOUNDARY TO  
THE NORTH OVER BRASIL. AS THE FRONT MEANDERS OVER CENTRAL  
ARGENTINA EARLY IN THE CYCLE IT IS TO FAVOR SCATTERED CONVECTION  
BETWEEN CORDOBA-MENDOZA...WITH ACCUMULATION OF 05-10MM/DAY. MOST  
ACTIVE IS EXPECTED ALONG THE OLD BOUNDARY TO THE NORTH. ACROSS  
NORTHERN SAO PAULO-MINAS GERAIS TO MATO GROSSO DO SUL THIS IS TO  
SUSTAIN SCATTERED DEEP CONVECTION WITH ACCUMULATION OF 25-50MM. ON  
TUESDAY-WEDNESDAY THERE IS A LULL IN ACTIVITY...WITH MAXIMA  
DECREASING TO 20-35MM/DAY. BUT LATER IN THE WEEK IT INCREASES  
AGAIN TO 25-50MM/DAY. HIGHER AMOUNTS ARE EXPECTED ACROSS  
PARAGUAY-MATO GROSSO SUL IN BRASIL...WITH MAXIMA OF 50-100MM ON  
WEDNESDAY AND 75-125MM ON THURSDAY.  
 
ALSO AT MID LEVELS...A SHORT WAVE TROUGH ON THE NORTHERN STREAM  
MEANDERS OVER THE EASTERN PACIFIC WHILE REMAINING JUST WEST OF  
CENTRAL OF CHILE. LATER ON TUESDAY/EARLY WEDNESDAY MORNING THE MID  
LEVEL VORTEX IS TO MEANDER EAST INTO CENTRAL REGIONS OF CHILE...TO  
SPILL ACROSS THE ANDES TO CENTRAL ARGENTINA LATER IN THE DAY. THIS  
IS TO FAVOR SCATTERED CONVECTION...WITH MAXIMA OF 20-30MM ON  
WEDNESDAY AND ON THURSDAY. THIS IS TO THEN INTERACT WITH  
AFOREMENTIONED FRONT OVER BRASIL TO PROVIDE DYNAMICAL SUPPORT TO  
ORGANIZED CONVECTION.  
 
NORTH OVER THE CONTINENT...AT 200 HPA...A HIGH OVER NORTHERN  
BOLIVIA-RONDONIA IN BRASIL ANCHORS A RIDGE OVER NORTHERN SOUTH  
AMERICA. A TROUGH TO THE NORTHEAST EXTENDS OVER NORTHEAST BRASIL  
WHILE CENTERING ON A CLOSED LOW NEAR RECIFE. THIS WEAKENS TO AN  
OPEN TROUGH ON WEDNESDAY...TO THEN RELOCATE OFF THE COAST INTO THE  
CENTRAL ATLANTIC ON WEDNESDAY-THURSDAY. AS IT MOVES AWAY THE RIDGE  
IS TO BECOME DOMINANT FEATURE OVER THE CONTINENT TO THE NORTH OF  
20S. THE RIDGE ALOFT IS VENTING DEEP CONVECTION ACROSS NORTHERN  
SOUTH AMERICA. OVER NORTHERN BOLIVIA-SOUTHERN PERU-ACRE/RONDONIA  
IN BRASIL THIS IS TO SUSTAIN MODERATE TO HEAVY CONVECTION...WITH  
MAXIMA OF 25-50MM. THROUGH WEDNESDAY THIS DECREASES TO  
15-25MM/DAY. OVER NORTHERN PERU-SOUTHERN COLOMBIA/AMAZONAS IN  
BRASIL EXPECTING MAXIMA OF 20-40MM. ON TUESDAY-WEDNESDAY THIS  
INCREASES TO 30-60MM AS AN MCS IS LIKELY TO FORM ALONG THIS  
BOUNDARY. THROUGH THURSDAY MORNING THIS DECREASES TO 15-25MM/DAY.  
OTHER CONVECTION IS EXPECTED ACROSS PARA IN BRASIL TO FAVOR  
RAINFALL AMOUNTS OF 15-25MM/DAY.  
 
OVIEDO...SHN (ARGENTINA)  
VANNUCCI...SMN (ARGENTINA)  
RIVAS...SENAMHI (PERU)  
DAVISON...WPC (USA)  

 
 
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