Prognostic Meteorological Discussion
Issued by NWS
Issued by NWS
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887 FXSA20 KWBC 192103 PMDSA SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS NWS WEATHER PREDICTION CENTER COLLEGE PARK MD 503 PM EDT THU SEP 19 2024 GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/ SOUTH AMERICAN FORECAST BULLETIN FOR 19 SEP 2024 AT 2100 UTC: SOME AREAS IN AND AROUND NORTHEASTERN ARGENTINA AND URUGUAY OBSERVED SEVERE WEATHER TODAY...AS IT WAS EXPECTED. HOWEVER...THE ACTIVE AND SEVERE WEATHER RISK CONTINUES TONIGHT INTO FRIDAY FOR PORTIONS MAINLY OVER URUGUAY...SOUTHERN BRAZIL AND SOUTHERN PARAGUAY...THOUGH PORTIONS OF NORTHEASTERN ARGENTINA MAY ALSO OBSERVE ACTIVE WEATHER INTO LATE THIS AFTERNOON. CENTRAL TO SOUTHERN CHILE...WILL BE SUBJECT TO HIGHER THAN NORMAL MOISTURE FOR THE NEXT FEW DAYS...STARTING MAINLY FRIDAY AFTERNOON...AS AN ATMOSPHERIC RIVER AFFECTS THE AREA. FOR TODAY INTO EARLY FRIDAY...THERE WILL BE DIRECT ONSHORE FLOW INTO CENTRAL TO SOUTH CENTRAL CHILE. THERE IS RAIN THAT IS FORECAST ACROSS THE LOWER ELEVATIONS OF THESE SECTIONS OF CHILE...WHILE SNOW IS FORECAST OVER THE HIGHEST ELEVATIONS. THERE IS AN ELEVATED RISK OF SEVERE WEATHER EVENT OVER ARGENTINA INTO URUGUAY FOR THE REST OF TODAY INTO EARLY FRIDAY...LARGE HAIL AND STRONG GUSTY WINDS HAVE ALREADY BEEN REPORTED OVER THE AREA. THE STRONG STORMS THAT ARE FORECAST TONIGHT INTO FRIDAY COULD PRODUCE FREQUENT LIGHTNING...STRONG GUSTY WINDS...HEAVY RAIN AND SOME HAIL. BASED ON THE LATEST GUIDANCE...THE MODERATE RISK OF SEVERE WEATHER WILL BE MAINLY OVER SOUTHERN BRAZIL INTO PARAGUAY ON FRIDAY INTO SATURDAY. THE STRONG GUSTY WINDS AND THE RISK OF HAIL IS DUE TO THE COMBINATION OF A WARM AND NORTHERLY LOW LEVEL JET OVER THE CONTINENT INTO THE AREA...JUST AHEAD OF A FRONTAL BOUNDARY THAT IS MOVING NORTHEAST. IN THE MID LEVELS...ALTHOUGH THE STRONG MID LEVEL TROUGH HAS PASSED...THERE IS STILL A MID LEVEL SHORTWAVE TROUGH OVER THE AREA. THERE IS ALSO AN UPPER TROUGH WITH STRONG WINDS THAT ARE PROVIDING SOME DIVERGENCE...THOUGH THERE IS ALSO DIFLUENCE EXPECTED IN THE UPPER LEVELS OVER THE AREA THROUGH FRIDAY NIGHT. THE GRO2T SEVERITY ALGORITHM DOES INDICATE A MODERATE TO ELEVATED RISK FOR SEVERE WEATHER THROUGH EARLY FRIDAY...THEN SLIGHT TO MODERATE ON FRIDAY OVER PARAGUAY AND SOUTHERN BRAZIL...THEN ALSO SLIGHT TO MODERATE OVER ARGENTINA IN AND AROUND THE BUENOS AIRES PROVINCE. THE PRECIPITABLE WATER IS FORECAST TO BE NEAR TO BELOW NORMAL TONIGHT INTO EARLY FRIDAY ACROSS CENTRAL TO SOUTHERN CHILE...THEN INCREASE ON FRIDAY ONWARD...AS AN ATMOSPHERIC RIVER MOVES IN AND AFFECTS PORTIONS OF CENTRAL CHILE. THE MODEL GUIDANCE CONTINUES TO OVERESTIMATE THE PRECIPITATION OVER THE AREA...AS IT HAS BEEN DOING OVER THE PAST FEW DAYS. FOR THAT REASON...THE FORECAST PRECIPITATION OVER CENTRAL TO SOUTHERN CHILE WILL BE LOWER THAN WHAT THE MODEL GUIDANCE IS SUGGESTING. OUR FORECAST SUGGESTS MAX VALUES NEAR 20-40MM TODAY AND FRIDAY...WHILE SATURDAY INTO SUNDAY UP TO 50MM IS FORECAST. THE REASON FOR THE HIGHER AMOUNT ON SATURDAY INTO FRIDAY IS BECAUSE THERE WILL BE A MID LEVEL TROUGH MOVING INTO THE AREA AS WELL AS STRONG UPPER LEVEL WINDS...WHICH WILL CAUSE AN INCREASE IN INSTABILITY. THE GFS MODEL IS VERY AGGRESSIVE WITH THE AMOUNTS...WHILE THE ECMWF IS MORE MODEST...THEREFORE AT THIS TIME WE SHOWED AN INCREASE IN PRECIPITATION BUT SPLITTING THE DIFFERENCE BETWEEN THE MODELS. WE WILL ADJUST AS WE FIND MORE CONSISTENCY AMONG THE MODELS. IT IS WORTH MENTIONING THAT THERE IS ALSO A CHANCE OF ISOLATED THUNDERSTORMS ON FRIDAY INTO EARLY SATURDAY ACROSS CENTRAL CHILE. AS MENTIONED ABOVE...PORTIONS OF EAST CENTRAL ARGENTINA IN TO SOUTHERN BRAZIL AND SOUTHERN PARAGUAY ARE AT RISK OF OBSERVING SEVERE WEATHER THIS EVENING INTO THE WEEKEND...THOUGH MORE ISOLATED WITH EACH PASSING DAY. THE PRECIPITATION FORECAST SUGGESTS A MAXIMA NEAR 35-70MM FOR SOUTHERN BRAZIL...NORTHERN URUGUAY AND SOUTHERN PARAGUAY...WITH EASTERN AND NORTHEASTERN ARGENTINA OBSERVING UP TO AROUND 35MM. PRECIPITATION FORECAST FROM FRIDAY INTO SATURDAY...SUGGESTS LITTLE RAIN OVER ARGENTINA....BUT SOUTHERN BRAZIL AND SOUTHERN PARAGUAY IS FORECAST TO OBSERVE UP TO 35-70MM OF RAIN...WITH THE ECMWF MODEL HAVING A LARGER COVERAGE AREA IN THE FORECAST COMPARED TO THE GFS MODEL. THEN SATURDAY INTO SUNDAY...UP TO 25MM IS FORECAST OVER SOUTHERN BRAZIL...BUT AREAS IN AND AROUND BUENOS AIRES ARE FORECAST UP TO 35MM WITH A SLIGHT RISK OF SVR WEATHER. PERU IS FORECAST TO OBSERVE SOME RAINFALL EACH DAY BUT THE AMOUNTS WILL BE RELATIVELY MODEST. GENERALLY MAX VALUES OF 25MM OR LESS ARE FORECAST EACH DAY OVER PERU...WITH THE CHANCE OF ISOLATED THUNDERSTORMS. THAT BEING SAID...NORTHERN PERU COULD OBSERVE UP TO 35MM ON SATURDAY INTO SUNDAY. ALAMO...(WPC) $$