518  
FXSA20 KWBC 231806  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
105 PM EST TUE JAN 23 2018  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/  
 
MODEL DISCUSSION (VALID FROM JAN 23 AT 0000 UTC): AMPLIFYING MID  
LEVEL TROUGH MOVES ACROSS THE SOUTHERN CONE OF SOUTH AMERICA EARLY  
IN THE CYCLE...WITH AXIS TO MOVE OFF THE COAST OF PATAGONIA LATER  
THIS EVENING. AS IT PULLS ACROSS THE WESTERN ATLANTIC THIS IS TO  
PHASE WITH A SHORT WAVE TROUGH TO THE NORTH...TO EVOLVE INTO A  
BROAD TROUGH BETWEEN 55W-30W LATER ON WEDNESDAY. THIS IS TO THEN  
MEANDER EAST ACROSS 30W LATER ON THURSDAY. AT LOW LEVELS THE  
TROUGH ALOFT SUSTAINS A PROGRESSIVE FRONT THAT SURGES ACROSS THE  
BUENOS AIRES PROVINCE TO CENTRAL ARGENTINA LATER TODAY. ON  
WEDNESDAY IT MOVES ACROSS URUGUAY-NORTHERN ARGENTINA...WHERE IT IS  
TO THEN REINFORCE AN OLD BOUNDARY TO THE NORTH. THE LATTER EXTENDS  
WEST ACROSS THE SOUTH ATLANTIC TO SOUTHERN BRASIL-NORTHERN  
PROVINCES IN ARGENTINA. THIS BOUNDARY IS EXPECTED TO PERSIST  
THROUGH THE END OF THE WEEK. ALSO AT LOW LEVELS...A NORTHERLY FLOW  
ACROSS PARAGUAY IS TO FAVOR A WARM/MOIST ADVECTIVE PATTERN...WITH  
MOISTURE TO CONVERGE ALONG THE FRONT AS IT MEANDERS OVER SOUTHERN  
BRASIL-NORTHERN ARGENTINA. AT 250 HPA A SUBTROPICAL JET EXTENDS  
ACROSS CENTRAL CHILE AND ARGENTINA...FAVORING AN UPPER DIVERGENT  
PATTERN THAT IS TO VENT DEEP CONVECTION ALONG THIS BOUNDARY. OVER  
SOUTHERN BRASIL-CORRIENTES-MISIONES IN ARGENTINA EXPECTING  
ACCUMULATION OF 30-60MM THROUGH WEDNESDAY MORNING. ON WEDNESDAY TO  
THURSDAY ENVIRONMENTAL CONDITIONS WILL BECOME HIGHLY FAVORABLE FOR  
A MESO SCALE CONVECTIVE SYSTEM TO DEVELOP ACROSS NORTHERN  
ARGENTINA ALONG THIS BOUNDARY...WITH MAXIMA OF 40-80MM IN  
POTENTIALLY SEVERE CONVECTION. ON THURSDAY THIS DECREASES TO  
20-30MM/DAY.  
 
PROGRESSIVE MID LEVEL TROUGH WILL FOLLOW...WITH AXIS TO RACE  
ACROSS 110W LATER ON WEDNESDAY...AND ACROSS 90W ON THURSDAY. ON  
FRIDAY EVENING IT REACHES THE SOUTHERN/CENTRAL REGIONS OF  
CHILE...TO QUICKLY MOVE ACROSS THE ANDES INTO PATAGONIA-LA PAMPA  
IN ARGENTINA ON SATURDAY. NOTE THAT DURING THE 00 UTC CYCLE THE  
GFS FAVORED A DEEPER PERTURBATION THAT ON PREVIOUS RUNS...WHILE  
THE 06 AND 12 UTC RUNS OF THE MODEL FAVOR A WEAKER PERTURBATION.  
THIS VARIABILITY AMONG THE DETERMINISTIC MODELS IS ALSO EVIDENT  
AMONG THE GLOBAL ENSEMBLE MEMBERS...THUS EXPECTING FURTHER  
ADJUSTMENTS TO THIS SYSTEM IN THE NEXT FEW RUNS. AT LOW LEVELS IT  
IS TO SUSTAIN A PROGRESSIVE FRONT THAT REACHES SOUTHERN CHILE  
LATER ON THURSDAY. AS IT APPROACHES THE SOUTHERN CONE THE LOW/MID  
LEVEL TROUGH WILL SUSTAIN A SUBTROPICAL CONNECTION...FAVORING  
ADVECTION OF WARM/MOIST AIR ACROSS THE EASTERN PACIFIC TO THE  
SOUTHERN REGIONS OF CHILE. THIS WILL HELP SUSTAIN SCATTERED  
CONVECTION WITH MAXIMA OF 15-20MM ON THURSDAY TO FRIDAY...WHILE ON  
FRIDAY TO SATURDAY EXPECTING MAXIMA OF 10-15MM.  
 
NORTH OVER THE CONTINENT...AT 200 HPA...A HIGH OVER PARAGUAY IS TO  
ANCHOR THE SUBTROPICAL RIDGE OVER MID SECTIONS OF SOUTH AMERICA.  
THIS PATTERN IS TO HOLD DURING THE NEXT FOUR TO FIVE DAYS. TWO  
TROUGHS BOUNDS THE NORTHERN FRINGES OF THIS RIDGE. ONE EXTENDS TO  
THE NORTHEAST ALONG A CLOSED LOW AT 09S 86W TO NORTHERN  
PERU...WHILE THE OTHER EXTENDS OVER NORTHEAST BRASIL WHILE  
CENTERING ON A CLOSED LOW AT 09S 46W. THE TROUGHS TO THE NORTH ARE  
TO INTERACT WITH THE NEAR EQUATORIAL TROUGH OVER BRASIL. ON THE  
NORTHEAST STATES OF BRASIL EXPECTING RAINFALL AMOUNTS OF 20-35MM  
OVER THE NEXT TWO DAYS...INCREASING TO 20-45MM ON THURSDAY THROUGH  
SATURDAY MORNING. OVER PARA AND AMAZONAS IN BRASIL INITIALLY  
EXPECTING MAXIMA OF 30-60MM IN SCATTERED DEEP CONVECTION. THROUGH  
THURSDAY THIS DECREASES TO 20-35MM/DAY. THE RIDGE ALOFT IS TO ALSO  
VENT CONVECTION ON THE SOUTHERN SIERRA OF PERU AND THE ALTIPLANO  
OF BOLIVIA. INITIALLY EXPECTING MAXIMA OF 20-30MM...WHILE ON  
WEDNESDAY THE MAXIMA WILL PEAK AT 10-15MM. THROUGH SATURDAY THIS  
IS TO AGAIN INCREASES TO 15-25MM/DAY.  
 
OVIEDO...SHN (ARGENTINA)  
POMIER...SENAMHI (BOLIVIA)  
DAVISON...WPC (USA)  

 
 
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