SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
139 PM EST FRI DEC 06 2024

GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/

NOTE: ALL SOUTH AMERICAN DESK FORECAST PRODUCTS WILL BE PAUSED
UNTIL MONDAY DECEMBER 16TH. APOLOGIES FOR THE INCONVENIENCE.

SOUTH AMERICAN FORECAST BULLETIN FOR 06 DEC 2024 AT 1745 UTC:

OF INTEREST IS A HEAVY RAINFALL EVENT DEVELOPING IN EASTERN
PORTIONS OF CENTRAL SOUTH AMERICA...PARTICULARLY IN SOUTHERN
BRASIL...NORTHEAST ARGENTINA AND EASTERN PARAGUAY. A REMARKABLE
ASPECT IS PRECIPITABLE WATER VALUES FORECAST TO EXCEED THE 65-70MM
RANGE IN THE PANTANAM/PARAGUAY. AS THIS EXTREMELY MOIST AIR MASS
IS ADVECTED BY AN ACTIVE SALLJ ON FRIDAY AND SATURDAY...EXPECT
SEVERAL ROUNDS WITH HEAVY RAINFALL. NOTE THAT THREE-DAY TOTALS
COULD EXCEED 200-300MM IN PORTIONS OF SOUTHEAST
PARAGUAY...MISIONES-ARGENTINA AND PARANA/SANTA CATARINA IN BRASIL.

IN THE MID-UPPER TROPOSPHERE...A TROUGH IS PROPAGATING EASTWARD
NEAR 50-55W. THIS IS SUSTAINING A SURFACE FRONT THAT IS EXPECTED
TO REACH NORTHERN RIO GRANDE DO SUL...MISIONES-ARGENTINA AND
SOUTHERN PARAGUAY BY FRIDAY EVENING. THE FRONT IS FORECAST TO
INTERACT WITH AN ACTIVE SALLJ OVERNIGHT TO TRIGGER HEAVY
CONVECTION AND POSE A RISK FOR MCS FORMATION BETWEEN SOUTHERN
PARAGUAY AND SANTA CATARINA/RIO GRANDE DO SUL...WHERE
ACCUMULATIONS ARE FORECAST TO REACH 15-30MM/DAY AND MAXIMA OF
50-100MM. A SECOND FRONT IS FORECAST TO ARRIVE FROM THE SOUTH AND
PROVIDE ANOTHER ROUND OF ENHANCE LOW-LEVEL MOISTURE CONVERGENCE ON
SATURDAY. AS THIS ENTERS IN PHASE WITH A PEAK IN AVAILABLE
MOISTURE EXPECT THE RAINFALL EVENT TO PEAK. THIS WILL FAVOR
20-40MM/DAY AND SCATTERED MAXIMA OF 75-150MM ON SATURDAY THROUGH
EARLY SUNDAY ACROSS CENTRAL/EASTERN PARAGUAY INTO PARANA/SANTA
CATARINA IN BRASIL. NOTE A RISK FOR MCS FORMATION AND ISOLATED
SEVERE CONVECTION. ONSUNDAY...THE FRONT IS FORECAST TO STALL
ACROSS PARANA...EASTERN PARAGUAY/PANTANAL/SOUTHEAST BOLIVIA.
ALTHOUGH LOW-LEVEL MOISTURE FLUS CONVERGENCE IS FORECAST TO START
DECREASING...THE INFLUENCE OF THE BASE OF A LARGE UPPER TROUGH
WILL PROVIDE SUFFICIENT DYNAMICS TO FAVOR MAXIMA OF 40-80MM FROM
EASTERN PARAGUAY INTO PARANA/NORTHERN SANTA CATARINA...INCLUDING A
RISK FOR MCS FORMATION. NOTE THAT ISOLATED HIGHER AMOUNTS ARE
POSSIBLE.

THE ANALYSIS OF THE UPPER LEVELS IN TROPICAL SOUTH AMERICA SHOWS A
WELL ESTABLISHED CELL OF THE BOLIVIAN HIGH LOCATED IN EASTERN
BOLIVIA. TO THE NORTHEAST...A STRONG CLOSED LOW/VCAN CENTERS OVER
SOUTHERN CEARA IN NORTHEAST BRASIL. THE VCAN IS FORECAST TO
RETROGRADE THROUGH THE FORECAST CYCLE TO CENTER OVER CENTRAL PARA
BY MONDAY MORNING. THIS WILL YIELD TO A WESTWARD MIGRATION OF THE
REGION OF ENHANCED VENTILATION/UPPER DIVERGENCE THAT ESTABLISHES
BETWEEN THE VCAN AND THE BOLIVIAN HIGH...WHICH IS EXPECTED TO
FAVOR ACTIVE DIURNAL CONVECTION IN WESTERN PORTIONS OF THE AMAZON
BASIN THROUGH THE FORECAST CYCLE. IN THE LOWER
LEVELS...NORTHEASTERLY WINDS ARE ALREADY PRESENT ACROSS THE
NORTHERN AMAZON ESPECIALLY IN CENTRAL COLOMBIA AND
RORAIMA/GUIANAS. THIS WILL CONTINUE TO FAVOR THE ESTABLISHMENT OF
THE NEAR EQUATORIAL TROUGH (NET) ACROSS EQUATORIAL PORTIONS OF THE
AMAZON BASIN...WHICH WILL HIGHLIGHT LOW-LEVEL MOISTURE CONVERGENCE
AND RESULTING DEVELOPMENT OF AFTERNOON CONVECTION. THE LARGE SCALE
ANALYSIS SHOWS THAT MJO CONDITIONS ARE STILL UPPER
CONVERGENT...WHICH GENERALLY FAVORS MORE CONSERVATIVE
PRECIPITATION AMOUNTS THAN WHAT MODELS SUGGEST. ACCORDINGLY...THE
FORECAST CHARTS REFLECT AMOUNTS THAT ARE SLIGHTLY LOWER THAN THOSE
INDICATED BY MODEL GUIDANCE. IN TERMS OF DAILY
ACCUMULATIONS...DIURNAL CONVECTION ON FRIDAY WILL FAVOR MAXIMA OF
15-30MM ACROSS MOST OF THE CENTRAL AND WESTERN AMAZON BASIN.
MODERATE AMOUNTS ARE EXPECTED ALONG THE EASTERN SLOPES OF THE
ANDES IN PERU AND BOLIVIA...WHERE EXPECT MAXIMA OF 25-50MM IN SOME
LOCATIONS. ON SATURDAY...EXPECT MAXIMA OF 20-40MM STRECHING FROM
THE NORTHERN CHACO IN PARAGUAY AND SOUTHERN BOLIVIA TO
RONDONIA/ACRE/AMAZONAS IN BRASIL AND EASTERN PERU. ALONG THE
EASTERN ANDES OF PERU EXPECT MAXIMA OF 25-50MM. ON SUNDAY EXPECT
SCATTERED MAXIMA OF 20-40MM IN FAR WESTERN BRASIL...NORTHEAST
PERU...AS WELL AS ALONG THE ANDEAN FOOTHILLS IN SOUTHERN PERU AND
NORTHERN BOLIVIA.

GALVEZ...(WPC)