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South American Forecast Discussion
 
(Latest Discussion - Issued 1717Z Sep 21, 2018)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
117 PM EDT FRI SEP 21 2018

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

NOTE: A PROLONGED PERIOD OF SEVERAL ROUNDS OF SEVERE CONVECTION IS
EXPECTED IN NORTHEASTERN ARGENTINA/URUGUAY/SOUTHERN BRASIL AND
PARAGUAY INTO EARLY NEXT WEEK. THE MOST CRITICAL PERIOD IS
EXPECTED ON SATURDAY...WHEN EXTREME INSTABILITY AND FAVORABLE
DYNAMICS WILL FAVOR SEVERE CONVECTION DEVELOPING IN ENTRE
RIOS...THE NORTHERN BUENOS AIRES PROVINCE AND NORTHERN AND WESTERN
URUGUAY.

MODEL DISCUSSION (VALID FROM SEP 21 AT 0000 UTC): GLOBAL MODELS
ARE IN FAIRLY GOOD AGREEMENT THROUGH 84-96 HRS...WHEN CONFIDENCE
STARTS TO DECREASE GRADUALLY.

THE MOST REMARKABLE EVENT THROUGH THE FORECAST CYCLE IS THE
DEVELOPMENT OF SEVERE CONVECTION IN THE RIO DE LA PLATA BASIN
DURING EARLY HOURS OF SATURDAY. LIFTED INDEX VALUES ARE FORECAST
TO BE AS LOW AS -12...SUPPORTED BY BOUNDARY LAYER DEWPOINT VALUES
OF 24C IN ENTRE RIOS...AND A COLD SHORT WAVE MID-LEVEL TROUGH
PROPAGATING THROUGH THE REGION. CAPE VALUES ARE AS HIGH AS 4500
J/KG. THE EVENT IS ACCOMPANIED BY CONVERGENCE IN THE EXIT OF A
LOW-LEVEL JET FROM THE NORTH. THE POTENTIAL FOR SEVERE CONVECTION
IS HIGH...INCLUDING A HIGH RISK FOR THE OCCURRENCE OF HAIL. IN
TERMS OF ACCUMULATIONS...EXPECTING DEEP CONVECTION TO START
DEVELOPING EARLY ON SATURDAY MORNING IN CENTRAL PORTIONS OF THE
BUENOS AIRES PROVINCE. THIS WILL SUSTAIN MAXIMA OF 15-20MM/DAY
FROM MIDNIGHT INTO EARLY HOURS OF SATURDAY. DURING
SATURDAY-SUNDAY...EXPECTING ACCUMULATIONS OF 40-80MM/DAY IN ENTRE
RIOS...NORTHERN/WESTERN URUGUAY AND IN SOUTHWESTERN RIO GRANDE DO
SUL. IN NORTHERN AND EASTERN PORTIONS OF THE BUENOS AIRES PROVINCE
AS WELL AS SOUTHERN AND EASTERN URUGUAY EXPECTING MAXIMA OF
20-35MM.

AFTER THIS PERIOD...THE SURFACE FRONT WILL MOVE NORTHWARD VERY
LITTLE. BY SUNDAY AFTERNOON IS WILL POSITION ACROSS CENTRAL
URUGUAY INTO NORTHERN SANTA FE/NORTHERN SANTIAGO DEL ESTERO. UPPER
JET DYNAMICS WILL BE FAVORABLE TO CONTINUE FAVORING STRONG
ASCENT...AND LOW-LEVEL JET DYNAMICS WILL CONTINUE SUSTAINING
ENHANCED LOW-LEVEL MOISTURE CONVERGENCE. THIS WILL SUSTAIN
RAINFALL ACCUMULATIONS OF 40-80MM/DAY ACROSS SOUTHERN
CORRIENTES/NORTHERN ENTRE RIOS INTO EXTREME NORTHERN URUGUAY AND
SOUTHERN RIO GRANDE DO SUL...UNDER THE INFLUENCE OF SEVERAL ROUNDS
OF CONVECTION. ACROSS SANTIAGO DEL ESTERO/NORTHERN SANTA FE
EXPECTING MAXIMA OF 15-25MM. ON SUNDAY-MONDAY...MODELS ARE
FORECASTING A SHORT WAVE TROUGH CROSSING THE ANDES TO INTERACT
WITH A STRENGTHENING CHACO LOW. UNDER THE INFLUENCE OF THE SHORT
WAVE MID/UPPER TROUGH...ENHANCED UPPER JET DYNAMICS...INSTABILITY
AND A PRECIPITABLE WATER POOL OF 50-55MM...EXPECTING HEAVY RAIN
PRODUCING THUNDERSTORMS DEVELOP IN CHACO/CORRIENTES IN
NORTHEASTERN ARGENTINA TO SUSTAIN MAXIMA OF 75-125MM. DOWNSTREAM
ACROSS RIO GRANDE DO SUL AND EXTREME NORTHERN URUGUAY EXPECTING
MAXIMA OF 20-40MM.

SANCHEZ...SMN (ARGENTINA)
SALAZAR...SENAMHI (BOLIVIA)
GALVEZ...WPC (USA)