725
FXSA20 KWBC 091644
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1244 PM EDT TUE SEP 09 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z SEP 09). GOOD AGREEMENT AMONG
THE GLOBAL MODELS THROUGH 72 HRS...THEY THEN START TO DIVERGE ON
PATTERN EVOLUTION ACROSS THE EASTERN PACIFIC. IN THIS AREA...THE
GLOBAL ENSEMBLE MODELS CLUSTER QUITE WELL...WITH A SMALL SPREAD
AMONG THE MEMBERS. BUT THE DETERMINISTIC SOLUTION(S) VARIES TOO
MUCH FROM THE ENSEMBLE MEAN...WHICH CREATES HIGH UNCERTAINTY AND
LOW CONFIDENCE IN THE MEDIUM RANGE FORECAST.
CONSISTENT WITH PREVIOUS FORECAST...A 500 HPA TROUGH WILL MOVE
ACROSS 80W BY 24 HRS...AND THROUGH 48 HRS IT WILL PUSH EAST ALONG
67W/70W. AT 72 HRS IT WILL MOVE ALONG 60W/64W...WITH A LOW TO
CLOSE NEAR 42S 60W. NOTE THAT IN THIS CYCLE THE MODELS FAVOR A
DEEPER/SLOWER LOW THAN ON PREVIOUS RUNS...WITH THE LOW TO MEANDER
NEAR 40S 50W THROUGH 120-132 HRS. AS IT ENTERS THE SOUTHERN CONE
BETWEEN 24-48 HRS...THIS SYSTEM WILL SUPPORT/INDUCE HEIGHT FALLS
OF 25-50GPM. BUT AS IT MOVES ACROSS BUENOS AIRES PROVINCE/URUGUAY
ON DAY 03...AND AS A RESULT OF THE RAPID INTENSIFICATION OF THIS
VORTEX...HEIGHT FALLS ARE FORECAST TO INCREASE TO 75-150GPM. AT
LOW LEVELS A POLAR FRONT EXTENDS ACROSS SOUTHERN BRASIL-
PARAGUAY...WHERE IT WILL MEANDER THROUGH 60 HRS. THIS WILL FAVOR
SCATTERED CONVECTION ACROSS SOUTHERN BRASIL-SAO PAULO...WITH
DAILY RAINFALL AMOUNTS OF 15-25MM. THIS WILL PERSIST THROUGH 24
HRS...WITH A SURGE IN ACTIVITY BY 48-60 HRS...WHEN MAXIMA ALONG
THE STATIONARY FRONT IS TO PEAK AT 20-40MM.
SIMULTANEOUSLY...ANOTHER FRONT WILL MOVE ACROSS PATAGONIA THROUGH
24-36 HRS. BY 48-72 HRS THE FRONT WILL UNDULATE NORTH ACROSS
BUENOS AIRES PROVINCE-CORDOBA IN ARGENTINA. BETWEEN 60-72 HRS IT
WILL MERGE WITH/REINFORCE THE OLD FRONT OVER SOUTHERN BRASIL-
NORTHERN ARGENTINA/PARAGUAY. AS THEY COMBINE...AND A LOW OCCLUDES
OFF THE SOUTHERN COAST OF BRASIL...THE FRONT WILL THEN RACE NORTH
INTO SOUTHEAST BRASIL-MATO GROSSO DO SUL-NORTHERN BOLIVIA. AS THE
FRONT ENTERS SOUTHERN CHILE IT WILL FAVOR SCATTERED RAINFALL WITH
ACCUMULATION OF 10-20MM...WITH ACTIVITY TO ALSO BUILD NORTH
ACROSS CENTRAL CHILE THROUGH 42-48 HRS. OVER SOUTHERN BRASIL-
MESOPOTAMIA VALLEY-SOUTHEAST PARAGUAY MAXIMA WILL PEAK AT 30-
60MM...WITH LOCALLY HIGHER AMOUNTS POSSIBLE. NOTE SLIGHT RISK OF
SEVERE CONVECTION ON DAY 03...WHEN FAVORABLE JET DYNAMICS AND MID
LEVEL COLD AIR INCURSION WILL DESTABILIZE THE AIR MASS. OVER
NORTHERN BOLIVIA INTO SOUTHERN PERU/RONDONIA BRASIL...PREFRONTAL
CONVECTION WILL FAVOR MAXIMA OF 10-20MM/DAY LATER IN THE PERIOD.
THE NEXT MID LEVEL TROUGH IS TO MOVE ACROSS 120W AND SOUTH OF 35S
BY 24 HRS. THROUGH 48 HRS IT WILL MOVE ACROSS 100W/110W...
MEANWHILE PUSHING AGAINST AN INDUCED RIDGE OFF THE COAST OF
CHILE. THROUGH 96 HRS THE TROUGH WILL MOVE ALONG 85W/90W. AT THIS
TIME/POINT...THE MODELS START TO DIVERGE ON HOW THIS TROUGH IS
GOING TO EVOLVE. AS THE TROUGH CLASHES WITH THE BLOCKING
RIDGE...THE UKMET FORECASTS THE TROUGH TO RAPIDLY WEAKEN AS
ENERGY SHEARS SOUTH ACROSS THE DRAKE PASSAGE. THE GFS/ECMWF
FORECAST THE TROUGH TO SURVIVE THE COLLISION...WHILE THE RIDGE
WEAKENS. BUT THEY DIVERGE ON HOW STRONG THE SHORT WAVE TROUGH IS
TO REMAIN...AND HOW FAST IT IS TO PROPAGATE. SO FURTHER
CORRECTIONS TO THIS PATTERN ARE EXPECTED. THE ASSOCIATED FRONT
WILL ENTER EXTREME SOUTHERN CHILE BY 84 HRS...WHERE IT WILL
MEANDER THROUGH 84-96 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 48-72 HRS...WITH TROUGH PATTERN TO PERSIST THROUGH 120 HRS.
SCATTERED CONVECTION OVER NORTHWEST BRASIL-NORTHERN PERU/SOUTHERN
COLOMBIA WILL PERSIST THROUGH 60-72 HRS...WITH DAILY MAXIMA OF
15-30MM. LOCALLY HIGHER AMOUNTS ARE POSSIBLE OVER NORTHERN PERU
THROUGH 24 HRS WITH MAXIMA OF 20-40MM.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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