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South American Forecast Discussion
 
(Latest Discussion - Issued 1633Z May 22, 2018)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1233 PM EDT TUE MAY 22 2018

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

MODEL DISCUSSION (VALID FROM MAY 22 AT 0000 UTC): MID/UPPER LEVEL
TROUGH EXTENDS ACROSS MID SECTIONS OF SOUTH AMERICA...WITH AXIS
TRAVERSING URUGUAY-PARAGUAY TO BOLIVIA EARLY IN THE CYCLE. LATER
TODAY THE TROUGH WILL MOVE OFF THE COAST OF BRASIL/URUGUAY...WITH
AXIS TO GRADUALLY DEEPEN AS IT NEARS 30W LATER ON THURSDAY. AT LOW
LEVELS THIS SUSTAINS A DEEPENING LOW OFF THE COAST OF
URUGUAY/SOUTHERN BRASIL...WITH A FRONT TO THE WEST-NORTHWEST
ACROSS SOUTHERN BRASIL. ON WEDNESDAY-THURSDAY A  BUILDING POLAR
RIDGE OVER ARGENTINA-PARAGUAY IS TO DISPLACE THE POLAR TROUGH
FARTHER EAST INTO THE SOUTH ATLANTIC...WHILE ASSOCIATED SURFACE
FRONT IS TO TRAIL ALONG THE SOUTHEAST COAST OF BRASIL. AS IT
MEANDERS OVER EASTERN SAO PAULO-RIO DE JANEIRO THIS IS TO SUSTAIN
LIGHT COASTAL CONVECTION...WITH MAXIMA OF 05-10MM ON
WEDNESDAY-THURSDAY. ALSO...UNDER THE INFLUENCE OF THE LOW LEVEL
RIDGE...A DRYER AIR MASS IS TO THEN ADVECT ACROSS MID SECTIONS OF
SOUTH AMERICA. THIS PATTERN IS EXPECTED TO HOLD TROUGH THE END OF
THE WEEK.

AN OLD FRONTAL BOUNDARY LIES TO THE NORTH...EXTENDING WEST ACROSS
THE SOUTH ATLANTIC TO ESPIRITO SANTO/SOUTHERN MINAS GERAIS IN
BRASIL. A PREFRONTAL SHEAR LINE TO THE NORTH EXTENDS ACROSS
SOUTHERN BAHIA. THIS IS ENHANCING COASTAL CONVERGENCE BETWEEN
NORTHERN ESPIRITO SANTO AND EASTERN BAHIA. THIS PATTERN IS TO HOLD
DURING THE NEXT FEW DAYS. MEANWHILE... EXPECTING RAINFALL AMOUNTS
OF 20-35MM/DAY...WITH LOCALLY HIGHER AMOUNTS HIGHLY PROBABLE DUE
TO TOPOGRAPHICAL FORCING.

OVER THE EASTERN PACIFIC...A BROAD/LOW AMPLITUDE MID LEVEL RIDGE
DOMINATES THE FLOW TO THE EAST OF 90W AND NORTH OF 40S. AS THE
RIDGE HOLDS...IT IS TO STEER A DEEPENING TROUGH TOWARDS SOUTHERN
PORTIONS OF CHILE. THIS IS EXPECTED TO FOLLOW A RAPID RATE OF
INTENSIFICATION... INDUCING HEIGHT FALLS OF 250-300 GPM AS IT
NEARS SOUTH COAST OF CHILE ON FRIDAY. AT LOW LEVELS THIS WILL
SUSTAIN A LOW THAT IS TO DEEPEN AT A NEAR EXPLOSIVE RATE...WITH
THE UKMET AND ECMWF SHOWING POSSIBILITY OF A  METEOROLOGICAL BOMB.
AS IT MOVES ACROSS THE SOUTHERN CONE THE DEEP CYCLONE WILL BE
ACCOMPANIED BY TROPICAL STORM INTENSITY WINDS OF 40-60KT. UNDER
FAVORABLE JET DYNAMICS THIS IS TO SUSTAIN MODERATE TO HEAVY
RAINFALL AMOUNTS BETWEEN PUERTO MONTT AND ISLA DE CHILOE...WITH
MAXIMA OF 20-35MM ON THURSDAY AND 20-40MM ON FRIDAY. NOTE THAT THE
STABILITY INDICES CONTINUE TO SHOW RISK OF THUNDERSTORMS OVER
SOUTHERN CHILE ON THURSDAY-FRIDAY AND SATURDAY.

NORTH OVER THE CONTINENT...AT 200 HPA...A BROAD/LOW AMPLITUDE
RIDGE CONTINUES TO DOMINATE THE FLOW TO THE NORTH OF 10S. THIS IS
TO VENT DIURNAL CONVECTION ALONG THE NORTH COAST OF BRASIL AND
EQUATORIAL SOUTH AMERICA. ALONG THE NORTH COAST OF BRASIL
EXPECTING RAINFALL AMOUNTS OF 05-10MM/DAY EARLY IN THE
CYCLE...INCREASING TO 15-25MM ON THURSDAY-FRIDAY. OVER
AMAPA-NORTHERN PARA EXPECTING ACCUMULATION OF 25-50MM EARLY IN THE
CYCLE...DECREASING TO 15-30MM/DAY ON THURSDAY. OVER NORTHERN
AMAZONAS-RORAIMA IN WESTERN BRASIL TO SOUTHERN COLOMBIA-EASTERN
ECUADOR/NORTHERN PERU ACTIVITY IS TO BECOME BETTER ORGANIZED LATER
IN THE WEEK. EARLY IN THE CYCLE EXPECTING MAXIMA OF 20-35MM.
THROUGH THURSDAY-FRIDAY THIS INCREASES TO MAXIMA OF 30-60MM IN
SCATTERED DEEP CONVECTION.

BALBINO...INMET (BRASIL)
DIAZ...INUMET (URUGUAY)
DAVISON...WPC (USA)