313
FXSA20 KWBC 151533
PMDSA
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS HYDROMETEOROLOGICAL PREDICTION CENTER CAMP SPRINGS MD
1132 AM EDT FRI AUG 15 2008
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
MODEL COMPARISON (VALID FROM 00Z AUG 15). THE SOUTHERN STREAM
PATTERN ACROSS THE DOMAIN CONTINUES TO BAFFLE THE GLOBAL
MODELS...AS THEY ARE FORCED TO MAKE CYCLE TO CYCLE CORRECTIONS
TO THE SHORT WAVE PATTERN. THIS CONTINUES TO LEAD TO
HIGH UNCERTAINTY AND LOW CONFIDENCE IN THE FORECAST
BEYOND 72/84 HRS.
CONSISTENT WITH THE PREVIOUS FORECAST...A SOUTHERN STREAM TROUGH
AMPLIFIES INTO THE SOUTHERN CONE EARLY THIS PERIOD. AS IT ENTERS
THE CONTINENT...IT IS MERGING WITH A NORTHERN STREAM VORTEX OVER
CENTRAL CHILE/ARGENTINA. THE MID LEVEL PERTURBATIONS THEN COMBINE
INTO ONE TROUGH ALONG 60W TO 25S BY 48 HRS...AND THROUGH 72
HRS THEY RAPIDLY EJECT ACROSS 45W. DURING THE NEXT 24 HRS THESE
ARE TO FAVOR HEIGHT FALLS IN EXCESS OF 150-200GPM OVER CENTRAL
CHILE AND SOUTHERN/CENTRAL PROVINCES OF ARGENTINA. AT 250 HPA THE
POLAR AND SUBTROPICAL JETS ACCOMPANY THIS INCURSION...WITH BOTH
DIVERGING AS THEY ENTER CENTRAL CHILE. THIS IN-TURN...IS
PROVIDING VENTILATION ALOFT TO ORGANIZED DEEP CONVECTION ACROSS
CHILE. THE SLOW TO EVOLVE TROUGH PATTERN OVER THE SOUTHERN CONE
IS TO CONTINUE TAKING ADVANTAGE OF ABOVE NORMAL SEA SURFACE
TEMPERATURES OVER THE PACIFIC TO SUSTAIN A MOIST FLUX...WHICH
WILL CONTRIBUTE TO BOUNDARY LAYER MIXING RATIO VALUES OF 08-
10G/KG ACROSS CHILE SOUTH OF LA SERENA. POOLS OF HIGHER MOISTURE
CONTENT WILL PEAK AT 11-12G/KG. THE HEIGHT OF THE 0C DEG ISOTHERM
IS TO DROP TO 2.5KM EARLY IN THE PERIOD...PRESENTING A HIGH RISK
OF ORGANIZED SNOWFALL AMOUNTS ACROSS THE ANDES OF CHILE. AT LOW
LEVELS MODELS PROJECT A SIMILAR EVOLUTION...WITH A BROAD TROUGH
PATTERN OVER THE SOUTHERN CONE...WITH A POLAR FRONT TO MOVE NORTH
ACROSS LA PAMPA-SOUTHERN MENDOZA TO A WAVE/LOW OFF THE CENTRAL
COAST OF CHILE. AT 48 HRS IT WILL LIE ACROSS URUGUAY/SOUTHERN
BRASIL TO PARAGUAY...TO MEANDER NORTH INTO SOUTHERN BRASIL-
PARAGUAY/BRASIL BORDER BY 72-96 HRS...WHERE IT WILL MEANDER. OVER
CENTRAL CHILE MAXIMA IS TO RANGE BETWEEN 30-60MM/DAY...WITH
LOCALLY HIGHER AMOUNTS QUITE LIKELY DUE TO TOPOGRAPHICAL FORCING.
ACROSS CENTRAL ARGENTINA ONLY EXPECT LIGHT CONVECTION...WITH A
SURGE IN ACTIVITY ACROSS SOUTHERN BRASIL-NORTHERN PROVINCES OF
ARGENTINA/PARAGUAY BY 36-60 HRS...WITH MAXIMA TO PEAK AT 30-
60MM. LIGHT TO MODERATE CONVECTION IS TO THEN PERSIST OVER
SOUTHERN BRASIL THROUGH 108 HRS...WITH DAILY MAXIMA TO RANGE
BETWEEN 10-20MM.
ON THE SOUTHERN STREAM...A PROGRESSIVE TROUGH WILL MOVE ALONG
100W AND SOUTH OF 45S BY 24 HRS...AND THROUGH 48-60 HRS IT WILL
ENTER THE DRAKE PASSAGE/SOUTHERN CONE. IT CONTINUES EAST ALONG
55W/60W AND SOUTH OF 45S BY 72 HRS...TO THEN RAPIDLY EJECT ACROSS
THE SOUTH ATLANTIC. THE ASSOCIATED FRONT ENTERS SOUTHERN
CHILE/PATAGONIA BY 48-60 HRS...AND MOVES NORTH INTO LA PAMPA BY
72 HRS...TO THEN MEANDER THROUGH 96-108 HRS. THIS IS TO ALSO
FAVOR SCATTERED PRECIPITATION OVER SOUTHERN CHILE WITH
ACCUMULATION OF 05-10MM/DAY...WITH SCATTERED SNOWFALL OVER TIERRA
DEL FUEGO.
LATER IN THE PERIOD ANOTHER NORTHERN STREAM TROUGH IS TO ENTER
CENTRAL CHILE/SOUTHERN CONE. THE MID LEVEL TROUGH IS TO SLOWLY
PUSH EAST INTO ARGENTINA BY 108-120 HRS. AS IT ENTERS THE
CONTINENT IT WILL EVOLVE INTO A BROAD TROUGH PATTERN...BUT THE
MODELS DIVERGE ON THE INTENSITY/AMPLITUDE OF THIS SYSTEM...WITH
HIGH UNCERTAINTY PROJECTED BY THE GLOBAL ENSEMBLE MEMBERS. SO
FURTHER CORRECTIONS TO THIS PATTERN ARE EXPECTED IN THE NEXT FEW
RUNS.
AT 200 HPA...THE MODELS INITIALIZE TWO CELLS OF THE SUBTROPICAL
RIDGE...WITH ONE OVER THE EASTERN PACIFIC AND THE OTHER OVER
NORTHERN BRASIL. THROUGH 72-96 HRS THE WESTERNMOST COLLAPSES
WHILE THE ONE OVER THE CONTINENT BUILDS/INTENSIFIES. BY THE END
OF THE PERIOD A STRONG HIGH WILL CENTER OVER BRASIL NEAR 15S
55W...WITH THE RIDGE ALSO BUILDING SOUTH ALONG THE ATLANTIC COAST
OF SOUTH AMERICA. TROPICAL CONVECTION OVER NORTHERN BRASIL WILL
REMAIN SPARSE...WITH MOST ACTIVE EXPECTED ALONG AND NORTH OF THE
EQUATOR...WITH DAILY MAXIMA TO PEAK AT 15-30MM IN CLUSTERS.
ACROSS ECUADOR...WESTERN PORTIONS...DAILY MAXIMA WILL PEAK AT 05-
10MM/DAY IN SEA BREEZE CONVERGENCE. AT LOW LEVELS A FRONT ENTERS
SOUTHERN BRASIL-PARAGUAY...WHERE IT WILL MEANDER THROUGH 24-36
HRS. THIS BOUNDARY WILL REMAIN ACTIVE FROM SOUTHERN PARAGUAY-
UPPER MESOPOTAMIA TO SOUTHERN BRASIL...WHERE IT WILL SUPPORT
MAXIMA OF 10-15MM/DAY THROUGH 36-48 HRS.
GUZMAN...SENAMHI (BOLIVIA)
URRA...DMC (CHILE)
DAVISON...HPC (USA)
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