807  
FXSA20 KWBC 181718  
PMDSA  
 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS  
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD  
118 PM EDT TUE MAR 18 2025  
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/DATA/NCCF/COM/GFS/PROD/  
 
SOUTH AMERICAN FORECAST BULLETIN FROM 18 MARCH 2025 AT 17:15 UTC  
 
ANALYSIS AND FORECAST OF LARGE SCALE CONDITIONS  
 
MJO AND TROPOSPHERIC WAVES: THE MJO CONTINUES PROPAGATING ACROSS  
PHASE 3, CONSISTENT WITH LARGE SCALE UPPER CONVERGENCE ESTABLISHED  
OVER SOUTH AMERICA. A TROPOSPHERIC KELVIN WAVE IS FORECAST TO  
ARRIVE DURING THE WEEKEND/EARLY NEXT WEEK, TO PROVIDE SOME MINOR  
ENHANCEMENT IN THE COVERAGE AND INTENSITY OF DEEP CONVECTION IN  
TROPICAL REGIONS.  
 
SST: IMPORTANT POSITIVE ANOMALIES CONTINUE ALONG EQUATORIAL  
PORTIONS OF THE EASTERN PACIFIC AND THE GULF OF PANAMA. THESE ARE,  
HOWEVER, SUPERFICIAL AND COULD BE RAPIDLY ALTERED BY CHANGES IN  
SURFACE WINDS. STILL, THEY WILL CONTINUE TO FUEL CONVECTION AND  
ASSOCIATED RAINFALL AMOUNTS IN ECUADOR AND WEST COLOMBIA.  
 
SYNOPTIC ANALYSIS AND FORECAST FOR TROPICAL SOUTH AMERICA  
 
ANALYSIS: A CELL OF THE SUBTROPICAL UPPER RIDGE CONTINUES  
STRENGTHENING OFF THE COAST OF SOUTHERN PERU. SPEED DIVERGENCE  
NORTH OF THE RIDGE IS VENTING WIDESPREAD DEEP CONVECTION IN FAR  
WEST BRASIL/SE COLOMBIA AND NE PERU. ANOTHER CELL CENTERS OVER  
ESPIRITO SANTO BUT HAS LIMITED IMPACTS IN PRECIPITATION GIVEN THE  
PRESENCE OF A MID-LEVEL RIDGE AND ASSOCIATED DRY AIR OVER EAST  
BRASIL. OVER THE EQUATORIAL ATLANTIC, THE SUBEQUATORIAL UPPER  
RIDGE EXTENDS ALONG 05-07N. ENHANCED DIFFLUENCE AND DIVERGENCE  
SOUTH OF THIS AXIS IS VENTING CONVECTION ALONG THE ATLANTIC ITCZ,  
LOCATED MOSTLY OFF THE NORTH COAST OF BRASIL AND INTO THE AMAZON  
DELTA.  
 
WESTERN TROPICAL SOUTH AMERICA FORECAST: THE UPPER RIDGE IS  
FORECAST TO REAMIN ALMOST STATIONARY OFF THE SOUTHERN COAST OF  
PERU THROUGH WEDNESDAY, MOVING EAST INTO SOUTH PERU ON THURSDAY.  
THIS WILL SUSTAIN UPPER DIVERGENCE IN MOST OF WEST BRASIL, PERU  
AND ECUADOR THROUGH THE FORECAST CYCLE. IN THE LOW-LEVELS, A MOIST  
AIR MASS OF TOTAL PRECIPITABLE WATER EXCEEDING 55MM IS PROPAGATING  
ACROSS THE CENTRAL AMAZON. IT IS FORECAST TO REACH THE ANDES OF  
ECUADOR, NORTH PERU AND SOUTH COLOMBIA ON THURSDAY, TO FAVOR AN  
INCREASE IN RAINFALL AMOUNTS. IN THE PACIFIC BASIN, AN INCREASE IN  
THE ONSHORE COMPONENT OF LOW-LEVEL WINDS IS EXPECTED ON WEDNESDAY,  
TO FAVOR MODERATE RAINFALL AMOUNTS IN ECUADOR ONCE AGAIN. AN  
ADDITIONAL INCREASE IN ONSHORE FLOW AND IN PRECIPITABLE WATER WILL  
SUPPORT HIGHER ACCUMULATIONS ON THURSDAY ACROSS THE ENTIRE  
ECUADORIAN COAST AND FAR SOUTHWEST COLOMBIA.  
 
NOTEWORTHY QUANTITATIVE PRECIPITATION FORECASTS:  
-PACIFIC BASIN OF ECUADOR: ISOLATED MAXIMA OF 20 - 35MM ON  
TUESDAY, INCREASING TO MAXIMA OF 25 - 50MM ON WEDNESDAY AND TO 35  
- 70MM ON THURSDAY, WITH A RISK OF HEAVY THUNDERSTORMS.  
-WEST AND CENTRAL AMAZON: MAXIMA OF 20 - 45MM IN AMAZONAS-BRASIL  
INTO PERU/ORIENTE ECUATORIANO ON TUESDAY. ON WEDNESDAY MAXIMA IN  
THE 20 - 50MM/DAY RANGE. ON THURSDAY, MAXIMA OF 25 - 50MM WEST  
AMAZONAS INTO SOUTH COLOMBIA AND NORTH PERU.  
-ALTIPLANO/PERUVIAN ANDES: MAXIMA GENERALLY IN THE 15 - 20MM/DAY  
RANGE OR SEASONAL VALUES.  
 
EASTERN AND SOUTHERN TROPICAL SOUTH AMERICA FORECAST: OF INTEREST  
IS A MID-UPPER TROUGH FORECAST TO AMPLIFY WHILE PROPAGATING ACROSS  
SOUTH BRASIL AND PARAGUAY/SE BOLIVIA LATE ON TUESDAY. THIS WILL  
FIRST YIELD TO AN INCREASE IN VENTILATION IN THE SOUTHWEST AMAZON  
INTO SOUTHERN MINAS GERAIS/SAO PAULO ON TUESDAY. AS THE TROUGH  
MOVES EAST ON WEDNESDAY, ITS DIVERGENT TIER WILL ENHANCE ASCENT IN  
SOUTHEAST BRASIL TO FOCUS HEAVY PRECIPITATION IN RIO DE  
JANEIRO/SOUTHERN MINAS GERAIS AND EAST SAO PAULO IN COMBINATION  
WITH MOIST SOUTHERLY FLOW ALONG COASTAL AREAS. ON THURSDAY, EXPECT  
THE HEAVIEST PRECIPITATION IN ESPIRITO SANTO/SE MINAS GERAIS. IN  
NORTHERN/NORTHEAST BRASIL, SCATTERED CONVECTION AND MODERATE  
AMOUNTS ARE EXPECTED ON A DAILY BASIS IN ASSOCIATION WITH THE  
ATLANTIC ITCZ AND THE NET. A PERTURBATION IN THE EASTERLY TRADES  
IS FORECAST TO FAVOR AN INCREASE IN RAINFALL TOTALS FROM PIAUI  
INTO EAST PARA ON THURSDAY, AS IT IS EXPECTED TO ASSOCIATE WITH  
PRECIPITABLE WATER VALUES APPROACHING 60MM AND ENHANCED TRADE WIND  
CONVERGENCE.  
 
NOTEWORTHY QUANTITATIVE PRECIPITATION FORECASTS:  
-BOLIVIA/PARAGUAY/MG SUL: MAXIMA IN THE 20 - 50MM/DAY RANGE ON  
TUESDAY DECREASING THEREAFTER.  
-SOUTHEAST BRASIL: MAXIMA OF 20 - 35MM/DAY ON TUESDAY INCREASING  
TO MAXIMA OF 40 - 80MM IN RIO DE JANEIRO/S MINAS GERAIS AND S  
ESPIRITO SANTO ON WEDNESDAY. MAXIMA OF 30 - 60MM IN N RIO DE  
JANEIRO/ESPIRITO SANTO/SE MINAS GERAIS ON THURSDAY.  
-NORTH COAST OF BRASIL/NORTHERN NORDESTE: MAXIMA OF 20 - 45MM IN  
MARANHAO/AMAZON DELTA ON TUESDAY AND WEDNESDAY. INCREASES TO  
MAXIMA OF 25 - 50MM ON THURSDAY.  
 
SYNOPTIC ANALYSIS AND FORECAST FOR EXTRATROPICAL SOUTH AMERICA:  
 
ANALYSIS: A BROAD TROUGH IN THE BELLINGSHAUSEN SEA IS HOSTING A  
POTENT NEGATIVELY TILTED SHORT WAVE TROUGH NEAR 90W AND SOUTH OF  
45S ON TUESDAY MORNING. THIS INCLUDES THE COLD EXIT OF AN UPPER  
JET, WHICH IS HIGHLIGHTING ASCENT. THE MID-UPPER TROUGH AND JET  
ARE INTERACTING WITH AN ATMOSPHERIC RIVER OF TOTAL PRECIPITABLE  
WATER VALUES NEAR 30MM, AND LOW-LEVEL WINDS EXCEEDING 55KT SOUTH  
OF THE FRONT. THIS SYSTEM WILL HAVE AN IMPORTANT IMPACT IN  
PRECIPITATION IN AUSTRAL CHILE AS IT MOVES INLAND LATER TODAY.  
OTHERWISE, GENERALLY FAIR WEATHER AND DRY CONDITIONS ARE PRESENT  
FROM NORTHERN ARGENTINA-RIO GRANDE DO SUL INTO CENTRAL PATAGONIA.  
AN EXCEPTION IS A FRONT PROPAGATING ACROSS THE EASTERN BUENOS  
AIRES PROVINCE, WHICH IS SUSTAINING STRONG THUNDERSTORMS THIS  
MORNING.  
 
FORECAST: THE SYSTEM OF INTEREST IS THE ATMOSPHERIC  
RIVER/NEGATIVELY TILTED UPPER TROUGH, FORECAST TO ENTER AUSTRAL  
CHILE LATER ON TUESDAY. THE LARGEST ACCUMULATION IS EXPECTED IN  
INTERIOR PORTIONS OF SOUTHERN AYSEN/FAR NORTHERN MAGALLANES ON  
TUESDAY, ALTHOUGH REMAINING CONVECTIVE STABILITY WILL SUSTAIN COLD  
AIR CUMULUS SHOWERS THROUGH THURSDAY, TO PRODUCE LESSER AMOUNTS.  
GENERALLY FAIR WEATHER IS EXPECTED ELSEWHERE WITH THE EXCEPTION OF  
ISOLATED DIURNAL THUNDERSTORMS IN THE ARGENTINEAN CENTRAL AND  
NORTHERN ANDES ON A DAILY BASIS; AND THE BUENOS AIRES PROVINCE ON  
TUESDAY WITH THE FRONT. A SHORT WAVE TROUGH CROSSING THE ANDES  
WILL FAVOR AN INCREASE IN THE COVERAGE AND INTENSITY OF DIURNAL  
CONVECTION ALONG EASTERN FLANKS OF THE ANDES ON THURSDAY,  
ESPECIALLY BETWEEN SOUTH BOLIVIA AND CENTRAL ARGENTINA. THIS WILL  
ALSO HIGHLIGHT PRECIPITATION IN NORTHEAST PATAGONIA SUSTAINING A  
RISK FOR HEAVY THUNDERSTORMS ON THURSDAY INTO EARLY FRIDAY.  
 
NOTEWORTHY QUANTITATIVE PRECIPITATION FORECASTS:  
AUSTRAL CHILE: MAXIMA OF 25 - 50MM ON TUESDAY FROM NORTH  
MAGALLANES INTO CENTRAL AYSEN. MAXIMA OF 20 - 35MM IN THE SAME  
REGION ON WEDNESDAY. MAXIMA OF 15 - 25MM AND MOUNTAIN SNOW FROM  
CENTRAL AYSEN INTO WEST MAGALLANES ON THURSDAY.  
INTERIOR NW ARGENTINA: MAXIMA OF 20 - 35MM ON THURSDAY.  
NE PATAGONIA: MAXIMA OF 15 - 20MM ON THURSDAY WITH A RISK FOR  
HEAVY TS.  
 
FOR ASSOCIATED QPF GRAPHICAL INFORMATION:  
HTTPS://WWW.WPC.NCEP.NOAA.GOV/INTERNATIONAL/SAM_DAY1-3.SHTML  
 
GALVEZ...(WPC)  
LEDESMA...(WPC)  
 
 
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