Prognostic Meteorological Discussion
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000
FXSA20 KWBC 171642
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1242 PM EDT FRI APR 17 2015

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

MODEL DISCUSSION (VALID FROM APRIL 17 AT 0000 UTC): GLOBAL MODELS
CONTINUE ADJUSTING THEIR FORECAST. THIS IS STILL RESULTING IN AN
EARLY AND RAPID DECREASE OF FORECAST CONFIDENCE. CONFIDENCE IS
HIGH THROUGH 72-84 HRS AND RAPIDLY DECREASES AFTERWARDS ESPECIALLY
ON THE EVOLUTION OF SHORTER WAVES.

POTENT MID-LATITUDE UPPER TROUGH OVER SOUTH-CENTRAL ARGENTINA WILL
EXIT INTO THE SOUTHERN ATLANTIC BY EARLY SATURDAY. THIS TROUGH IS
SUSTAINING A SURFACE FRONT THAT WILL EXTEND ACROSS SOUTHERN RIO
GRANDE DO SUL-CHACO-SANTIAGO DEL ESTERO BY FRIDAY EVENING...TO
SANTA CATARINA-SOUTHERN PARAGUAY-FORMOSA-SALTA BY SATURDAY WHERE
IT IS TO STALL AND MEANDER DURING THE NEXT SEVERAL DAYS AS IT
WEAKENS. THE FRONT IS SUSTAINING A PRE-FRONTAL SQUALL LINE THAT
WILL ENHANCE CONVECTIVE DEVELOPMENT AHEAD OF THE MAIN FRONT TO
AFFECT MOST OF NORTHERN ARGENTINA/SOUTHWESTERN PARAGUAY THROUGH
EARLY SATURDAY. CONVECTIVE INSTABILITY REMAINS VERY HIGH AS WELL
AS MOISTURE CONVERGENCE...AIDED BY A PAMPERO JET FROM THE SOUTH
AND A LOW-LEVEL JET FROM THE NORTH EXITING IN THE PARAGUAYAN
CHACO. THIS WILL LEAD TO SEVERE CONVECTION WITH ACCUMULATIONS
PEAKING THROUGH EARLY SATURDAY AT MAXIMA OF 75-125MM/DAY. AMOUNTS
ARE TO RAPIDLY DECREASE AFTERWARDS. EXPECTING SHOWERS ACROSS THE
RIO DE LA PLATA BASIN AND SOUTHERN BUENOS AIRES PROVINCE WITH
MAXIMUM ACCUMULATIONS OF 15-30MM/DAY THROUGH EARLY SATURDAY
DECREASING AFTERWARDS.

A SUBTROPICAL MOISTURE CONNECTION IS ARRIVING INTO SOUTHERN CHILE
AND MOISTURE CONVERGENCE WILL PEAK INTO EARLY SATURDAY. THIS WILL
ACCUMULATIONS INCREASING TO MAXIMA OF 15-30MM/DAY THROUGH EARLY
SATURDAY. DURING SATURDAY RAINFALL WILL SPREAD NORTHWARD AS WELL
DEFINED SURFACE COLD FRONT RACES TO EXTEND NEAR TALCA/LINARES BY
SATURDAY EVENING. LARGEST RAINFALL AMOUNTS ARE EXPECTED BETWEEN
TALCA AND VALDIVIA/PUERTO MONTT WITH MAXIMA OF 15-20MM/DAY PEAKING
AT INLAND LOCATIONS. BY SUNDAY FRONT WILL EXTEND NEAR
CURICO/RANCAGUA TO START WEAKENING. THIS WILL LEAD TO
ACCUMULATIONS BELOW 05MM/DAY BETWEEN TALCA AND CONCEPCION. AMOUNTS
ARE TO DECREASE AFTERWARDS.

NORTH OVER THE CONTINENT AND AT 200 HPA...AN UNSEASONABLY STRONG
SUBTROPICAL RIDGE CONTINUES CENTERING OVER MATO GROSSO IN BRASIL.
TO THE NORTH THE SUBEQUATORIAL RIDGE IS FORECAST TO RELOCATE
WESTWARD. AS THIS HAPPENS...BEST VENTILATION WILL CONTINUE
MIGRATING NORTHWESTWARD INTO ECUADOR/COLOMBIA/NORTHERN PERU WHERE
IT WILL REMAIN THROUGH EARLY NEXT WEEK. THIS WILL LEAD TO LARGEST
RAINFALL AMOUNTS IN AMAZONIAN DIURNAL CONVECTION TO CONCENTRATE IN
NORTHWESTERN BRASIL WHERE EXPECTING DAILY MAXIMA OF 25-50MM/DAY
THROUGH EARLY SUNDAY. AMOUNTS WILL START DECREASING AFTERWARDS.
OTHER AREA OF ACTIVE CONVECTION WILL BE NORTHERN/EASTERN BOLIVIA
AND CENTRAL/SOUTHERN PERU EARLY IN THE CYCLE. ENHANCED LOW-LEVEL
CONVERGENCE AND INSTABILITY WILL LEAD TO DAILY MAXIMA IN THE ORDER
OF 15-35MM/DAY THROUGH EARLY SUNDAY DECREASING AFTERWARDS. TO THE
NORTHWEST...SEASONALLY ACTIVE CONVECTION WILL CONTINUE DOTTING THE
ANDES OF ECUADOR AND NORTHERN PERU WHERE EXPECTING MAXIMA OF
15-20MM/DAY THROUGH EARLY SATURDAY DECREASING DURING SATURDAY INTO
SUNDAY. ACCUMULATIONS ARE TO START INCREASING ONCE AGAIN INTO
EARLY NEXT WEEK. EXPECTING ISOLATED HIGHER AMOUNTS IN THE WESTERN
FOOTHILLS WHERE THE DIURNAL SEA BREEZE CONVERGENCE WILL CONTINUE
TRIGGERING ISOLATED STRONG THUNDERSTORMS AIDED BY THE LOCAL SST
WARMING.

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





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