268
FXSA20 KWBC 171647
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1246 PM EDT MON MAR 17 2025
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/DATA/NCCF/COM/GFS/PROD/
SOUTH AMERICAN FORECAST BULLETIN FROM 17 MARCH 2025 AT 18 UTC
ANALYSIS AND FORECAST OF LARGE SCALE CONDITIONS
MJO AND TROPOSPHERIC WAVES: THE MJO IS PROPAGATING ACROSS PHASE 3,
WHICH IS CONSISTENT WITH THE CURRENT LARGE SCALE UPPER CONVERGENCE
ESTABLISHED OVER SOUTH AMERICA. A TROPOSPHERIC KELVIN WAVE IS
PROPAGATING ACROSS THE CENTRAL PACIFIC, BUT IT IS NOT FORECAST TO
ARRIVE UNTIL THE WEEKEND/EARLY NEXT, WHEN IT IS LIKELY TO PROVIDE
SOME MINOR ENHANCEMENT TO AREAS AND THE INTENSITY OF DEEP
CONVECTION IN TROPICAL PORTIONS OF SOUTH AMERICA.
SST: IMPORTANT POSITIVE ANOMALIES CONTINUE ALONG EQUATORIAL
PORTIONS OF THE EASTERN PACIFIC AND NOW HAVE EXTENDED INTO THE
GULF OF PANAMA. NOTE, HOWEVER, THAT THESE WARM ANOMALIES ARE
MOSTLY SUPERFICIAL AND COULD BE RAPIDLY ALTERED BY CHANGES IN
SURFACE WINDS. IN FACT, COOL ANOMALIES HAVE EMERGED ALONG THE
PERUVIAN COAST IN RESPONSE TO STRONG SOUTHEASTERLY TRADES IN THE
LAST FEW DAYS.
SYNOPTIC ANALYSIS AND FORECAST FOR TROPICAL SOUTH AMERICA
ANALYSIS: THE SUBTROPICAL UPPER RIDGE IS CENTERING IN TWO CELLS.
ONE IS LOCATED OFF THE COAST OF SOUTHERN PERU WHILE THE OTHER
ELONGATES ZONALLY OVER SOUTHEAST BRASIL. THE IMPACT OF THESE
RIDGES IN VENTILATING CONVECTION IS SOMEWHAT LIMITED BY THE
PRESENCE OF A MID-LEVEL RIDGE EXTENDING OVER SOUTHEAST
BOLIVIA/NORTHERN PARAGUAY INTO ESPIRITO SANTO IN BRASIL. THE MOST
ACTIVE CONVECTION AND PRECIPITATION IS OCCURRING IN ASSOCIATION
WITH THE ITCZ AND NET IN EQUATORIAL PORTIONS OF SOUTH AMERICA,
PARTICULARLY ALONG THE NORTH COAST OF BRASIL INTO FAR NORTHWEST
BRASIL, SOUTHERN VENEZUELA AND COLOMBIA. THE PACIFIC BASIN OF
ECUADOR CONTINUES EXPERIECING DIURNAL CONVECTION, BUT STRONG UPPER
DIVERGENCE IN THE SOUTHEAST FLANK OF THE SUBEQUATORIAL RIDGE, HAS
FAVORED A NORTHWARD EXTENSION OF AREAS OF DEEP CONVECTION INTO
COLOMBIA DURING THE LAST FEW DAYS.
WESTERN TROPICAL SOUTH AMERICA FORECAST: THE WESTERN CELL OF THE
SUBTROPICAL UPPER RIDGE IS FORECAST TO REMAIN OFF THE COAST OF
SOUTHERN PERU THROUGH THE FORECAST CYCLE. LOW-LEVEL WINDS IN THE
AMAZON BASIN ARE FORECAST TO DEVELOP A NORTHEASTERLY CONPONENT,
WHICH WILL TRANSPORT HIGHER VALUES OF PRECIPITABLE WATER INTO
SOUTHERN AND WESTERN PORTIONS OF THE AMAZON BY WEDNESDAY. THIS, IN
COMBINATION WITH A DECREASE IN UPPER DIVERGENCE OVER COLOMBIA,
WILL YIELD TO A GENERAL SHIFT OF THE AREAS OF HEAVIEST
PRECIPITATION FROM THE NORTHWEST AMAZON/COLOMBIA/VENEZUELA EARLY
IN THE CYCLE TO PERU/ORIENTE ECUATORIANO, NORTH BOLIVIA AND WEST
BRASIL BY WEDNESDAY. HIGHER VALUES OF AVAILABLE MOISTURE BY
MID-WEEK WILL ALSO FAVOR A SLIGHT INCREASE IN ACCUMULATION IN
DIURNAL CONVECTION ACROSS THE ALTIPLANO AND ANDES OF PERU,
ALTHOUGH SEASONAL AMOUNTS ARE EXPECTED. IN THE PACIFIC BASINS OF
ECUADOR/FAR NORTHWEST PERU, VARIABILITY IN THE LOW-LEVEL WINDS
WILL CONTINUE TO PLAY A ROLE IN MODULATING DIURNAL CONVECTION.
ENHANCED ONSHORE FLOW AND HIGHER VALUES OF PRECIPITABLE WATER ARE
EXPECTED ON MONDAY AND WEDNESDAY, WHICH WOULD LIKELY YIELD TO
HIGHER ACCUMULATION DURING THESE DAYS.
IMPORTANT RAINFALL ACCUMULATION FORECASTS:
-PACIFIC BASIN OF ECUADOR: ON MONDAY, MAXIMA OF 25 - 50MM,
DECREASING TO MAXIMA OF 20 - 35MM ON TUESDAY, INCREASING AGAIN TO
MAXIMA OF 25 - 50MM ON WEDNESDAY.
-WESTERN AMAZON: INCREASE TO MAXIMA OF 25 - 50MM FROM COLOMBIA
INTO NORTH PERU ON TUESDAY, WHILE IN THE PERUVIAN AMAZON/ACRE
EXPECT MAXIMA OF 20 - 35MM. ON WEDNESDAY EXPECT MAXIMA OF 25 -
50MM FROM SOUTHWEST COLOMBIA/ORIENTE ECUATORIANO INTO CENTRAL
PERU/ACRE, WHILE IN SOUTH PERU, NORTH BOLIVIA AND RONDONIA EXPECT
MAXIMA OF 20 - 35MM.
-ALTIPLANO/PERUVIAN ANDES: GRADUAL INCREASE TO SUSTAIN SEASONAL
DAILY ACCUMULATIONS BY WEDNESDAY.
-CENTRAL AMAZON BASIN: MAXIMA OF 25 - 50MM IN NW BRASIL ON MONDAY
AND MAXIMA OF 20 - 35MM IN PARA. MAXIMA OF 20 - 35MM IN PARA/EAST
AMAZONAS ON TUESDAY. MAXIMA OF 20 - 45MM ON WEDNESDAY.
EASTERN AND SOUTHERN TROPICAL SOUTH AMERICA FORECAST: A SYSTEM OF
INTEREST IS A MID-UPPER TROUGH PROPAGATING IN EXTRATROPICAL SOUTH
AMERICA ALONG 70-67W AND SOUTH OF 25S ON MONDAY. THIS TROUGH IS
FORECAST TO START AMPLIFYING ACROSS PARAGUAY AND SOUTHEAST BOLIVIA
LATE ON TUESDAY. AS IT AMPLIFIES, IT WILL AID WITH THE VENTILATION
OF CONVECTION FROM SOUTHERN BOLIVIA INTO MINAS GERAIS/ESPIRITO
SANTO ON TUESDAY. DURING WEDNESDAY IT IS FORECAST TO PROPAGATE
ACROSS SOUTHERN AND SOUTHWEST BRASIL, PLACING ITS DIVERGENT TIER
OVER SOUTHEAST BRASIL. THIS WILL FAVOR LOCALLY HEAVY PRECIPITATION
IN EASTERN RIO DE JANEIRO, SOUTHEAST MINAS GERAIS AND ESPIRITO
SANTO IN COMBINATION WITH A WEAK SURFACE FRONT AND ASSOCIATED
SOUTHERLY WINDS IN THE SERRA DO MAR OF SAO PAULO AND RIO DE
JANEIRO. FURTHER NORTH, EXPECT ACTIVE CONVECTION TO PERSIST ACROSS
THE NORTH COAST OF BRASIL AND INTERIOR NORDESTE ALTHOUGH A MILD
DECREASING TREND IN ACCUMULATION IS EXPECTED. CONVERSELY, AN
INCREASE IN LOW-LEVEL MOISTURE IN CENTRAL BRASIL WILL FAVOR A MILD
INCREASING TREND IN DIURNAL CONVECTION, TO FAVOR SEASONAL
ACCUMULATION.
IMPORTANT RAINFALL TOTAL FORECASTS:
NORTH COAST OF BRASIL/NORTHERN NORDESTE: MAXIMA OF 25 - 50MM ON
MONDAY, DECREASING TO MAXIMA OF 20 - 40MM THEREAFTER.
BOLIVIA/PARAGUAY: MAXIMA OF 20 - 45MM IN CENTRAL BOLIVIA AND WEST
PARAGUAY/FAR NORTH ARGENTINA ON MONDAY. ON TUESDAY EXPECT MAXIMA
OF 25 - 50MM IN SOUTH BOLIVIA AND NORTH PARAGUAY/PANTANAL, WHILE
IN CENTRLA PARAGUAY AND CENTRAL MATO GROSSO DO SUL EXPECT MAXIMA
OF 20 - 40MM, DECREASING THEREAFTER.
SOUTHEAST BRASIL: MAXIMA OF 20 - 35MM ON MONDAY AND TUESDAY,
INCREASING TO 25 - 50MM IN RIO DE JANEIRO, EAST SAO PAULO AND
SOUTHEAST MINAS GERAIS ON WEDNESDAY.
SYNOPTIC ANALYSIS AND FORECAST FOR EXTRATROPICAL SOUTH AMERICA:
ANALYSIS: AN UPPER TROUGH IS CROSSING PATAGONIA ON MONDAY EXERTING
LIMITED EFFECTS ON REGIONAL PRECIPITATION. ANOTHER SHORT WAVE
UPPER TROUGH IS PROPAGATING EASTWARD ACROSS THE RIO DE LA PLATA.
THIS IS SUSTAINING WEAK CYCLOGENESIS OFF THE COAST OF URUGUAY AND
THE BUENOS AIRES PROVINCE, WITH LIMITED IMPACTS IN CONTINENTAL
PRECIPITATION. AN EXCEPTION IS ONSHORE FLOW IN RIO GRANDE DO SUL,
WHICH IS STIMULATING THE DEVELOPMENT OF DIURNAL CONVECTION IN THE
LAGOA DOS PATOS REGION. OF INTEREST, ANOTHER MID-UPPER TROUGH IS
APPROACHING FROM THE WEST WHILE PROPAGATING ALONG 100W AND SOUTH
OF 38S. IT ASSOCIATES WITH AN ATMOSPHERIC RIVER THAT CONTAINS
TOTAL PRECIPITABLE WATER VALUES EXCEEDING 25MM, AND WILL LIKELY
HAVE IMPACTS IN AUSTRAL CHILE ON TUESDAY.
FORECAST: THE PRIMARY SYSTEM OF INTEREST FOR PRECIPITATION IS THE
ATMOSPHERIC RIVER, FORECAST TO ENTER AUSTRAL CHILE ON TUESDAY
MORNING, ACCOMPANIED BY A NEGATIVELY TILTED SHORT WAVE UPPER
TROUGH. THIS WILL FAVOR A PEAK IN ACCUMULATION ON TUESDAY THROUGH
EARLY WEDNESDAY, WHEN EXPECT AMOUNTS OF XX - XXMM/DAY AND MAXIMA
OF XXMM. ALTHOUGH THE MAIN EVENT IS FORECAST TO FADE THEREAFTER,
COLD AIR CUMULUS SOUTH OF THE ATMOSPHERIC RIVER WILL FAVOR
INTERMITTENT PRECIPITATION ON AUSTRAL CHILE ON WEDNESDAY INTO
EARLY THURSDAY, WHEN EXPECT MAXIMA IN THE XX - XXMM/DAY RANGE.
OTHERWISE, EXPECT GENERALLY DRY CONDITIONS IN AREAS SOUTH OF
NORTHERN ARGENTINA AND SOUTHEAST PARAGUAY, GIVEN LIMITED AMOUNTS
OF AVAILABLE MOISTURE THROUGH THE FORECAST CYCLE.
IMPORTANT RAINFALL TOTAL FORECASTS:
AUSTRAL CHILE: MAXIMA OF 25 - 50MM DEVELOPING ON TUESDAY AS
ATMOSPHERIC RIVER ARRIVES. DECREASES TO MAXIMA OF 20 - 35MM ON
WEDNESDAY.
FOR ASSOCIATED QPF GRAPHICAL INFORMATION:
HTTPS://WWW.WPC.NCEP.NOAA.GOV/INTERNATIONAL/SAM_DAY1-3.SHTML
GALVEZ...(WPC)
LEDESMA...(WPC)
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