344  
FXSA20 KWBC 191754  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
1254 PM EST TUE DEC 19 2017  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM DEC 19 AT 0000 UTC): THE GLOBAL  
MODELS REMAIN IN GENERAL AGREEMENT ON SYNOPTIC PATTERN EVOLUTION  
DURING THE NEXT SIX DAYS. DIFFERENCES AMONG THE MODELS LIMIT TO  
SPEED/AMPLITUDE OF SHORT WAVE PERTURBATIONS ON THE NORTHERN AND  
SOUTHERN STREAM FLOW ENTERING THE FORECAST DOMAIN LATER IN THE  
WEEK. OVERALL CONFIDENCE IN THE FORECAST IS HIGH.  
 
BROAD MID LEVEL TROUGH ON THE CENTRAL SOUTH ATLANTIC SUSTAINS AN  
ELONGATED SURFACE FRONT THAT EXTENDS WEST ACROSS THE SOUTHERN  
STATES OF BRASIL TO NORTHERN ARGENTINA. A MID LEVEL SHORT WAVE  
TROUGH PULLS ACROSS SOUTHERN BRASIL EARLY IN THE CYCLE...TO THEN  
MEANDER OFF THE COAST WHILE GRADUALLY WEAKENING LATER ON THURSDAY.  
AS THE TROUGH ALOFT WEAKENS THE SURFACE FRONT OVER THE CONTINENT  
IS FORECAST TO FRONTOLIZE EARLY ON THURSDAY MORNING.  
MEANWHILE...OVER SOUTHERN BRASIL-MESOPOTAMIA VALLEY IN ARGENTINA  
AND URUGUAY THIS IS TO FAVOR MODERATE SCATTERED CONVECTION WITH  
DAILY MAXIMA OF 20-40MM. LOCALLY HIGHER AMOUNTS ARE POSSIBLE OVER  
SOUTHERN BRASIL DUE TO STRONG TOPOGRAPHICAL FORCING.  
 
CONSISTENT WITH PREVIOUS FORECAST...A DEEPENING MID/UPPER LEVEL  
TROUGH IS TO DOMINATE THE FLOW OVER THE SOUTHEAST PACIFIC...WITH  
AXIS TO AMPLIFY NORTH BETWEEN 100W-65W TO 33S LATER THIS EVENING.  
AS IT NEARS THE SOUTHERN CONE...THE TROUGH WILL FOCUS ITS SHORT  
WAVE ENERGY OVER CHILE/ARGENTINA TO THE SOUTH OF 40S...WHERE IT IS  
TO INDUCE HEIGHT FALLS IN EXCESS OF 200GPM. ON  
WEDNESDAY-TO-THURSDAY 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 SUSTAINS A BROAD TROUGH OVER THE SOUTHEAST PACIFIC THAT  
CENTERS ON A DEEP OCCLUDED LOW OVER THE BELLINGSHAUSEN SEA. POLAR  
FRONTS WILL REVOLVE AROUND THIS TROUGH...WITH ONE ENTERING  
SOUTHERN CHILE-PATAGONIA IN ARGENTINA EARLY IN THE CYCLE. THIS IS  
TO SUSTAIN A SUBTROPICAL CONNECTION...ADVECTING WARM/MOIST AIR  
THAT CONVERGES ON THE SOUTHERN REGIONS OF CHILE. THE FRONT THEN  
CONTINUES ACROSS PATAGONIA LATER TODAY...AND ON WEDNESDAY IT MOVES  
NORTH ACROSS LA PAMPA TO THE CENTRAL PROVINCES IN ARGENTINA-LA  
SERENA CHILE...WHERE IT IS TO STALL. AS IT MOVES ACROSS LA PAMPA  
TO CENTRAL ARGENTINA THIS BOUNDARY WILL BECOME THE FOCUS OF LOW  
LEVEL MOISTURE CONVERGENCE. ON THE SOUTHERN REGIONS OF CHILE THIS  
IS TO SUSTAIN MODERATE CONVECTION WITH ACCUMULATION OF 20-30MM  
THROUGH WEDNESDAY MORNING...AND 20-40MM ON WEDNESDAY TO THURSDAY.  
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.  
 
A DEEPENING SHORT WAVE TROUGH ON THE SOUTHERN STREAM IS TO CLOSELY  
FOLLOW...WITH MID LEVEL TROUGH TO MOVE ACROSS 90W AND SOUTH OF 35S  
ON THURSDAY. THIS ENTERS THE SOUTHERN REGIONS OF CHILE ON THURSDAY  
EVENING...AND ON FRIDAY IT MOVES ACROSS PATAGONIA TO LA  
PAMPA/BUENOS AIRES PROVINCE IN ARGENTINA. ALTHOUGH THE GLOBAL  
MODELS AGREE ON THE TIMING OF THIS FEATURE...THE ECMWF AND UKMET  
GENERALLY FAVOR A DEEPER TROUGH THAN WHAT THE GFS SUGGESTS. THE  
ENSEMBLE MEMBERS SPLIT BETWEEN THESE OPTIONS...WITH THE EUROPEAN  
ENSEMBLES SIDING WITH THE ECMWF/UKMET MODELS WHILE THE NCEP  
ENSEMBLES PROVIDE SUPPORT TO THE GFS SOLUTION. THUS EXPECTING  
FURTHER CORRECTION TO THIS FEATURE IN THE NEXT FEW RUNS. AT LOW  
LEVELS...THIS WILL SUSTAIN A SURFACE FRONT OVER SOUTHERN CHILE  
EARLY ON THURSDAY MORNING...TO TRIGGER MODERATE CONVECTION WITH  
MAXIMA OF 20-40MM. THIS THEN RACES NORTH ACROSS PATAGONIA-LA PAMPA  
ON FRIDAY MORNING... MERGING WITH THE OLD BOUNDARY TO THE NORTH  
LATER IN THE DAY. AS IT RACES ACROSS CENTRAL ARGENTINA THIS WILL  
SUSTAIN MODERATE TO HEAVY CONVECTION ON THURSDAY WITH MAXIMA OF  
25-50MM. ON FRIDAY THIS IS TO SUSTAIN AN MCS TO THE NORTH...WITH  
MAXIMA INCREASING TO 40-80MM IN POTENTIALLY SEVER CONVECTION. MOST  
ACTIVE IS TO CLUSTER ACROSS URUGUAY-ENTRE RIOS-CORDOBA IN  
ARGENTINA.  
 
NORTH OVER THE CONTINENT...AT 200 HPA...A CLOSED HIGH CENTERING  
OVER BOLIVIA IS TO ANCHOR A BROAD RIDGE OVER THE CONTINENT TO THE  
NORTH OF 35S AND WEST OF 50W. 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...WITH  
LOCALLY HIGHER AMOUNTS HIGHLY PROBABLE. THE RIDGE  
ALOFT...MEANWHILE...WILL VENT ORGANIZED CONVECTION ACROSS  
BOLIVIA-SOUTHERN JUNGLE OF PERU-ACRE/RONDONIA-AMAZONAS IN  
BRASIL...WITH MAXIMA OF 35-70MM/DAY EXPECTED THROUGH THURSDAY  
MORNING. ON THURSDAY-SATURDAY THIS DECREASES TO 20-40MM/DAY. 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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