244
FXSA20 KWBC 101640
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1240 PM EDT WED SEP 10 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
NOTE: ON DAY 01...THE GLOBAL MODELS SHOW FAVORABLE CONDITIONS FOR
SEVERE CONVECTION...TO AFFECT BETWEEN ASUNCION-CORRIENTES-
MONTEVIDEO TO SANTA CATARINA IN BRASIL. CAPE VALUES PEAK AT
4000J/KG ON THE 10/21UTC...WHILE THE TOTALS-TOTALS REACH 52 AND
THE SSI DROP TO -8. CONVECTIVE INHIBITORS...HOWEVER...ARE RATHER
HIGH. BUT WITH LOW LEVEL TEMPERATURES IN THE 30S AND DEW POINT
TEMPERATURES CLOSE TO 20C...THIS MIGHT BE ENOUGH TO OVERCOME THE
NEGATIVE AREAS.
MODEL COMPARISON (VALID FROM 00Z SEP 10). GFS MODEL MADE TIMING
CORRECTIONS IN FAVOR OF FASTER PROGRESSION ACROSS THE EASTERN
PACIFIC INTO THE SOUTHERN CONE...WITH SYSTEMS NOW ARRIVING
APPROXIMATELY 12 HRS SOONER THAN PREVIOUSLY FORECASTED. BUT IN
COMPARISON WITH THE UKMET/ECMWF...SOME SHORT WAVE PATTERN
DIFFERENCES STILL PERSIST. THE GLOBAL ENSEMBLES ADD TO THE
CONFUSION...RATHER THAN POINTING TOWARDS A CLEAR SOLUTION...AS
THEY DIVIDE IN THREE OPPOSITE CAMPS. SO CONFIDENCE IN THE
MEDIUM/LONG RANGE FORECAST IS LOW.
ALTHOUGH THE MODELS DIVERGE OVER THE EASTERN PACIFIC...ACROSS THE
ATLANTIC THEY TEND TO AGREE QUITE WELL THROUGH 108 HRS.
DOMINATING THE FLOW...A MID LEVEL PERTURBATION WILL PUSH EAST
ACROSS THE SOUTHERN CONE THROUGH 30-36 HRS...WITH SHORT WAVE
ENERGY FOCUSING ACROSS NORTHERN CHILE TO CENTRAL/NORTHERN
ARGENTINA AND THEN INTO THE MESOPOTAMIA VALLEY/SOUTHERN BRASIL.
IN THIS AREA...INITIALLY EXPECT HEIGHT FALLS IN THE RANGE OF 40-
60GPM. BUT THROUGH 60 HRS...AS THE MID LEVEL TROUGH DEEPENS OVER
THE WESTERN ATLANTIC...THE HEIGHT FALLS WILL INCREASE TO 60-
80GPM. BY 72 HRS A DEEP LOW WILL CENTER NEAR 40S 53W...EXTENDING
A TROUGH NORTH ACROSS MESOPOTAMIA VALLEY TO PARAGUAY. THE LOW
WILL SLOWLY MOVE ALONG 40S 42W BY 120 HRS...WITH A SHORT WAVE
TROUGH PERSISTING OVER URUGUAY/SOUTHERN BRASIL. THE TROUGH
SUPPORTS A FRONT OVER NORTHERN PATAGONIA/LA PAMPA INTO CENTRAL
CHILE. THE FRONT MOVES NORTH ACROSS CENTRAL ARGENTINA BY 24
HRS...WHILE SIMULTANEOUSLY A LOW DEEPENS OVER THE WESTERN
ATLANTIC/OFF THE COAST OF ARGENTINA. THROUGH 48 HRS THE LOW WILL
OCCLUDE...DRIVING THE FRONT FARTHER NORTH INTO SOUTHERN
BRASIL/NORTHERN ARGENTINA. AS IT REACHES MID SECTIONS OF SOUTH
AMERICA...THIS FRONT WILL MERGE WITH/REPLACE AN OLD FRONT... AS
THE LATTER EXTENDS ACROSS RIO GRANDE DO SUL/MESOPOTAMIA VALLEY TO
NORTHERN PROVINCES OF ARGENTINA. BY 72 HRS...AS THE BOUNDARIES
COMBINE...THEY ARE TO MOVE NORTH ACROSS SAO PAULO-MATO GROSSO DO
SUL TO NORTHERN BOLIVIA.
A LOW LEVEL JET ACROSS EASTERN BOLIVIA/PARAGUAY WILL SUSTAIN A
MOIST/WARM INFLOW THAT CONVERGES ALONG THE STATIONARY FRONT OVER
BRASIL-PARAGUAY/NORTHERN ARGENTINA...RESULTING IN BOUNDARY LAYER
DEW POINT TEMPERATURES OF 16C-18C. THIS WILL FUEL ORGANIZED
CONVECTION ACROSS SOUTHERN BRASIL-MESOPOTAMIA VALLEY AND
SOUTHEAST BRASIL...WITH RAINFALL MAXIMA OF 30-60MM ON DAY 01 AND
HIGH RISK OF SEVERE CONVECTION. ON DAY 02 MAXIMA DECREASE TO 25-
50MM/DAY...WHILE EXPANDING NORTHWESTWARD INTO CENTRAL PARAGUAY.
ON DAY 03 MAXIMA DECREASES TO 15-30MM/DAY...WHILE ON DAY 04 IT
WILL PEAK AT 05-10MM/DAY. ACTIVITY IS TO SURGE ACROSS CENTRAL/
NORTHERN BOLIVIA...WITH MAXIMA OF 15-30MM...TO START ON DAY 02
AND PERSIST THROUGH DAY 03. ON DAYS 02-04 ACTIVITY IS TO
GRADUALLY INCREASE ON THE SOUTHERN SIERRA/JUNGLE OF PERU.
THE NEXT MID LEVEL TROUGH IS TO MOVE ACROSS 100W/110W AND SOUTH
OF 35S BY 24 HRS. AS IT PUSHES EAST...IT WILL COLLIDE WITH A
BLOCKING RIDGE OFF THE COAST OF CHILE. THROUGH 72 HRS THE TROUGH
WILL MOVE ALONG 85W/90W. AT THIS TIME/POINT...THE MODELS CONTINUE
TO DIVERGE ON HOW THIS TROUGH IS GOING TO EVOLVE. AS THE TROUGH
CLASHES WITH THE BLOCKING RIDGE...THE UKMET AND ECMWF FORECAST
THE TROUGH TO RAPIDLY WEAKEN AS ENERGY SHEARS SOUTH ACROSS THE
DRAKE PASSAGE. THE GFS NOW FORECASTS THE TROUGH TO WEAKEN... BUT
IT CARRIES MORE ENERGY THAN THE OTHER TWO MODELS. SO WE EXPECT
FURTHER CORRECTIONS TO THIS PATTERN IN THE NEXT FEW RUNS OF THE
MODEL. THE ASSOCIATED FRONT WILL ENTER EXTREME SOUTHERN CHILE BY
60 HRS...WHERE IT WILL MEANDER THROUGH 72 HRS. AS IT ENTERS
SOUTHERN CHILE IT WILL SUSTAIN RAINFALL MAXIMA OF 15-25MM/DAY.
AT 200 HPA...A DISORGANIZED SUBTROPICAL RIDGE WILL REMAIN OVER
NORTHERN SOUTH AMERICA. THIS IS TO GRADUALLY ALLOW FOR THE
NORTHWARD AMPLIFICATION OF A TROUGH ACROSS BRASIL ALONG 48W-50W
BY 24-48 HRS. ANOTHER TROUGH WILL AMPLIFY NORTH ALONG 55W/60W BY
96 HRS. SCATTERED CONVECTION OVER NORTHWEST BRASIL-NORTHERN
PERU/SOUTHERN COLOMBIA WILL PERSIST. A SURGE IN ACTIVITY OVER
SOUTHERN PERU/NORTHERN BOLIVIA WILL ASSOCIATE WITH A POLAR
FRONT/PREFRONTAL TROUGH.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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