THE WEATHER PREDICTION CENTER

COLLEGE PARK, MD


SOUTH AMERICAN MODEL DISCUSSION




SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
145 PM EDT WED JUL 18 2018

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

MODEL DISCUSSION (VALID FROM JUL 18 AT 0000 UTC): GLOBAL MODELS
AND ENSEMBLE MEMBERS START GRADUALLY DISAGREEING ON THE EVOLUTION
OF THE FORECAST AFTER 96 HRS...ESPECIALLY TO THE SOUTH OF 35S-40S.
CONFIDENCE DECREASES MORE RAPIDLY AFTER 120 HRS.

THE SYSTEM OF INTEREST IS THE LARGE AND POTENT TROUGH SLOWLY
CROSSING THE ANDES OF CHILE...ARGENTINA...AND BOLIVIA THROUGH THE
CYCLE. UNDER THE INFLUENCE OF A BLOCKING RIDGE TO ITS EAST...THE
TROUGH IS FORECAST TO CROSS SLOWLY...AND EVOLVE INTO A NEGATIVELY
TILTED SYSTEM BY FRIDAY EVENING...WHILE EXTENDING ALONG/OFF THE
COAST OF THE BUENOS AIRES PROVINCE INTO URUGUAY/RIO GRANDE DO SUL.
MOISTURE IS TOO LIMITED TO SUSTAIN LARGE RAINFALL
AMOUNTS...YET...UNDER THE INFLUENCE OF STRONG DYNAMICS AND
EASTERLY MOIST FLOW IN NORTHERN PATAGONIA...EXPECTING LARGE AREAS
OF LIGHT-TO MODERATE PRECIPITATION TO HOLD THROUGH LATE FRIDAY.
THROUGH EARLY THURSDAY...LARGEST ACCUMULATIONS WILL CLUSTER IN
NORTHERN PATAGONIA AND IN URUGUAY/ENTRE RIOS/CORRIENTES...WHERE
EXPECTING MAXIMA OF 20-40MM. ALSO DURING THIS PERIOD...EXPECTING
ACCUMULATIONS OF 15-20CM OF SNOW ACROSS THE ANDES OF CENTRAL CHILE
AND ARGENTINA...AND INTERIOR PORTIONS OF THE NORTHERN PATAGONIA IN
RIO NEGRO AND NEUQUEN. BY THURSDAY-FRIDAY EXPECTING A PEAK IN
ACCUMULATIONS ACROSS NORTHEASTERN ARGENTINA INTO THE RIO DE LA
PLATA/URUGUAY...WHERE SCATTERED THUNDERSTORMS WILL SUSTAIN MAXIMA
OF 35-70MM/DAY. ACTIVITY WILL BE ENHANCED BY A PEAK IN UPPER
DYNAMICS...BUT ALSO BY INTERACTIONS BETWEEN THE PROGRESSING
SURFACE COLD FRONT AND THE LOW-LEVEL JET FROM THE NORTH. SOUTH
ACROSS NORTHERN PATAGONIA EXPECTING MAXIMA OF 20-40MM/DAY. BY
FRIDAY-SATURDAY...STRONGEST CONVECTION WILL SUSTAIN MAXIMA OF
15-25MM/DAY ACROSS SOUTHEASTERN PARAGUAY INTO MISIONES/CORRIENTES
AND SOUTHERN BRASIL. AS OCCLUDED LOW EXITS THE COAST...STRONG
SOUTHERLY FLOW IN SOUTHERN PORTIONS OF THE BUENOS AIRES PROVINCE
WILL SUSTAIN MAXIMA OF 15-20MM.

ALSO DURING THIS PERIOD...POTENT COLD FRONT WILL ARRIVE INTO
CENTRAL BOLIVIA ACCOMPANIED BY A SOUTHERLY JET. THIS WILL INTERACT
WITH ANOTHER TROUGH APPROACHING FROM THE WEST TO CROSS THE
ALTIPLANO DURING SATURDAY TO SUSTAIN ACTIVE CONVECTION AND
UNSEASONABLY LARGE ACCUMULATIONS. BY FRIDAY-SATURDAY EXPECTING
MAXIMA OF 20-40MM/DAY ACROSS SOUTHEASTERN PERU INTO CENTRAL
BOLIVIA. IN THE ALTIPLANO AND CENTRAL PERO EXPECTING ACCUMULATIONS
OF 05-10MM WITH A MIX OF RAIN AND SNOW...INCLUDING SNOW
ACCUMULATIONS IN HIGHER ELEVATIONS. A MORE SIGNIFICANT EVENT IS
EXPECTED BY SATURDAY SUNDAY...AS NEGATIVELY TILTED UPPER TROUGH
CROSSES THE ALTIPLANO ACCOMPANIED BY A MOIST AIR MASS. THIS WILL
LEAD TO MODERATE PRECIPITATION AND SIGNIFICANT SNOW FALL
ACCUMULATIONS IN HIGHER TERRAIN. EXPECTING AMOUNTS OF 15-20CM OF
SNOW IN EXTREME SOUTHERN PERU AND ACROSS ELEVATIONS ABOVE
4200/4300 METERS ABOVE SEA LEVEL...IN THE CENTRAL AND NORTHERN
BOLIVIAN ALTIPLANO. ALSO DURING THIS PERIOD...CONVECTION EAST OF
THE ANDES WILL LEAD TO MAXIMA OF 20-35MM. AS SHEAR LINE
ORGANIZES...ENHANCED CONVECTION IN NORTHWESTERN BRASIL...SOUTHERN
COLOMBIA AND NORTHERN PERU WILL SUSTAIN MAXIMA OF 25-50MM/DAY.
AMOUNTS WILL DECREASE AFTER.

ALSO IN ASSOCIATION WITH THIS UPPER TROUGH...AS IT CROSSES INTO
SOUTHERN BRASIL IT WILL INTERACT WITH MOIST AIR MASS AND SURFACE
FRONT IN RIO GRANDE DO SUL TO SUSTAIN AMOUNTS OF 15-25MM/DAY.

ANOTHER POTENT UPPER TROUGH ARRIVES INTO SOUTHERN CHILE ON
FRIDAY-SATURDAY. THIS WILL BE INITIALLY ACCOMPANIED BY A WEAK
SUBTROPICAL MOISTURE CONNECTION...BUT GIVEN THE LIMITED AMOUNTS OF
AVAILABLE MOISTURE AND RAPID PROPAGATION OF THE FRONT...EXPECTING
MAXIMA OF 10-15MM ONLY ON FRIDAY-SATURDAY. A PEAK IN ACTIVITY IS
E4XPECTED DURING SATURDAY-SUNDAY...AIDED BY INSTABILITY ASSOCIATED
BY THE UPPER TROUGH. THIS WILL SUSTAIN COLD AIR CUMULUS AND THE
RISK FOR ISOLATED THUNDERSTORMS WITH ACCUMULATIONS OF 15-30MM/DAY.

DIAZ...INUMET (URUGUAY)
SANCHEZ...SMN (ARGENTINA)
DAVISON/GALVEZ...WPC (USA)





Last Updated: 145 PM EDT WED JUL 18 2018