759  
FXSA20 KWBC 231615  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1214 PM EDT THU MAR 23 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM MARCH 23 AT 0000 UTC): MODELS ARE  
SLOWLY REACHING A CONSENSUS ON HOW PATTERN OVER THE SOUTHERN CONE  
IS GOING TO EVOLVE DURING THE MEDIUM RANGE PERIOD. MODELS ARE NOW  
TRENDING IN SUPPORT OF A DEEPER/SLOWER TROUGH TO REMAIN OFF THE  
SOUTH COAST OF CHILE. AS THE MODELS FALL IN AGREEMENT...OUR  
CONFIDENCE IN THE FORECAST IS HIGHER THAN BEFORE.  
 
DEEPENING MID/UPPER LEVEL TROUGH MOVES ACROSS THE EASTERN PACIFIC  
EARLY ON FRIDAY MORNING. AS IT APPROACHES THE SOUTHERN CONE...THE  
TROUGH WILL THEN CLASH WITH A RIDGE TO THE EAST. THE RIDGE IS TO  
EXTEND SOUTH ALONG 60W TO THE ANTARCTIC PENINSULA. AS THEY  
COLLIDE...THE TROUGH WILL SPLIT IN TWO...WITH A SHORT WAVE TROUGH  
TO THE SOUTH TO QUICKLY SHEAR UNDER THE RIDGE AS IT STREAMS ACROSS  
THE DRAKE PASSAGE ON SATURDAY EVENING. THE REMNANTS TO THE  
NORTH... MEANWHILE...ARE TO MEANDER OFF THE SOUTH COAST OF CHILE  
WHILE EVOLVING INTO A CLOSED LOW JUST WEST OF ISLA DE CHILOE ON  
SATURDAY. AT LOW LEVELS...THE TROUGH ALOFT WILL SUSTAIN A  
PROGRESSIVE FRONT THAT ENTERS SOUTHERN CHILE EARLY ON FRIDAY  
MORNING. DEEPENING MID LEVEL LOW WILL THEN SUPPORT CYCLOGENESIS  
ALONG THE MEANDERING FRONT AS IT LIES OFF THE SOUTH COAST OF  
CHILE. THE LOW IS FORECAST TO GRADUALLY OCCLUDE ON SATURDAY. THE  
SURFACE LOW AND ASSOCIATED FRONT WILL THEN MOVE EAST ACROSS  
PATAGONIA/LA PAMPA IN ARGENTINA ON SUNDAY. AS IT ENTERS SOUTHERN  
CHILE ON FRIDAY THIS IS TO ENHANCE MOISTURE CONVERGENCE TO TRIGGER  
SCATTERED CONVECTION WITH ACCUMULATION OF 05-10MM. ON SATURDAY TO  
SUNDAY THIS INCREASES TO 15-20MM...WITH SIMILAR AMOUNTS OVER  
NORTHERN PATAGONIA-LA PAMPA-SOUTHERN CUYO IN ARGENTINA ON SUNDAY.  
 
AS THE MID LEVEL LOW/TROUGH DEEPENS TO THE SOUTH...IT IS TO  
INTERACT WITH A MEANDERING SHORT WAVE TROUGH OFF THE CENTRAL COAST  
OF CHILE. THE SHORT WAVE TROUGH IS TO GRADUALLY WEAKEN THROUGH  
SATURDAY EVENING WHILE SHEARING SHORT WAVE ENERGY ACROSS THE  
CENTRAL/NORTHERN ANDES INTO NORTHERN ARGENTINA/MID SECTIONS OF  
SOUTH AMERICA. THE INFLOW OF SHORT WAVE ENERGY IS TO INITIALLY  
FAVOR SCATTERED CONVECTION ACROSS RIO DE LA PLATA BASIN/SOUTHERN  
STATES OF BRASIL...WITH ACCUMULATION OF 10-15MM/DAY EXPECTED  
THROUGH FRIDAY EVENING. A NORTHERLY LOW LEVEL JET ACROSS EASTERN  
BOLIVIA/PARAGUAY IS TO THEN ENHANCE MOISTURE TRANSPORT/CONVERGENCE  
ACROSS THE NORTHWEST PROVINCES IN ARGENTINA ON  
SATURDAY-SUNDAY...WITH PRECIPITABLE WATER CONTENT TO INCREASE TO  
25-50MM. THIS WILL COMBINE WITH INFLOW OF SHORT WAVE ENERGY ALOFT  
TO TRIGGER SCATTERED CONVECTION ACROSS PARAGUAY-SOUTHERN BOLIVIA  
AND NORTHERN ARGENTINA...WITH RAINFALL MAXIMA OF 20-35MM IN  
POTENTIALLY SEVERE CONVECTION. ON SUNDAY THIS IS TO INCREASE TO  
MAXIMA OF 20-45MM AS ENHANCED BY A SUBTROPICAL JET MAXIMA  
STREAMING ACROSS MID SECTIONS OF SOUTH AMERICA.  
 
MODELS THEN FORECAST THE NEXT MID/UPPER LEVEL TROUGH TO STREAM  
ACROSS THE SOUTH PACIFIC...NEARING 120W LATER ON FRIDAY...AND  
PULLING ACROSS 95W ON SATURDAY. BUILDING RIDGE TO THE WEST IS TO  
THEN SUSTAIN THE RAPID AMPLIFICATION OF THIS TROUGH ALONG THE  
SOUTH COAST OF CHILE ON SATURDAY. AS THE RIDGE TO THE WEST  
UNDERCUTS THE LONG WAVE TROUGH...IT IS FORECAST TO EVOLVE INTO A  
CLOSED LOW THAT MEANDERS OFF THE SOUTH COAST OF CHILE ON  
SUNDAY-MONDAY. NOTE THAT THIS IS A MUCH SLOWER EVOLUTION THAN WHAT  
THE MODELS PREVIOUSLY SUGGESTED. AT LOW LEVELS IT IS TO SUSTAIN A  
PROGRESSIVE FRONT THAT REACHES SOUTHERN CHILE LATER ON SUNDAY. IN  
A SIMILAR EVOLUTION TO THE FORMER...A LOW ALONG THIS AXIS IS TO  
RAPIDLY OCCLUDE ON SUNDAY WHILE MEANDERING OFF THE SOUTH COAST OF  
CHILE. THE DEEPENING TROUGH WILL THEN SUSTAIN A SUBTROPICAL  
CONNECTION...ENHANCING MOISTURE CONVERGENCE ACROSS SOUTHERN  
PORTIONS OF CHILE...WHERE WE EXPECT MAXIMA OF 15-20MM.  
 
PATTERN OVER NORTHERN SOUTH AMERICA IS TO REMAIN UNDER INFLUENCE  
OF THE SUBTROPICAL RIDGE THROUGHOUT THE MEDIUM RANGE PERIOD...WITH  
AXIS TO ANCHOR ON A 200 HPA HIGH THAT IS TO MEANDER BETWEEN MATO  
GROSSO IN BRASIL AND NORTHERN BOLIVIA. THE RIDGE ALOFT IS TO VENT  
EQUATORIAL TROUGH/ITCZ RELATED CONVECTION OVER NORTHERN SOUTH  
AMERICA. MOST ACTIVE EXPECTED TO THE NORTH OF 05S. ALONG THE NORTH  
COAST OF BRASIL THIS IS TO GENERALLY FAVOR SCATTERED DEEP  
CONVECTION WITH DAILY MAXIMA OF 20-35MM. OVER NORTHERN  
PARA-AMAZONAS/RORAIMA IN BRASIL TO SOUTHERN COLOMBIA EXPECTING  
MAXIMA OF 25-50MM DURING THE NEXT FEW DAYS. LESSER AMOUNTS TO THE  
SOUTH WILL AFFECT SOUTHERN AMAZONAS/ACRE-RONDONIA AND THE CENTRAL  
STATES OF BRASIL...WITH DAILY MAXIMA IN THIS AREA TO PEAK AT  
20-30MM. ON THE SIERRA OF PERU AND HIGH JUNGLE THE DAILY MAXIMA  
WILL PEAK AT 10-15MM/DAY. HIGHER AMOUNTS...HOWEVER...ARE EXPECTED  
ALONG THE NORTH COAST AND WESTERN ECUADOR...WHERE UNDER INFLUENCE  
OF THE SOUTHERN BRANCH OF THE EASTERN PACIFIC ITCZ EXPECTING  
ORGANIZED DEEP CONVECTION OVER THE NEXT COUPLE OF DAYS. THROUGH  
FRIDAY MORNING THIS IS TO FAVOR MAXIMA OF 50-100MM...WHILE ON  
FRIDAY TO SATURDAY THE MAXIMA IS TO PEAK AT 35-70MM. WEAKENING OF  
THE ONSHORE FLOW ON SATURDAY-SUNDAY WILL RESULT IN A FURTHER  
REDUCTION...WITH MAXIMA OF 25-50MM.  
 
ABDEMUR...FAA (ARGENTINA)  
TEPES...DMC (CHILE)  
DAVISON...WPC (USA)  

 
 
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