624
FXSA20 KWBC 231640
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1240 PM EDT TUE SEP 23 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z SEP 23). MODELS ARE MAKING SHORT
WAVE PATTERN CORRECTIONS OVER THE EASTERN PACIFIC...AND THEY ARE
SLOWLY REACHING AN AGREEMENT. THE GFS IS MAKING CORRECTIONS TO
THE SOUTHERN STREAM IN FAVOR OF HIGHER AMPLITUDE PATTERN TO
GRADUALLY ESTABLISH...WHILE BOTH EUROPEAN MODELS SHOW
INTENSIFICATION OF THE NORTHERN STREAM PATTERN OVER CHILE/
ARGENTINA. ALTHOUGH FURTHER CHANGES/CORRECTIONS TO THE MODEL ARE
EXPECTED IN THE NEXT COUPLE OF RUNS...CONFIDENCE IN THE FORECAST
IS INCREASING.
AT 500 HPA...ON THE NORTHERN STREAM...A BROAD TROUGH REMAINS OFF
THE SOUTHEAST COAST OF BRASIL...CENTERING ON A MEANDERING LOW
NEAR 30S 43W. THROUGH 24 HRS THE LOW FILLS...WITH THE REMAINING
OPEN TROUGH TO MOVE EAST ACROSS 40W/45W TO 20S. BY 48 HRS THE
TROUGH WILL MOVE ALONG 30W/35W...WITH A NEW LOW FORMING ALONG
THIS AXIS NEAR 38S 31W. THIS WILL THEN CONTINUE EAST ACROSS 39S
19W BY 96 HRS. THE TROUGH SUPPORTS A SURFACE FRONT THAT
ORIGINATES ON AN OCCLUDED LOW OFF THE COAST OF BRASIL...WITH
BOUNDARY ENTERING THE CONTINENT OVER RIO DE JANEIRO...THEN WEST
ACROSS SOUTHERN MINAS-MATO GROSSO TO NORTHERN BOLIVIA. THROUGH 24
HRS THE FRONT WILL MEANDER NORTH TO ESPIRITO SANTO...WHILE THE
TRAILING END REMAINS NEARLY STATIONARY ACROSS MATO GROSSO/MATO
GROSSO DO SUL-NORTHERN BOLIVIA. THE SURFACE LOW/TROUGH WILL THEN
PULL TO THE EAST-SOUTHEAST AS THE CYCLE PROGRESSES...WHILE THE
FRONT MEANDERS OVER ESPIRITO SANTO/MINAS GERAIS-MATO GROSSO DO
SUL TO BOLIVIA. CONVECTION ALONG THIS BOUNDARY IS BECOMING
SPARSE...WITH MOST ACTIVE TO CONCENTRATE OVER ESPIRITO SANTO/
SOUTHERN BAHIA AND EASTERN MINAS GERAIS WHERE IT WILL FAVOR
MAXIMA OF 05-10MM/DAY OVER THE NEXT THREE DAYS.
ANOTHER TROUGH LIES OFF THE COAST OF CHILE...WITH MID LEVEL AXIS
EXTENDING NORTH ALONG 80W. THIS TROUGH IS TO PUSH EAST AGAINST A
BLOCKING RIDGE OVER THE CONTINENT. UNDER INFLUENCE OF THIS
RIDGE...THE TROUGH WILL SHEAR EAST ACROSS PATAGONIA BY 24 HRS. A
NORTHERN STREAM VORTEX WILL LAG...TO MOVE ACROSS NORTHWEST
ARGENTINA EARLY THIS PERIOD...AND TO SOUTHEAST BRASIL BY 72 HRS.
IT WILL THEN DEEPEN THROUGH 120 HRS AS A SHORT WAVE RIDGE BUILDS
TO THE WEST ACROSS PARAGUAY/CHACO ARGENTINO. AS THE SHORT WAVE
TROUGH MOVES ACROSS THE NORTHERN ANDES OF CHILE...IT WILL
FAVOR/TRIGGER ISOLATED CONVECTION ON THE NORTHERN ANDES OF
ARGENTINA (JUJUY-SALTA) BY 48 HRS...WITH ACCUMULATION OF 05-
10MM/DAY. MOST SIGNIFICANT WILL BE THE INTERACTION BETWEEN THIS
TROUGH AND THE AFOREMENTIONED FRONT OVER BRASIL...WHERE THE SHORT
WAVE TROUGH IS TO TRIGGER A FRONTAL LOW TO CENTER OVER SOUTHERN
MINAS GERAIS/NORTHERN SAO PAULO. THIS WAVE DEEPENS THROUGH 72-96
HRS...SUPPORTING MODERATE TO HEAVY RAINS ACROSS MINAS GERAIS/
GOIAS WITH MAXIMA OF 30-60MM...WHILE OVER NORTHERN SAO
PAULO/SERRA DO MAR THE MAXIMA WILL PEAK AT 20-40MM IN MIXED
RAINFALL (STRATIFORM/CONVECTIVE).
THE MODELS CONTINUE TO FORECAST ANOTHER MID LEVEL TROUGH TO MOVE
ACROSS THE PACIFIC ALONG 90W BY 48 HRS. AS IT NEARS 80W...IT WILL
CLASH WITH A BUILDING RIDGE OVER CHILE/ARGENTINA. UNDER INFLUENCE
OF THIS RIDGE...MODELS STARTING TO AGREE ON A GRADUAL EVOLUTION
OF THIS AXIS...WITH TROUGH TO SPLIT IN TWO BY MID CYCLE. AT THAT
TIME...THE SOUTHERN HALF WILL START TO MOVE ACROSS THE DRAKE
PASSAGE/SOUTHERN CHILE-PATAGONIA...TO CROSS INTO THE SOUTH
ATLANTIC BY 108 HRS. SOME DIFFERENCES REMAIN ON THE AMPLITUDE OF
THIS AXIS...WITH BOTH EUROPEAN MODELS STILL FAVORING A SLIGHTLY
STRONGER/DEEPER TROUGH THAN THE GFS. ON THE NORTHERN STREAM...
HOWEVER...THEY START TO AGREE ON A STRONG VORTEX TO REMAIN OFF
THE CENTRAL COAST OF CHILE BY 72 HRS...AND THROUGH 96-108 HRS IT
WILL MOVE INTO CENTRAL CHILE/CUYO IN ARGENTINA...WHERE IT WILL
MEANDER THROUGH 120 HRS. THE NORTHERN STREAM TROUGH WILL SUPPORT
A POLAR FRONT THAT ENTERS CENTRAL CHILE BY 72-84 HRS...AND MOVES
INTO PATAGONIA THROUGH 96-108 HRS...TO THEN REMAIN OVER CENTRAL
PROVINCES OF ARGENTINA. STRONG JET DYNAMICS ARE TO ACCOMPANY THIS
SYSTEM...WHICH WILL HELP SUSTAIN THE BAROCLINICITY AND STRONG
THERMAL GRADIENT. AS A RESULT...IT HAS A HIGH POTENTIAL TO
TRIGGER ORGANIZED RAINFALL ACROSS PATAGONIA AND CENTRAL PROVINCES
OF ARGENTINA...WITH RISK OF SEVERE CONVECTION OVER CORDOBA-
MENDOZA. ON DAY 04 RAINFALL MAXIMA WILL PEAK AT 10-20MM ALONG THE
CENTRAL ANDES...TO INCREASE TO 25-50MM/DAY OVER CENTRAL ARGENTINA
AND 20-40MM OVER PATAGONIA/LA PAMPA.
NORTH OVER THE CONTINENT...AT 200 HPA....A SHORT WAVE TROUGH
ALONG 50W/55W SPLITS THE SUBTROPICAL RIDGE IN TWO CELLS...WITH
ONE OVER THE EASTERN PACIFIC AND THE OTHER OVER THE WESTERN
TROPICAL ATLANTIC/NORTHEAST BRASIL. THE SHORT WAVE TROUGH WILL
GRADUALLY MOVE EAST ACROSS MINAS GERAIS IN BRASIL BY 36-48 HRS.
BY THAT TIME...THE WESTERN CELL OF THE SUBTROPICAL RIDGE
RELOCATES TO AMAZONAS IN BRASIL...WHERE IT BUILDS/INTENSIFIES 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. OTHER CONVECTION IS
EXPECTED OVER MINAS GERAIS IN BRASIL...WHERE THE AFOREMENTIONED
SHORT WAVE TROUGH...AND A SECONDARY VORTEX ARE TO TRIGGER
SCATTERED DEEP CONVECTION BY MID CYCLE. IN THIS REGION THE MODELS
ARE PROJECTING MODERATE TO HEAVY RAINFALL AMOUNTS BY 72-108
HRS...AND THEY HAVE REMAIN FAIRLY CONSISTENT ON THEIR FORECAST OF
THIS EVENT.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
CLICK HERE TO GO TO PREVIOUS BULLETINS.
Main Text Page