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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1648Z Apr 21, 2022)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1248 PM EDT THU APR 21 2022

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

LAST NO AMENDS...NEXT DISCUSSION ON MONDAY APRIL 25.

FORECAST DISCUSSION FROM APR 21/16 UTC: PERSISTENT MID/UPPER LEVEL
RIDGE DOMINATES THE FLOW OVER THE PACIFIC BETWEEN 90W-120W WHILE
ANCHORING ON A MEANDERING MID LEVEL HIGH NEAR 35S 107W. AS THE
RIDGE HOLDS TO THE WEST...SHORT WAVE PERTURBATIONS STREAMING TO
THE SOUTH OF THIS AXIS WILL TEND TO AMPLIFY AS THEY LIFT ACROSS
THE SOUTHERN REGIONS OF CHILE INTO ARGENTINA. AT AN INTERVAL
RANGING BETWEEN 36-48 HRS...THE SUCCESSIVE SHORT WAVE
PERTURBATIONS WILL HELP SUSTAIN THE GRADUAL AMPLIFICATION OF A
LONG WAVE TROUGH ACROSS THE BELLINGSHAUSEN SEA INTO THE SOUTHERN
CONE THROUGH THE WEEKEND TO EARLY NEXT WEEK. THE EVOLVING LONG
WAVE AXIS WILL THEN FAVOR A LONG FETCH COLD ADVECTIVE PATTERN
ACROSS THE SOUTHEAST PACIFIC TO THE SOUTHERN CONE...WITH COLD
ANTARCTIC AIR ADVECTING ACROSS SOUTHERN CHILE TO PATAGONIA/LA
PAMPA IN ARGENTINA.

AT LOW LEVELS...PROGRESSIVE FRONTAL LOWS WILL LIFT ACROSS THE
SOUTHEAST PACIFIC TO THE SOUTHERN OF SOUTH AMERICA DURING THE NEXT
COUPLE OF DAYS...TO COMBINE IN SUPPORT OF A PERSISTENT BAROCLINIC
BOUNDARY ACROSS URUGUAY/NORTHERN ARGENTINA INTO SOUTHERN BRASIL.
THIS BOUNDARY WILL BE THE FOCUS OF LOW LEVEL MOISTURE CONVERGENCE
DURING THE NEXT THREE DAYS. THROUGH THE DAY TODAY INTO THE LATE
EVENING HOURS...UNDER THE INFLUENCE OF A SUBTROPICAL JET
MAXIMA...THIS WOULD LIKELY FAVOR HEAVY RAINFALL AMOUNTS ACROSS
CORRIENTES/MISIONES IN ARGENTINA-SOUTHEAST PARAGUAY INTO RIO
GRANDE DO SUL IN SOUTHERN BRASIL. IN THIS AREA EXPECTING MAXIMA OF
35-70MM IN POTENTIALLY SEVERE CONVECTION...WHILE ON FRIDAY TO
SATURDAY EXPECTING ADDITIONAL RAINFALL AMOUNTS OF 20-40MM.

THESE WILL BE FOLLOWED BY AN INTENSE FRONTAL LOW THAT REACHES THE
SOUTHERN REGIONS OF CHILE LATER ON FRIDAY...WITH AXIS EVOLVING
INTO A BROAD/DEEP OCCLUSION OVER THE SOUTHERN CONE/SOUTHWEST
ATLANTIC DURING THE WEEKEND. THE DEEPENING FRONTAL LOW AND THE
ENSUING PRESSURE GRADIENT WILL SUSTAIN STRONG BOUNDARY LAYER WINDS
35-60KT STARTING ON FRIDAY AND LASTING THROUGH SUNDAY. FAVORING
COLD ADVECTION OVER WARMER THAN NORMAL WATERS OFF THE SOUTH COAST
OF CHILE...THE ENSUING CONVECTIVE INSTABILITY WILL COMBINE WITH
TOPOGRAPHICAL LIFTING ACROSS THE SOUTHERN CONE TO TRIGGER HEAVY
RAINFALL AMOUNTS OVER CHILE BETWEEN CONCEPCION IN CENTRAL CHILE
AND ISLA DE CHILOE TO THE SOUTH. THIS INCREASES FROM 25-50MM
DURING THE DAY TODAY TO 35-70MM/DAY ON FRIDAY AND SATURDAY...AND
AGAIN ON MONDAY AS ANOTHER FRONTAL LOW LIFTS ACROSS THE SOUTHERN
CONE. OVER ARGENTINA MOST ACTIVE IS EXPECTED IN ASSOCIATION WITH
THE FRONTAL LOW TO THE SOUTH AS IT MEANDERS ACROSS PATAGONIA...BUT
LATER IN THE CYCLE IT WILL INTERACT WITH THE OLD/AFOREMENTIONED
BAROCLINIC BOUNDARY TO THE NORTH TO FAVOR HEAVY RAINFALL PRODUCING
MCS ACROSS URUGUAY-ENTRE RIOS/CORRIENTES IN ARGENTINA AND URUGUAY.
THROUGH EARLY NEXT WEEK THE ACTIVITY WILL THEN BUILD EAST AND
NORTH ACROSS SOUTHERN BRASIL...MEANWHILE TRIGGERING MAXIMA OF
75-125MM.

NORTH OVER THE CONTINENT...THE UPPER LEVEL FLOW SPLITS BETWEEN
CELL OF THE SUBTROPICAL RIDGE OVER BRASIL-BOLIVIA-PERU AND A CELL
OF THE SUBEQUATORIAL RIDE TO THE NORTH. AS THE UPPER FLOW SPLITS
BETWEEN THESE RIDGES...IT WILL RESULT IN A BROAD AREA OF UPPER
LEVEL DIVERGENCE OVER EQUATORIAL SOUTH AMERICA. THIS WILL VENT
CONVECTION TO THE NORTH AND IT IS LIKELY TO ENHANCE CONVECTION IN
ASSOCIATION OF EASTERLY WAVES STREAMING ALONG THE ATLANTIC
ITCZ/NEAR EQUATORIAL TROUGH. EARLY IN THE CYCLE THESE WILL SUSTAIN
A HEAVY RAINFALL PRODUCING MCS ACROSS AMAPA-NORTHERN PARA IN
BRASIL AND THE GUIANAS...WITH MAXIMA IN THIS AREA PEAKING AT
75-150MM THROUGH FRIDAY MORNING. FRIDAY AND SATURDAY EXPECTING AN
ADDITIONAL RAINFALL AMOUNTS OF 20-40MM. AS ACTIVITY ACROSS PARA IN
BRASIL WEAKENS...CONVECTION ACROSS NORTHERN AMAZONAS-RORAIMA TO
THE NORTHERN JUNGLE OF PERU IS EXPECTED TO INTENSIFY. THIS
INCREASES FROM 15-20MM/DAY EARLY IN THE CYCLE TO 20-45MM/DAY ON
SUNDAY THROUGH TUESDAY.

DAVISON...WPC (USA)