003
FXSA20 KWBC 241704
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
104 PM EDT WED SEP 24 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z SEP 24). ON THE SHORT RANGE
FORECAST THE GLOBAL MODELS MAINTAIN GOOD CONTINUITY THROUGH 60-72
HRS. ON THE MEDIUM RANGE THE OPPOSITE IS TRUE...AS THEY MAKE
PATTERN CORRECTIONS TO DAYS 04 AND 05. THE GFS...IN A REVERSAL TO
PREVIOUS RUNS...NOW FORECASTS A STRONGER/HIGHER AMPLITUDE TROUGH
TO ENTER THE SOUTHERN CONE BY 84-96 HRS...WHICH IS SIMILAR TO
WHAT THE EUROPEAN MODELS PREVIOUSLY SUGGESTED. BUT BY DOING
SO...IT SHIFTS THE BULK OF THE ENERGY TO PATAGONIA/DRAKE
PASSAGE...AND NOW DOWNPLAYS IMPORTANCE OF A NORTHERN STREAM
VORTEX ENTERING CENTRAL/NORTHERN ARGENTINA. THE EUROPEAN MODELS
DO THE OPPOSITE... NOW FORECASTING A STRONGER/DEEPER NORTHERN
STREAM VORTEX TO ENTER MID SECTIONS OF SOUTH AMERICA. THE GLOBAL
ENSEMBLES CONCUR WITH THIS ASSESSMENT...WITH A LARGE MAJORITY OF
THE MEMBERS SUPPORTING THE EUROPEAN SOLUTION. BUT WE EXPECT
FURTHER CORRECTIONS TO THIS PATTERN...WHICH INTRODUCES A LOT OF
UNCERTAINTY TO THE MEDIUM RANGE FORECAST...AND LOW CONFIDENCE IN
THE FORECAST LATER IN THE PERIOD.
AT 500 HPA...ON THE NORTHERN STREAM...A TROUGH EXTENDS INTO
SOUTHEAST BRASIL...WITH AXIS TO MOVE EAST ACROSS 30W/35W BY 24
HRS...AND THROUGH 48 HRS IT WILL CONTINUE ACROSS 20W. A SECONDARY
VORTEX MOVES ACROSS NORTHWEST ARGENTINA/CHACO BY 24 HRS...AND
THROUGH 48 HRS IT WILL TRACK INTO SOUTHEAST BRASIL. AT 60-84 HRS
IT WILL THEN MOVE ACROSS SAO PAULO/RIO DE JANEIRO-SOUTHERN MINAS
GERAIS...TO EVOLVE INTO A BROAD/LOW AMPLITUDE TROUGH OFF THE
COAST BY THE END OF THE CYCLE. AT LOW LEVELS THE TROUGH SUPPORTS
A SURFACE FRONT THAT ORIGINATES ON AN OCCLUDED LOW OFF THE COAST
OF BRASIL...WITH BOUNDARY ENTERING THE CONTINENT OVER VITORIA...
THEN WEST ACROSS SOUTHERN MINAS-MATO GROSSO TO NORTHERN BOLIVIA.
THE SURFACE LOW/TROUGH WILL PULL TO THE EAST-SOUTHEAST AS THE
CYCLE PROGRESSES...WHILE THE FRONT MEANDERS OVER ESPIRITO SANTO/
MINAS GERAIS-MATO GROSSO DO SUL TO BOLIVIA THROUGH 24-36 HRS. AT
48 HRS...UNDER INFLUENCE OF THE MID/UPPER LEVEL SHORT WAVE
TROUGH...A WAVE/LOW WILL FORM ALONG THIS FRONT OVER NORTHERN SAO
PAULO...WITH LOW TO SLOWLY MOVE OFF THE COAST TO JUST EAST OF RIO
DE JANEIRO/NORTHERN SAO PAULO BY 72-96 HRS. THE ASSOCIATED
FRONT...MEANWHILE...IS TO MEANDER ACROSS ESPIRITO SANTO/MINAS
GERAIS TO MATO GROSSO DO SUL. EARLY IN THE PERIOD EXPECT ISOLATED
TO SCATTERED CONVECTION TO CONCENTRATE OVER ESPIRITO SANTO/
SOUTHERN BAHIA AND EASTERN MINAS GERAIS WHERE IT WILL FAVOR
MAXIMA OF 10-15MM/DAY. AS THE LOW/WAVE FORMS ALONG THE
FRONT...CONVECTION ACROSS MINAS GERAIS/SAO PAULO BRASIL WILL
BECOME MORE ACTIVE...WITH ACCUMULATION OF 20-40MM/DAY.
A HIGH AMPLITUDE TROUGH WILL FOLLOW...WITH MID LEVEL AXIS ACROSS
90W TO 20S BY 24 HRS. AS IT NEARS 80W BY 48-60 HRS...IT WILL
CLASH WITH A BUILDING RIDGE OVER CHILE/ARGENTINA. UNDER INFLUENCE
OF THIS RIDGE...THE TROUGH WILL SPLIT IN TWO PERTURBATIONS. THE
SOUTHERN HALF WILL MOVE ACROSS THE DRAKE PASSAGE/SOUTHERN CHILE-
PATAGONIA BY 72-84 HRS...INTO THE SOUTH ATLANTIC THROUGH 96-108
HRS. ON THE NORTHERN STREAM...BY 96-108 HRS...WE EXPECT A
STRONGER MID/UPPER LEVEL TROUGH TO ENTER CENTRAL CHILE/ARGENTINA
THAN WHAT THE GFS INDICATES...AS WE ARE MORE PARTIAL TO THE ECMWF
SOLUTION AS SUPPORTED BY THE GLOBAL ENSEMBLES. THE NORTHERN
STREAM TROUGH WILL SUPPORT A POLAR FRONT THAT ENTERS CENTRAL
CHILE BY 48-60 HRS...AND MOVES INTO PATAGONIA THROUGH 72 HRS. AT
96 HRS THE FRONT WILL EXTEND NORTHWEST INTO A LOW OVER BUENOS
AIRES PROVINCE/LA PAMPA TO CORDOBA/MENDOZA IN ARGENTINA. THE LOW
WILL OCCLUDE OVER BUENOS AIRES PROVINCE BY 120 HRS...WHILE THE
FRONT ACCELERATES ACROSS THE NORTHERN PROVINCES OF ARGENTINA TO
SOUTHERN BRASIL-PARAGUAY LATER IN THE PERIOD. AS THE FRONT
ACCELERATES...IT IS LIKELY TO TRIGGER A PREFRONTAL SQUALL LINE
AND SEVERE CONVECTION. ON DAY 03 RAINFALL MAXIMA WILL PEAK AT 10-
20MM ALONG THE CENTRAL ANDES...TO INCREASE TO 25-50MM/DAY OVER
CENTRAL ARGENTINA BY 84-96 HRS. HEAVY RAINS WILL BUILD AHEAD OF
THE FRONT AS IT ENTERS NORTHERN ARGENTINA-SOUTHERN BRASIL-
PARAGUAY LATER IN THE CYCLE.
NORTH OVER THE CONTINENT...AT 200 HPA....A SHORT WAVE TROUGH
ALONG 40W SPLITS THE SUBTROPICAL RIDGE IN TWO CELLS...WITH ONE
OVER WESTERN BRASIL/EASTERN PACIFIC AND THE OTHER OVER THE
WESTERN TROPICAL ATLANTIC/NORTHEAST BRASIL. THE SHORT WAVE TROUGH
WILL GRADUALLY MOVE EAST OF MINAS GERAIS/ESPIRITO SANTO IN BRASIL
BY 24 HRS. AS THE TROUGH MOVES AWAY...THE RIDGE OVER BRASIL/PERU
IS FORECAST TO BUILD/INTENSIFY AS THE CYCLE PROGRESSES. THE
INTENSIFICATION OF THIS RIDGE SIGNALS A STRONG SEASONAL
TRANSITION OVER THE CONTINENT. AS THE RIDGE INTENSIFIES...EXPECT
A SHARP INCREASE IN TROPICAL CONVECTION ACROSS WESTERN/NORTHWEST
PORTIONS OF BRASIL-PERUVIAN JUNGLE-SOUTHERN COLOMBIA AND ACROSS
MOST OF ECUADOR...WHERE DAILY MAXIMA WILL LIKELY EXCEED 20-
40MM/DAY. AN INCREASE IN ACTIVITY IS ALSO EXPECTED ACROSS
CENTRAL/SOUTHERN JUNGLE OF PERU INTO NORTHERN BOLIVIA/RONDONIA
BRASIL BY 96-120 HRS AS PREFRONTAL CONVECTION DEVELOPS IN THIS
AREA.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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