Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
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651 FXSA20 KWBC 212029 PMDSA SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 328 PM EST THU NOV 21 2024 GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/ SOUTH AMERICAN FORECAST BULLETIN FOR 21 NOV 2024 AT 2030 UTC: THE NORTHERN HALF OF THE SOUTH AMERICAN CONTINENT WILL OBSERVE SHOWERS AND THUNDERSTORMS EACH DAY. PARTICULARLY OVER THE AMAZON BASIN INTO PORTIONS OF BOLIVIA. EASTERN BRAZIL AS WELL AS ITS SOUTHEASTERN COAST WILL HAVE AN ENHANCED CHANCE OF SHOWERS AND THUNDERSTORMS OVER THE NEXT FEW DAYS. THE REASONS DIFFER...BUT MOST OF SOUTH AMERICA WILL BE ACTIVE WITH SHOWERS AND THUNDERSTORMS. THE DRIER AREAS WILL BE THE NORTHERN HALF OF CHILE AND MOST OF ARGENTINA. THERE IS A CHANCE OF DAILY SHOWERS AND THUNDERSTORMS OVER CENTRAL ARGENTINA...NEAR LA PAMPA...HOWEVER. SOUTHERN CHILE HAS A GOOD CHANCE OF SHOWERS IN THE LOWER ELEVATIONS WITH MOUNTAIN SNOW POSSIBLE...ESPECIALLY FOR THE REST OF TODAY INTO SATURDAY. THE BOLIVIAN HIGH IS PREVAILING...WHICH IS HELPING THE DEVELOPMENT OF CONVECTION ACROSS THE AMAZON...AS WELL AS CONTRIBUTING TO A TIGHT PRESSURE GRADIENT...THEREFORE STRONG UPPER LEVEL WINDS CENTRAL CHILE AND ARGENTINA. THE UPPER LEVEL WINDS ACROSS THE AMAZON BASIN WILL NOT BE AS STRONG AS THOSE ACROSS CENTRAL CHILE AND ARGENTINA...BUT IT WILL BE ENOUGH TO VENTILATE PULSE THUNDERSTORMS THAT WILL DEVELOP DUE TO THE INTERACTION OF DIURNAL HEATING...SFC TROUGHS...AND THE AVAILABLE MOISTURE. THE SHOWERS AND THUNDERSTORMS THAT ARE EXPECTED EACH DAY ACROSS THE AREA...ARE EXPECTED TO CAUSE RAINFALL GENERALLY IN THE 15-25MM RANGE...WITH ISOLATED HIGHER AMOUNTS REACHING 40-45MM. THERE IS ALSO A TROPOSPHERIC KELVIN WAVE OVER THE REGION...WHICH MAY CONTRIBUTE TO THE RAINFALL ACROSS THE NORTHWEST AMAZON BASIN. THIS IS WHY NORTHWEST BRASIL AND PORTIONS OF EASTERN PERU HAS A RAINFALL FORECAST MAXING OUT AT 20-45MM. ECUADOR ALSO HAS UP TO 45MM...WHILE ELSEWHERE IN PERU HAS UP TO 35MM IN THE FORECAST. THERE ARE SOME AREAS IN BOLIVIA THAT ARE FORECAST TO OBSERVE FORECAST MAX VALUES REACHING 40-80MM FOR THE REST OF TODAY INTO FRIDAY MORNING...THEN UP TO 50MM ON FRIDAY INTO SATURDAY MORNING...WITH LOWER AMOUNTS ON SATURDAY INTO SUNDAY. CURRENT SATELLITE IMAGERY INDICATES DEEPER CONVECTION OCCURRING OVER EASTERN BOLIVIA...CLOSE TO THE BORDER WITH PARAGUAY...AND THIS IS EXPECTED TO CONTINUE ACROSS THE NORTHERN AND EASTERN SECTIONS OF BOLIVIA INTO FRIDAY MORNING. A FRONTAL BOUNDARY...EXTENDING ACROSS THE ATLANTIC INTO EASTERN BRAZIL WILL PROVIDE CONSTANT MOISTURE TO THE EASTERN COAST OF BRAZIL FROM SAO PAULO TO ESPIRITO SANTO. THE HIGHER THAN NORMAL MOISTURE WILL PREVAIL OVER THE NEXT FEW DAYS...WHICH WILL CONTRIBUTE TO AMOUNTS OF RAIN UP TO 50MM TODAY. EVEN HIGHER AMOUNTS OF RAIN ARE EXPECTED FROM FRIDAY INTO EARLY SUNDAY FOR THE AFOREMENTIONED AREAS IN BRAZIL...WITH RAINFALL AMOUNTS MAXING OUT NEAR 70MM. THE ABOVE NORMAL MOISTURE WILL PREVAIL...BUT MID AND UPPER TROUGHS WILL START TO MOVE NORTH...AND PROVIDE STRONGER WINDS ALOFT...INCREASING INSTABILITY. EVEN THOUGH ARGENTINA IS EXPECTED TO BE GENERALLY DRY...A DEVELOPING COLD FRONT OVER THE CENTRAL PORTIONS WILL INTERACT WITH LOCALLY HIGHER THAN NORMAL MOISTURE AND A DIFFLUENT PATTERN IN THE MID AND UPPER LEVELS. THE MODEL GUIDANCE IS NOT VERY AGGRESSIVE WITH THE TOTAL RAINFALL AMOUNTS...WITH MAX TOTALS GENERALLY AROUND 15-35MM EACH DAY INTO THE WEEKEND...BUT THE STRONGER WINDS ALOFT WITH THE COLD MID LEVEL TEMPERATURES ARE FAVORABLE FOR A MARGINAL TO SLIGHT RISK OF STRONG THUNDERSTORMS EACH DAY INTO SUNDAY MORNING. THE NORTHERN HALF OF CHILE IS FORECAST TO REMAIN MAINLY DRY FOR THE NEXT FEW DAYS...BUT A STRONG LOW PRESSURE IN TEH PACIFIC WILL BRING ABOVE NORMAL MOISTURE TO THE SOUTHERN HALF OF THE COUNTRY TODAY AND FRIDAY INTO SATURDAY. THERE ARE ALSO STRONG MID AND UPPER LEVEL WINDS...WHICH WILL AFFECT THE AREA UNTIL SATURDAY. THIS WILL CAUSE SHOWERS AND THUNDERSTORMS OVER THE SOUTHERN PORTION OF CHILE...WITH MOUNTAIN SNOW. RAINFALL MAX VALUES TO 45MM ARE FORECAST EACH DAY UNTIL SATURDAY MORNING...THEN UP TO 25MM ON SATURDAY INTO SUNDAY AS DRIER AIR MOVES IN AND THE WINDS ALOFT WEAKEN. ALAMO...(WPC) $$