213
FXSA20 KWBC 291659
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1259 PM EDT MON SEP 29 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z SEP 29). IN GENERAL THE MODELS
ARE IN GOOD AGREEMENT EARLY IN THE PERIOD BUT SIGNIFICANT
DIFFERENCES APPEAR BY DAY 4 IN THE SOUTHERN STREAM AS THE MODELS
HAVE DIFFICULTY IN FORECASTING THE TIMING AND STRENGTH OF THE
SHORTWAVES. THE GFS SHOWS REASONABLE CONTINUITY WITH YESTERDAYS
RUNS. MODELS SHOW ONLY MINOR DIFFERENCES IN TIMING AND STRENGTH
OF THE NORTHERN STREAM SHORT WAVES. CONFIDENCE IS LOW AFTER DAY 3
IN THE SOUTHERN STREAM HOWEVER. THE ECMWF AND UKMET ARE
FORECASTING A STRONGER SOUTHERN STREAM SHORTWAVE APPROACHING THE
SOUTHERN CONE THAN THE GFS AND BY DAY 5 THE GLOBAL MODELS ARE OUT
OF PHASE WITH EACH OTHER.
AT 500 HPA... A NORTHERN STREAM SHORTWAVE TROUGH ALONG 85W AT 36
HRS IS FORECAST BY THE GLOBAL MODELS TO REACH CENTRAL CHILE IN 60
HRS THEN MOVE EAST ACROSS CENTRAL ARGENTINA. THE GFS IS A LITTLE
SLOWER WITH THIS SYSTEM... BUT IN GENERAL MODELS ARE IN
REASONABLE AGREEMENT. WITH HEIGHT FALLS OF 150 DM SYSTEM IS
EXPECTED TO SUPPORT RAINFALL OF 10-20MM PER DAY ON DAY 2 OVER
CENTRAL CHILE AND 15-25MM OVER CENTRAL ARGENTINA. AS THE TROUGH
CONTINUES EAST INTO THE PACIFIC EXPECT RAINFALL OF 15-25MM/DAY ON
DAY 3 ACROSS NORTHERN BUENOS AIRES PROVINCE. A SECOND NORTHERN
STREAM SHORTWAVE IS FORECAST BY THE GFS TO FOLLOW BY DAY 5. THIS
SYSTEM IS WEAKER AND LITTLE IF ANY PRECIPITATION IS EXPECTED AS
THIS SYSTEM ENTERS THE SOUTHERN CONE ON DAY 5.
A NEARLY STATIONARY SURFACE FRONT ACROSS SOUTHERN BRASIL AND
PARAGUAY WILL SERVE AS A FOCUS FOR CONVECTION LATER IN THE
PERIOD. WITH GOOD NORTHERLY INFLOW OF LOW LEVEL MOISTURE NORTH OF
THE FRONT PWS WILL INCREASE TO GREATER THAN 50MM BY DAY 3. JET
DYNAMICS WILL ALSO FAVOR CONVECTION LATE IN THE PERIOD. EXPECT
05-10MM/DAY OVER SAO PAOLO DAY 2 INCREASING TO 30-60MM/DAY ACROSS
SOUTHERN BRASIL AND PARAGUAY DAYS 3-5.
A STRONG SHORTWAVE TROUGH IS FORECAST TO MOVE EAST TO 90W SOUTH
OF 40W BY 60 HRS THEN CONTINUE EAST TO 80W BY 84 HRS. THE TROUGH
WILL MOVE INTO THE SOUTH ATLANTIC BY 96 HRS THEN MOVE RAPIDLY
EAST. AS THE TROUGH APPROACHES... EXPECT RAINFALL OVER SOUTHERN
CHILE OF 10-20MM ON DAYS 2 AND 3 DIMINISHING TO 05-10 DAYS 4 AND
5. THE GFS FORECASTS A FLAT RIDGE TO BUILD OVER PATAGONIA ON DAY
5 BEHIND THIS RIDGE BUT THIS SOLUTION IS NOT SUPPORTED BY THE
OTHER GLOBAL MODELS. BOTH THE ECMWF AND UKMET MODEL FAVOR A
STRONGER SOUTHERN STREAM SHORTWAVE MOVING INTO THE SOUTHEASTERN
PACIFIC OCEAN AND PATAGONIA ON DAY 5.
NORTH OVER THE CONTINENT...AT 200 HPA....THE SUBTROPICAL RIDGE
WILL DOMINATE THE FLOW OVER NORTHERN SOUTH AMERICA THROUGH THE
CYCLE. THE HIGH CENTER WILL LINGER OVER SOUTHERN PERU THROUGH DAY
3. AS THIS CENTER WEAKENS ON DAY 4 A NEW CENTER IS FORECAST TO
DEVELOP FARTHER EAST OVER EASTERN BRASIL/GOIAS. THE ASSOCIATED
EAST TO WEST RIDGE ALONG 08S-10S EARLY IN THE PERIOD WILL
GRADUALLY SHIFT SOUTH TO 11S-14S BY DAY 5. THE RIDGE WILL SUPPORT
CONVECTION ACROSS BRASIL TO PERU-ECUADOR/SOUTHERN COLOMBIA...
WITH MAXIMA OF 20-40MM/DAY CONCENTRATED ACROSS AMAZONAS-PARA-
PERUVIAN JUNGLE-ECUADOR AND SOUTHERN COLOMBIA.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
MOREYRA...SENAMHI PERU
DANAHER...HPC (USA)
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