616  
FXSA20 KWBC 181809  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
108 PM EST MON DEC 18 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM DEC 18 AT 0000 UTC): GLOBAL MODELS  
GENERALLY AGREE ON HOW PATTERN OVER THE EASTERN PACIFIC IS GOING  
TO EVOLVE DURING THE NEXT SIX DAYS. OVER THE ATLANTIC THEY FOLLOW  
A SIMILAR EVOLUTION THROUGH 72-96 HRS...THEN DISAGREE ON THE  
AMPLITUDE OF A LONG WAVE TROUGH STREAMING ACROSS THE SOUTHERN CONE  
TO THE WESTERN ATLANTIC. BOTH EUROPEAN MODELS FAVOR A  
DEEPER/HIGHER AMPLITUDE TROUGH THAN WHAT THE GFS SUGGESTS.  
ALTHOUGH THE MEAN OF THE GLOBAL ENSEMBLES FAVOR THE GFS  
SOLUTION...THE VARIABILITY AMONG SOME OF THE ENSEMBLE MEMBERS IS  
LARGE ENOUGH TO PUT THE FORECAST IN QUESTION.  
 
OVER THE EASTERN PACIFIC...DEEPENING MID LEVEL TROUGH DOMINATES  
THE FLOW...WITH AXIS TO AMPLIFY NORTH BETWEEN 110W-70W TO 35S. AS  
IT NEARS THE SOUTHERN CONE LATER ON TUESDAY...THE TROUGH IS TO  
FOCUS ITS SHORT WAVE ENERGY OVER CHILE/ARGENTINA TO THE SOUTH OF  
40S...WHERE IT IS TO SUSTAIN HEIGHT FALLS IN EXCESS OF 200GPM. ON  
WEDNESDAY THE TROUGH IS TO SLOWLY MOVE ACROSS THE SOUTHERN  
CONE/DRAKE PASSAGE TO THE FAR WESTERN ATLANTIC...DAMPENING AS IT  
CROSSES 40W ON THURSDAY. AT LOW LEVELS THIS WILL SUSTAIN A BROAD  
TROUGH OVER THE SOUTHEAST PACIFIC THAT IS TO CENTER ON A DEEP  
OCCLUDED LOW OVER THE BELLINGSHAUSEN SEA. POLAR FRONTS WILL EMBED  
IN THIS FLOW...WITH ONE TO ENTER SOUTHERN CHILE EARLY ON TUESDAY.  
AS IT NEARS THE SOUTHERN CONE THE FRONT WILL SUSTAIN A SUBTROPICAL  
CONNECTION...ADVECTING WARM/MOIST AIR THAT IS TO CONVERGE ON THE  
SOUTHERN REGIONS OF CHILE. THE FRONT THEN CONTINUES ACROSS  
PATAGONIA ON TUESDAY...AND ON WEDNESDAY IT MOVES NORTH ACROSS LA  
PAMPA TO THE CENTRAL PROVINCES IN ARGENTINA. OVER SOUTHERN CHILE  
THIS IS TO FAVOR MODERATE RAINFALL AMOUNTS ON  
TUESDAY-WEDNESDAY...WITH ACCUMULATION OF 20-30MM. DUE TO STRONG  
TOPOGRAPHICAL FORCING LOCALLY HIGHER AMOUNTS ARE HIGHLY PROBABLE.  
OVER CENTRAL ARGENTINA THE MAXIMA ON WEDNESDAY AFTERNOON WILL PEAK  
AT 20-40MM...WHILE ON THURSDAY MORNING IT INCREASES TO 25-50MM AS  
AN MCS FORMS ALONG THE SURFACE FRONT. ON BOTH DAYS THERE IS AN  
ELEVATED RISK OF SEVERE THUNDERSTORMS BUILDING AHEAD OF THE  
SURFACE FRONT...WITH FORECAST INDICES SHOWING A HIGHER POTENTIAL  
FOR LARGE HAIL ON WEDNESDAY.  
 
FARTHER NORTH...AN ELONGATED FRONT FLANKS THE EASTERN-NORTHERN  
FRINGES OF A SURFACE RIDGE ON THE NORTHERN STREAM OFF THE COAST OF  
ARGENTINA AND URUGUAY. THE FRONT STRETCHES ACROSS SOUTHERN STATES  
OF BRASIL TO NORTHERN ARGENTINA. THIS BOUNDARY...AS IT  
PERSISTS...WILL BE THE FOCUS OF LOW LEVEL MOISTURE CONVERGENCE. AT  
500 HPA...A SHORT WAVE TROUGH ON THE NORTHERN STREAM IS TO ENTER  
CENTRAL CHILE LATER TODAY...TO MOVE ACROSS RIO DE LA PLATA  
BASIN/SOUTHERN BRASIL ON TUESDAY EVENING. ON WEDNESDAY-THURSDAY  
THE SHORT WAVE PERTURBATION IS TO THEN MEANDER EAST ACROSS 40W. AS  
IT ENTERS THE CONTINENT THE MID LEVEL VORTEX IS TO INTERACT WITH  
THE SURFACE FRONT. ON THE SOUTHERN STATES OF BRASIL AND PARAGUAY  
THIS IS TO INITIALLY FAVOR MAXIMA OF 15-20MM. ON TUESDAY AND  
WEDNESDAY THIS IS TO BECOME BETTER ORGANIZED...TO SUSTAIN MODERATE  
TO HEAVY CONVECTION WITH MAXIMA OF 25-50MM.  
 
DEEPENING SHORT WAVE TROUGH ON THE SOUTHERN STREAM IS TO  
FOLLOW...WITH MID LEVEL TROUGH TO MOVE ACROSS 90W AND SOUTH OF 35S  
ON WEDNESDAY... EARING THE SOUTHERN REGIONS OF CHILE ON THURSDAY  
AFTERNOON. ON FRIDAY THIS IS TO MOVE ACROSS 60W TO THE CENTRAL  
PROVINCES IN ARGENTINA. THE ASSOCIATED FRONT RACES ACROSS THE  
SOUTHEAST PACIFIC TO SOUTHERN PORTIONS OF CHILE EARLY ON THURSDAY  
MORNING...MOVING ACROSS PATAGONIA TO THE WESTERN ATLANTIC LATER IN  
THE DAY. THIS IS TO INTERACT WITH THE AFOREMENTIONED BOUNDARY TO  
THE NORTH...DRIVING THIS FRONT FARTHER NORTH ACROSS RIO DE LA  
PLATA BASIN-CENTRAL ARGENTINA ON THURSDAY-FRIDAY. ON THE SOUTHERN  
REGIONS OF CHILE THE FRONT IS TO FAVOR SCATTERED CONVECTION WITH  
MAXIMA OF 15-25MM ON THURSDAY-FRIDAY...WHILE OVER CENTRAL  
ARGENTINA IT IS TO TRIGGER RAINFALL MAXIMA OF 25-50MM IN  
POTENTIALLY SEVERE CONVECTION. ON FRIDAY THIS INCREASES TO 40-80MM  
WHILE CLUSTERING ACROSS RIO DE LA PLATA BASIN/URUGUAY-NORTHERN  
PROVINCES IN ARGENTINA.  
 
NORTH OVER THE CONTINENT...AT 200 HPA...A CLOSED HIGH IS TO CENTER  
OVER BOLIVIA DURING THE NEXT FEW DAYS...MEANWHILE ANCHORING A  
RIDGE TO THE NORTH OF 30S AND WEST OF 40W/45W. AS THE RIDGE  
MEANDERS OVER THE CONTINENT...AN UPPER LEVEL TROUGH IS TO AMPLIFY  
ACROSS NORTHEAST BRASIL ON TUESDAY-WEDNESDAY. A LOW IS TO THEN  
CLOSE NEAR 10S 40W ON WEDNESDAY-THURSDAY. AS THE TROUGH DEEPENS IT  
IS TO THEN ENHANCE CONVECTIVE DEVELOPMENT ALONG THE NORTH COAST OF  
BRASIL...WHERE THE MAXIMA IS TO PEAK AT 15-30MM/DAY ON  
WEDNESDAY-FRIDAY. THE RIDGE ALOFT...MEANWHILE...WILL VENT  
ORGANIZED CONVECTION ACROSS BOLIVIA-SOUTHERN JUNGLE OF  
PERU-ACRE/RONDONIA-AMAZONAS IN BRASIL...WITH MAXIMA OF 35-70MM ON  
MONDAY THROUGH WEDNESDAY. ON THURSDAY THIS DECREASES TO  
20-40MM/DAY...WHILE ON FRIDAY THE MAXIMA WILL PEAK AT 20-30MM. THE  
RIDGE ALOFT IS TO ALSO VENT DEEP CONVECTION ACROSS THE SOUTHERN  
SIERRA OF PERU-ALTIPLANO OF BOLIVIA...WITH MAXIMA OF 10-15MM/DAY.  
ON WEDNESDAY CONVECTION IS TO BECOME BETTER ORGANIZED...WITH  
MAXIMA OF 15-30MM ALONG THE PERU/BOLIVIA BORDER...WHILE OVER THE  
ALTIPLANO TO THE SOUTH THE MAXIMA WILL PEAK AT 20-40MM.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
RIVAS...SENAMHI (PERU)  
DAVISON/GALVEZ...WPC (USA)  

 
 
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