251
FXSA20 KWBC 061650
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1249 PM EDT WED AUG 06 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z AUG 06). UNDER INFLUENCE OF
NUMEROUS MID/UPPER LATITUDE SHORT WAVE PERTURBATIONS...THE GLOBAL
MODELS START TO DIVERGE BY 96-108 HRS...WITH DIFFERENCES BECOMING
QUITE PRONOUNCED BY 132-144 HRS. DURING THE LATTER PART OF THE
CYCLE...THE SPREAD AMONG THE GLOBAL ENSEMBLE MEMBERS BECOME QUITE
LARGE. AS A RESULT...OUR CONFIDENCE IN THE FORECAST ON THE
SOUTHERN STREAM IS LOW TOWARDS THE END OF THE PERIOD. ALSO NOTE
THAT THE MEDIUM-LONG RANGE FORECAST PATTERN CONTINUES TO SUGGEST
POSSIBILITY OF A HIGH AMPLITUDE TROUGH PATTERN TO DOMINATE THE
SOUTHERN CONE BEYOND 144 HRS.
AT LOW LEVELS...A POLAR FRONT OVER THE WESTERN ATLANTIC IS TO
MOVE NORTH ACROSS SOUTHERN BRASIL EARLY THIS PERIOD...AND THROUGH
24-48 HRS IT IS TO MERGE WITH/REINFORCE AN OLD FRONT TO THE
NORTH. THE OLD FRONT...MEANWHILE...IS TO REMAIN QUASISTATIONARY
ACROSS SAO PAULO-MATO GROSSO DO SUL-CHACO PARAGUAYO TO CENTRAL/
NORTHERN BOLIVIA DURING THE NEXT SEVERAL DAYS. AT 250 HPA...AN
ANTICYCLONICALLY CURVED SUBTROPICAL JET LIES ACROSS BRASIL...
WHERE IT WILL REMAIN DURING THE NEXT 72-84 HRS. AT MID LEVELS...
SHORT WAVE PERTURBATIONS WILL STREAM ACROSS NORTHERN CHILE/
SOUTHERN BOLIVIA AND ACROSS PARAGUAY...WHICH ARE TO THEN INTERACT
WITH THE SURFACE FRONT TO TRIGGER ISOLATED TO SCATTERED
CONVECTION. THE JET ALOFT WILL SUSTAIN AREAS OF DIVERGENCE. THE
FAVORABLE JET DYNAMICS...PERSISTENT SURFACE BOUNDARIES...MOIST
NORTHERLY FLOW WILL CONTINUE FAVORING PERIODS OF DENSE/DEEP CLOUD
COVER AND ORGANIZED CONVECTION. MOST INTENSE IS EXPECTED BETWEEN
CENTRAL PARAGUAY/MATO GROSSO DO SUL BRASIL TO SAO PAULO/SANTA
CATARINA. IN THIS AREA WE EXPECT RAINFALL AMOUNTS OF 25-50MM/DAY
THROUGH 36 HRS...AND 30-60MM/DAY AT 36-84 HRS. BY 84-132 HRS A
SLIGHT DECREASE IN ACTIVITY IS EXPECTED WITH DAILY MAXIMA OF 20-
40MM. ON THE TAIL END OF THE FRONT...OVER NORTHERN BOLIVIA/
SOUTHERN PERU...EXPECT ISOLATED CONVECTION THROUGH 36 HRS. BY 36-
72 HRS EXPECT A SLIGHT INCREASE IN ACTIVITY WITH DAILY MAXIMA TO
PEAK AT 10-15MM.
ON THE SOUTHERN STREAM...A BROAD MID LEVEL TROUGH IS INITIALIZED
OVER THE SOUTHERN CONE/WESTERN ATLANTIC. THIS TROUGH IS TO SLOWLY
MOVE EAST TO EXTEND BETWEEN 90W-40W BY 24 HRS...WHILE THE AXIS
AMPLIFIES NORTH INTO THE SOUTHERN CONE TO 25S/30S. THE MEAN AXIS
RELOCATES BETWEEN 55W-15W BY 72 HRS...IT IS TO THEN SLOWLY DAMPEN
THROUGH 120 HRS. EARLY IN THE CYCLE IT WILL FOCUS IT SHORT WAVE
ENERGY/PERTURBATIONS OVER CHILE SOUTH OF 35S. AT LOW LEVELS IT
REFLECTS AS A BROAD TROUGH OVER THE WESTERN ATLANTIC/SOUTHERN
CONE...WITH AXIS TO ALSO MOVE EAST IN TANDEM WITH THE UPPER LEVEL
PERTURBATIONS. IN A TIGHT PRESSURE GRADIENT...EXPECT STRONG LOW
LEVEL WINDS OF 30-45KT ACROSS CHILE/PATAGONIA SOUTH OF 40S
THROUGH 36-48 HRS...THEN RAPIDLY SLACKEN BY 60-72 HRS. IN A MOIST
SOUTHWESTERLY FLOW...THIS SYSTEM WILL SUSTAIN SCATTERED
CONVECTION OVER SOUTHERN CHILE/SOUTHERN ANDES THROUGH 36-48
HRS...WITH MAXIMA OF 10-20MM/DAY EXPECTED THROUGH 36 HRS...AND
LESSER AMOUNTS AT 36-60 HRS. SNOWFALL WILL PERSIST ON THE
SOUTHERN ANDES DURING THE SAME PERIOD.
ON THE NORTHERN STREAM...A TROUGH ALONG 100W/110W WILL MOVE TO
90W BY 72 HRS...AND ACROSS 80W BY 96 HRS. AS IT NEARS CENTRAL
CHILE LATER THIS PERIOD IT WILL PHASE WITH A SOUTHERN STREAM
TROUGH ENTERING THE SOUTHERN CONE. THE GLOBAL MODELS DIFFER ON
THE INTENSITY/AMPLITUDE OF THE LATTER...WITH THE GFS FAVORING A
STRONGER/HIGHER AMPLITUDE TROUGH THAN THE EUROPEAN MODELS. AS A
RESULT OF THE LARGE VARIABILITY AMONG THE MODELS...CONFIDENCE IN
THE FORECAST DURING THE LATTER PART OF THE CYCLE IS LOW. AT LOW
LEVELS...AN ELONGATED FRONT IS TO SLOWLY MOVE ACROSS THE EASTERN
PACIFIC OVER THE NEXT FOUR TO FIVE DAYS. THIS SYSTEM WILL NEAR
CHILE BETWEEN CONCEPCION-PUERTO MONTT BY 48 HRS...AND IT IS TO
MEANDER ON THIS AREA THROUGH AT LEAST 84-96 HRS. AT 96-120 HRS IT
WILL UNDULATE NORTH ACROSS CENTRAL CHILE. IT IS TO INITIALLY
FAVOR MOSTLY LIGHT RAINFALL...WITH A GRADUAL INCREASE IN ACTIVITY
THROUGH 72-96 HRS WHEN DAILY MAXIMA WILL PEAK AT 10-15MM.
AT 200 HPA...THE SUBTROPICAL RIDGE CENTERS ON A HIGH NEAR 10S
55W...AND IT ENVELOPS CONTINENTAL AREA NORTH OF 15S. THROUGH 72
HRS THE HIGH WILL MOVE TO 10S 43W...WHILE THE RIDGE EXPANDS SOUTH
TO ENVELOP AREA NORTH OF 20S. A SHORT WAVE TROUGH IS TO SLOWLY
MOVE AGAINST THIS RIDGE...AND THROUGH 60 HRS IT WILL ENTER
NORTHERN CHILE/SOUTHERN PERU. BY 84-96 HRS IT WILL MOVE ACROSS
60W...TO EXTEND OVER PARAGUAY/EASTERN BOLIVIA. THE
INTENSIFICATION OF THE RIDGE ALOFT WILL FOLLOW A GRADUAL INCREASE
IN CONVECTION ACROSS PARA-AMAZONAS IN WESTERN BRASIL...WHERE
DAILY MAXIMA WILL PEAK AT 15-30MM. ACROSS NORTHEAST-NORTHERN
BRASIL...MEANWHILE...EXPECT WIDELY ISOLATED CONVECTION...WITH
PERIODS OF MODERATE TO HEAVY RAINFALL TO ASSOCIATED WITH TROPICAL
WAVES. AS THE SHORT WAVE TROUGH ENTERS NORTHERN CHILE/SOUTHERN
PERU...IT WILL TRIGGER SCATTERED CONVECTION ON THE SOUTHERN
SIERRA OF PERU AND THE ALTIPLANO OF BOLIVIA...WITH GOOD CHANCE
FOR SCATTERED SNOWFALL ON THE HIGHER ELEVATIONS.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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