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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1644Z Apr 15, 2015)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1244 PM EDT WED APR 15 2015

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

MODEL DISCUSSION (VALID FROM APRIL 15 AT 0000 UTC): GLOBAL MODELS
CONTINUE EXPERIENCING PROBLEMS IN THE LONG TERM RANGE. FORECAST
CONFIDENCE IS HIGH THROUGH 72-84 HRS...DECREASING AFTERWARDS.

SYSTEM OF CONCERN IS A MID LATITUDE TROUGH THAT IS TO MEANDER OFF
THE SOUTHERN-CENTRAL COAST OF CHILE THROUGH EARLY THURSDAY...WHEN
IT WILL START CROSSING THE ANDES CENTERING AT LATITUDE 40S.
MOISTURE IN THE CHILEAN SIDE IS LIMITED THUS EXPECTING AMOUNTS
BELOW THE 10MM/DAY RANGE AS THE TROUGH CROSSES. HOWEVER...THE
SYSTEM WILL ENCOUNTER A MUCH MORE UNSTABLE AIRMASS ONCE IT CROSSES
INTO ARGENTINA. EXPECTING CYCLOGENESIS IN PATAGONIA DURING
THURSDAY. THIS WILL PLACE SURFACE LOW JUST TO THE NORTHWEST OF
PENINSULA VALDEZ AND THE COLD FRONT INTO SOUTHERN LA PAMPA AND
SOUTHERN MENDOZA. INSTABILITY WILL BE HIGH. THIS WILL LEAD TO
SCATTERED THUNDERSTORM DEVELOPMENT WITH ACCUMULATIONS OF
50-10MM/DAY AND THE RISK FOR SEVERE WEATHER INTO EARLY FRIDAY.
DURING FRIDAY AND INTO EARLY SATURDAY...EXPECTING STRONGEST
CONVECTION TO EXTEND FROM NORTHERN ARGENTINA/SOUTHERN PARAGUAY
INTO SOUTHERN BRASIL AND URUGUAY WITH MAXIMA OF 40-80MM/DAY AND
STILL THE RISK OF SEVERE CONVECTION. AMOUNTS ARE TO DECREASE
RAPIDLY AFTERWARDS.

A SUBTROPICAL MOISTURE CONNECTION IS EXPECTED TO ARRIVE IN
SOUTHERN CHILE BY FRIDAY INTO EARLY SATURDAY. THIS WILL LEAD TO
ENHANCED MOISTURE CONVERGENCE BETWEEN LOS ANGELES AND AYSEN.
EXPECTING ACCUMULATIONS INCREASING TO MAXIMA OF 15-30MM/DAY.
AMOUNTS ARE TO DECREASE AFTERWARDS. NOTE THAT CONFIDENCE IN
SOUTHERN CHILE DECREASES RAPIDLY AFTER THIS PERIOD. THE NEXT FRONT
TO PRODUCE SIGNIFICANT PRECIPITATION IN THE AREA IS EXPECTED
DURING SUNDAY. THIS WILL LEAD TO LARGEST RAINFALL AMOUNTS BETWEEN
VALDIVIA AND AYSEN ONCE AGAIN WITH MAXIMA OF 15-20MM/DAY. LIGHTER
AMOUNTS ARE EXPECTED AS FAR NORTH AS CONCEPCION/CHILLAN AND AS FAR
SOUTH AS PUERTO NATALES.

NORTH OVER THE CONTINENT...AT 200 HPA...THE SUBEQUATORIAL RIDGE
CONTINUES WELL ESTABLISHED OVER MATO GROSSO. AN UPPER DIVERGENT
PATTERN IS TO CONTINUE ACROSS EQUATORIAL PARTS OF SOUTH AMERICA.
THIS WILL VENTILATE STRONG CONVECTION IN ASSOCIATION WITH THE
ITCZ/NET. ACTIVITY WILL BE FURTHER ENHANCED BY THE UPPER DIVERGENT
PHASE OF THE MADDEN-JULIAN OSCILLATION. EXPECTING DIURNAL AMOUNTS
IN GENERALLY THE 20-40MM/DAY RANGE FOR THE MAXIMA WITH AREAS OF
30-60MM/DAY SHIFTING FROM THE AMAZON DELTA EARLY IN THE CYCLE TO
NORTHWESTERN BRASIL/COLOMBIA LATER IN THE CYCLE. ACTIVE CONVECTION
WILL ALSO DOT THE ECUADORIAN COAST AND ANDES...ESPECIALLY EARLY IN
THE CYCLE. INSTABILITY AND VENTILATION WILL REMAIN HIGH THROUGH
EARLY NEXT WEEK...BUT IT APPEARS THAT LOW-LEVEL DYNAMICS WILL NOT
BE FAVORABLE FOR EXTREME RAINFALL. OTHER REGION WITH ACTIVE
CONVECTION WILL BE PARAGUAY...WHERE MODELS ARE SHOWING DEEP-LAYER
INSTABILITY REMAINING HIGH THROUGH THE CYCLE. THIS PATTERN OF
DEEP-LAYER CONVECTIVE INSTABILITY AND SHORT WAVE PERTURBATIONS
MOVING IN FROM THE NORTHWEST IN THE PERIPHERY OF THE SUBTROPICAL
RIDGE IS FAVORABLE FOR PERIODS WITH STRONG THUNDERSTORMS.
EXPECTING ACCUMULATIONS IN THE 15-30MM/DAY RANGE AFTER THE HEAVY
RAINFALL EVENT TAPERS OFF FRIDAY-EARLY SATURDAY.

SCHNEIDER...INMET (BRASIL)
CALLE...UNALM (PERU)
CHARPENTIER...DMC (CHILE)
DAVISON/GALVEZ...WPC (USA)