THE WEATHER PREDICTION CENTER

CAMP SPRINGS, MD


SOUTH AMERICAN MODEL DISCUSSION



 
SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER CAMP SPRINGS MD
1202 PM EDT THU MAY 14 2009
 
GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/
 
NOTE: THIS BULLETIN WILL NOT BE UPDATED AGAIN UNTIL TUES MAY
19...

MODEL COMPARISON (VALID FROM 00Z MAY 14) SHOWS GOOD AGREEMENT IN
GLOBAL MODELS WITH THEIR 500 HPA HEIGHTS OVER SOUTH AMERICA UNTIL
ABOUT 72 HOURS WHEN SIGNIFICANT DIFFERENCES BEGIN TO APPEAR WITH
THE UPPER LOW OVER THE SOUTH PACIFIC WHICH EVENTUALLY ENDS UP
EFFECTING THE STRENGTH OF THE RIDGE TO THE NORTH AND THE
DIRECTION OF THE FLOW ACROSS THE SOUTHERN CONE. BY DAY5 THE ECMWF
SHOWS MORE TROUGHING THAT EXTENDS FROM THE DRAKE PASSAGE BACK TO
THE NORTHWEST INTO THE CENTRAL SOUTH PACIFIC WHILE THE GFS IS
MUCH WEAKER. HOWEVER THE GFS ENSEMBLE MEAN DOES SUPPORT THE ECMWF
SOLUTION SO IT IS LIKELY THAT THE FLOW OVER THE SOUTHERN CONE
WILL BE MORE LIKE THE ECMWF...WHICH IS MORE ZONAL.  

AT 250 HPA....THE BOLIVIAN HIGH WILL REMAIN PRIMARILY OVER
CENTRAL PERU AND WESTERN BRASIL WITH RIDGING EXTENDING NORTHWEST
INTO THE PACIFIC. TO THE EAST A BROAD AREA OF DIFFLUENCE ALOFT
OVER THE AMAZON BASIN WILL SUPPORT TYPICAL MODERATE TO HEAVY DEEP
DIURNAL CONVECTION. AN UPPER TROUGH THAT EXTENDS FROM THE SOUTH
ATLANTIC INTO ARGENTINA WILL RAPIDLY PUSH TO THE EAST ACROSS
PARAGUA...URUGUAY...AND SOUTHERN BRASIL LEAVING AN ELONGATED
CUT-OFF LOW TO THE WEST OVER THE PACIFIC THAT AMPLIFIES BEFORE
BEING REABSORBED BACK INTO THE WESTERLIES AND MOVING TOWARDS THE
COAST OF CHILE. 

AT 500 HPA....OVER THE SOUTH ATLANTIC A VORTEX SOUTHEAST OF
TIERRA DEL FUEGO WILL WEAKEN NORTHERN STREAM ENERGY TO THE NORTH
LOWERS HEIGHTS OVER URUGUAY AND RIO GRANDE DO SUL...PARANA AND
SAO PAULO IN BRASIL. THIS SYSTEM WILL SLOWLY SHIFT TO THE EAST
WITH THE SOUTHERN VORTEX AGAIN DEEPENING BY 48 HRS.  MEANWHILE TO
THE WEST...A DEEPENING MID-LEVEL VORTEX WEST OF THE ANTARCTIC
PENINSULA WILL  LIFT NORTHWARD ALONG WITH AN ELONGATED VORTICITY
MAXIMA THAT CROSSES THE SOUTHERN CONE BY DAY3. THIS WILL HELP TO
BREAK DOWN THE 500 HPA PACIFIC RIDGE TO THE NORTH. AT THIS
SHORTWAVE IMPULSE MOVES INTO THE SOUTH ATLANTIC AND THE FLOW
ALOFT BECOMES MORE ZONAL THE UPPER TROUGH OVER THE SOUTH ATLANTIC
WILL QUICKLY MOVE TO THE EAST.

EARLY IN THE PERIOD THE RIDGE OVER THE CENTRAL SOUTH PACIFIC WILL
LIMIT THE NORTHWARD EXTENT OF ENERGY MOVING EASTWARD TO THE SOUTH
AND INTO SOUTHERN CHILE. THIS WILL ALSO HELP TO LIMIT THE AMOUNT
OF MOISTURE MOVING INLAND AND WILL SUPPORT ONLY LIGHT RAIN AND
SHOWERS OVER THE COASTAL REGIONS OF EXTREME SOUTHERN CHILE WHERE
MAX RAINFALL OF 10-20MM PER DAY IS ANTICIPATED. THE SHOWER
ACTIVITY WILL INCREASE SOMEWHAT IN AREAL EXTENT BY DAY 2 AS IT
SHIFTS SLIGHTLY NORTH IN RESPONSE TO LOWERING HEIGHT S AS THE
VORTEX TO THE SOUTH MOVES CLOSER TO THE COAST. BY DAY3 MAX
RAINFALL AMOUNTS OF 15-25MM PER DAY ARE EXPECTED AS THE SHORTWAVE
TROUGH PASSES INLAND AND MOVES INTO SOUTHERN PATAGONIA. 
STRONG...NEARLY ZONAL ONSHORE FLOW INTO THE REGION FOR THE
REMAINDER OF THE FORECAST PERIOD WILL CONTINUE TO PRODUCE
MODERATE PRECIPITATION WITH 15-25MM PER DAY FORECAST.

OVER SOUTHERN BRASIL THE COLD FRONT LIFTING UP ALONG THE COAST
WILL ACT AS THE PRIMARY FOCUSING MECHANISM FOR MODERATE TO HEAVY
SHOWERS AND THUNDERSTORMS. ABUNDANT DEEP LAYERED TROPICAL
MOISTURE STREAMING SOUTHEAST FROM AMAZONIA WILL COMBINE WITH
STRONG SHORTWAVE ENERGY ALOFT SUPPORTING MAX RAINFALL AMOUNTS OF
25-50MM PER DAY WHICH WILL DIMINISH ON DAY2 DUE TO LACK OF
SUPPORT ALOFT AS THE TROUGH AXIS SHIFTS OFFSHORE. FOR DAYS 3-7
RIDGING BUILDING INLAND AT THE SURFACE FROM THE SOUTH ATLANTIC
WILL LIMIT THE INPUT OF TROPICAL MOISTURE FROM THE NORTHWEST.

THE ATLANTIC ITCZ IS EXPECTED TO LINGER JUST ALONG THE COAST OR
SLIGHTLY INLAND...AS THE NET...OVER NORTHEAST BRASIL PROVIDING
STRONG DIVERGENCE ALOFT TO SUPPORT MODERATE CONVECTION THROUGH
THE FORECAST PERIOD.  PERSISTENT EASTERLY FLOW WILL PROVIDE AN
ABUNDANCE OF TROPICAL MOISTURE AND SUPPORT MAX RAINFALL AMOUNTS
OF 15-35MM PER DAY. OVER AMAZONIA BROADLY DIFFLUENT FLOW ALOFT
BETWEEN THE BOLIVIAN HIGH AND UPPER TROUGH TO THE SOUTH WILL
SUPPORT A BROAD AREA OF CONVECTIVE ACTIVITY WHERE DIURNAL HEATING
PROVIDES THE PRIMARY TRIGGER MECHANISM FOR THE THUNDERSTORMS. 

SEABRA...INMET (BRASIL)
VAZQUEZ...DINAC (PARAGUAY)
ROBSON...WPC (USA)




Last Updated: 1202 PM EDT THU MAY 14 2009