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South American Forecast Discussion
 
(Latest Discussion - Issued 1634Z Oct 26, 2021)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1234 PM EDT TUE OCT 26 2021

GFS DATA AT FTPPRD.NCEP.NOAA.GOV/PUB/DATA/NCCF/COM/GFS/PROD/

FORECAST DISCUSSION FROM OCT 26/17UTC: ON THE SOUTHERN STREAM
FLOW...AT 250 HPA...IN A TIGHT BELT OF MID LATITUDE WESTERLIES THE
ANALYSIS SHOWS A STRONG POLAR JET MAXIMA STREAMING ACROSS THE
SOUTHEAST PACIFIC INTO THE SOUTHERN REGIONS OF CHILE/PATAGONIA IN
ARGENTINA...WITH A WIND MAXIMA OF 200KT EMBEDDED IN THIS FLOW. THE
STRONG WINDS ARE TO PERSIST THROUGH WEDNESDAY MORNING...THEN
GRADUALLY WEAKEN AS A BROAD MID/UPPER LEVEL TROUGH LIFTING ACROSS
THE SOUTHEAST PACIFIC APPROACHES THE SOUTHERN CONE LATER IN THE
DAY/EARLY ON THURSDAY. AT LOW LEVELS...BROAD TROUGH DOMINATES THE
SOUTHEAST EAST PACIFIC WHILE A RIDGE REIGNS ON THE SUBTROPICAL
FLOW TO THE NORTH. THE RIDGE ANCHORS ON A CLOSED HIGH WEST OF THE
ARCHIPELAGO OF JUAN FERNANDEZ. THE ENSUING PRESSURE GRADIENT
FAVORS STRONG LOW LEVEL WINDS ACROSS THE SOUTHERN CONE...WITH 850
HPA WINDS OF 50-60KT. THE BROAD LOW LEVEL TROUGH IS TO PERSIST
THROUGH THURSDAY MORNING...WITH SUCCESSIVE FRONTS/FRONTAL WAVES TO
STREAM ACROSS SOUTHERN CHILE/PATAGONIA IN ARGENTINA. THIS WILL
ENHANCE MOISTURE TRANSPORT/CONVERGENCE ACROSS THE SOUTHERN
ANDES...WHILE UNDER THE INFLUENCE OF AN ATMOSPHERIC KELVIN WAVE
THE PWAT CONTENT OF 30MM IS 200 PERCENT OF NORMAL FOR THIS TIME OF
THE YEAR. THE STRONG LOW LEVEL FORCING AND FAVORABLE UPPER LEVEL
JET DYNAMICS WILL COMBINE IN SUPPORT OF AN EXTREME RAINFALL EVENT.
DURING THE NEXT THREE DAYS EXPECTING STORM TOTAL AMOUNTS OF
100-175MM...WITH THE MOST INTENSE ON WEDNESDAY WHEN THE MAXIMA
PEAKS AT 75-125MM. ON FRIDAY...AS THE MID/UPPER LEVEL TROUGH
ENTERS THE SOUTHERN CONE...THIS WILL THEN DRIVE THE SURFACE FRONT
NORTH ACROSS PATAGONIA. THIS BOUNDARY CONTINUES NORTH ACROSS LA
PAMPA/THE BUENOS AIRES PROVINCE TO CENTRAL ARGENTINA ON
SATURDAY...WHERE IT IS EXPECTED TO STALL. ACROSS CENTRAL ARGENTINA
THIS WILL THEN TRIGGER SCATTERED DEEP CONVECTION...WITH RAINFALL
MAXIMA OF 20-35MM ON SATURDAY...WITH POTENTIAL FOR LOCALLY HIGHER
AMOUNTS IN SEVERE CONVECTION.

ON THE NORTHERN STREAM...AT 200 HPA...A SHORT WAVE TROUGH
EXTENDING FROM NORTHWEST ARGENTINA TO WESTERN BRASIL SPLITS THE
SUBTROPICAL RIDGE IN TWO CELLS...WITH THE WESTERNMOST LOCATING
OVER THE EASTERN TROPICAL PACIFIC WHILE CENTERING AT 250 HPA ON A
CLOSED HIGH NEAR 15S 85W. THE EASTERNMOST...MEANWHILE...EXTENDS
OVER BRASIL BETWEEN BAHIA AND MINAS GERAIS. THROUGH WEDNESDAY THE
RIDGE OVER THE PACIFIC IS FORECAST TO GRADUALLY COLLAPSE...WHILE
THE ONE OVER BRASIL TENDS TO BUILD. AS THE LATTER INTENSIFIES...IT
WILL THEN STEER THE SHORT WAVE TROUGH ACROSS PARAGUAY TO SOUTHERN
BRASIL ON THURSDAY...WHILE SIMULTANEOUSLY STEERING A TUTT TO THE
NORTH ACROSS NORTHEAST BRASIL. UNDER THE INFLUENCE OF THE UPPER
LEVEL TROUGH/CONTINENTAL RIDGE...SCATTERED DEEP CONVECTION IS TO
CLUSTER BETWEEN RIO DE JANEIRO/SAO PAULO AND AMAZONAS IN
BRASIL/NORTHERN PERU-SOUTHERN COLOMBIA. IN THIS AREA EXPECTING
RAINFALL AMOUNTS OF 15-30MM/DAY WITH MAXIMA OF 20-45MM. THROUGH
THE END OF THE WEEK...AS THE SHORT WAVE TROUGH STARTS TO PROPAGATE
ACROSS MID SECTIONS OF SOUTH AMERICA...FOCUS OF THE ORGANIZED
CONVECTION WILL SHIFT TO NORTHWEST ARGENTINA-PARAGUAY...WHERE THE
ENVIRONMENTAL CONDITIONS WILL BECOME FAVORABLE FOR A HEAVY
RAINFALL PRODUCING MESO SCALE CONVECTIVE COMPLEX TO DEVELOP ON
SATURDAY TO SUNDAY. THIS WILL SUSTAIN RAINFALL AMOUNTS OF
40-80MM/DAY...WITH SOME OF THE MODELS SHOWING LOCALIZED MAXIMA OF
150-200MM AS IT BUILDS NORTH ACROSS PARAGUAY TO MATO GROSSO DO SUL
IN BRASIL.

DAVISON...WPC (USA)