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South American Forecast Discussion
 
(Caution: Version displayed is not the latest version. - Issued 1606Z Nov 25, 2014)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1106 AM EST TUE NOV 25 2014

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

NOTE: FORECAST PRODUCTS WILL NOT BE AVAILABLE ON NOVEMBER 27-28.

MODEL DISCUSSION (VALID FROM NOVEMBER 25 AT 00UTC): THE GLOBAL AND
ENSEMBLE GLOBAL MEMBERS CONTINUE TO AGREE ON PATTERN EVOLUTION
THROUGH 96-108 HRS. THEY THEN DIVERGE ON HOW MID LEVEL TROUGH
ENTERING THE SOUTHERN CONE IS GOING TO EVOLVE. THE GFS AND UKMET
TEND TO FAVOR A LOWER AMPLITUDE AND FASTER EVOLUTION THAT WHAT THE
ECMWF SUGGESTS. THE ENSEMBLE MEMBERS ALSO DIVERGE ON THIS
EVOLUTION...WITH THE CANADIAN AND EUROPEAN MEMBERS FAVORING THE
SLOWER ECMWF. BUT VARIABILITY AMONG THE MEMBERS IS TO LARGE TO
ESTABLISH CONFIDENCE IN THE FORECAST.

AT MID LEVELS...SHORT WAVE PERTURBATIONS IN THE SOUTHERN STREAM
FLOW  ARE TO DOMINATE OVER THE NEXT COUPLE OF DAYS...WITH VORTICES
FORECAST TO STREAM ACROSS SOUTHERN CHILE/ARGENTINA. AS THEY MOVE
ACROSS THE CONTINENT...THESE ARE TO PRESS AGAINST THE SUBTROPICAL
RIDGE TO THE NORTH...SUSTAINING A TIGHT GRADIENT ACROSS MID
SECTIONS OF SOUTH AMERICA. THIS...IN-TURN...WILL SUSTAIN A
SUBTROPICAL JET MAXIMA ACROSS NORTHERN ARGENTINA-SOUTHERN STATES
OF BRASIL OVER THE NEXT 60-72 HRS. THE JET ALOFT IS TO FAVOR A
DIVERGENT PATTERN ACROSS MID SECTIONS OF THE CONTINENT. AT LOW
LEVELS...A POLAR FRONT IS FORECAST TO STALL OVER SOUTHERN
BRASIL-NORTHERN ARGENTINA/ PARAGUAY...WHILE A PREFRONTAL TROUGH
EXTENDS ACROSS SAO PAULO-MATO GROSSO/PARAGUAY TO SANTA CRUZ
BOLIVIA. UNDER FAVORABLE JET DYNAMICS...WE NOW EXPECT HEAVY
CONVECTION ACROSS SAO PAULO-MINAS GERAIS IN BRASIL OVER THE NEXT
24-36 HRS...WITH RAINFALL AMOUNTS OF 30-60MM. A SECOND SURGE IN
ACTIVITY IS THEN EXPECTED BY 60-84 HRS...WHEN MAXIMA IS TO ONCE
AGAIN PEAK AT 30-60MM. AS THE LOW LEVEL TROUGH ESTABLISHES ACROSS
BRASIL/BOLIVIA...THIS IS TO THEN ENHANCE LOW LEVEL MOISTURE
CONVERGENCE ACROSS NORTHERN BOLIVIA TO SOUTHERN PERU...WITH
ENVIRONMENTAL CONDITIONS BECOMING FAVORABLE AND CONDUCIVE TO MCS
FORMATION.

DURING THE SECOND HALF OF THE FORECAST...THE GLOBAL MODELS
CONTINUE TO FORECAST A HIGHLY AMPLIFIED SINUSOIDAL WAVE PATTERN
ACROSS THE EASTERN PACIFIC/SOUTHERN CONE TO THE ATLANTIC. IN THIS
PATTERN A DEEP MID LEVEL TROUGH IS TO AMPLIFY ALONG 90W LATER ON
THURSDAY... AND THROUGH FRIDAY EVENING IT IS TO MOVE EAST ACROSS
SOUTHERN/ CENTRAL CHILE TO ARGENTINA...WHILE FAVORING HEIGHT FALLS
OF 150-200GPM. MODELS THEN DIVERGE ON HOW FAST THIS IS TO
PROPAGATE ACROSS THE SOUTHERN CONE...WITH THE UKMET AND GFS
FAVORING A FASTER PROGRESSION THAN THE ECMWF. BUT THIS WILL BE
DEPENDENT ON HOW DEEP THE TROUGH IS BY THE TIME IT MAKES LANDFALL
OVER CHILE. FOR NOW WE ARE SIDING WITH THE FASTER GFS-UKMET AS
SUGGESTED BY SOME OF THE GLOBAL ENSEMBLE MEMBERS...BUT FORECAST
UNCERTAINTY IS HIGH DURING THIS PERIOD. THE HIGHLY AMPLIFIED
TROUGH IS LIKELY TO SUSTAIN A SUBTROPICAL CONNECTION AS IT MOVES
TOWARDS THE SOUTHERN CONE OF SOUTH AMERICA. THIS WILL THEN FEED
CONVECTIVE ACTIVITY AS THE ASSOCIATED FRONT MOVES ACROSS SOUTHERN
CHILE TO PATAGONIA/LA PAMPA IN ARGENTINA-CENTRAL CHILE LATER ON
FRIDAY. OVER CHILE SOUTH OF TEMUCO IT IS TO TRIGGER SCATTERED
CONVECTION WITH ACCUMULATION OF 10-15MM...WHILE OVER CENTRAL CHILE
EXPECTING RAINFALL AMOUNTS OF 00-05MM. ACROSS LA PAMPA-CENTRAL
ARGENTINA EXPECTING ACCUMULATION OF 10-15MM AS THE FRONT SURGES
NORTH. BOUNDARY MOVES ACROSS URUGUAY/ SOUTHERN BRASIL TO NORTHERN
PROVINCES IN ARGENTINA LATER ON SATURDAY. DURING THAT PERIOD
CONDITIONS APPEAR FAVORABLE FOR AN MCS TO FORM ACROSS THE NORTHERN
PROVINCES IN ARGENTINA...WHERE IT IS LIKELY TO TRIGGER HEAVY
RAINFALL AMOUNTS AND POTENTIALLY SEVERE CONVECTION.

AT 200 HPA...THE SUBTROPICAL RIDGE DOMINATES CONTINENTAL AREA TO
THE NORTH OF 30S...WITH AXIS ANCHORING ON A CLOSED HIGH OVER
CENTRAL BRASIL. A RETROGRESSING TUTT LIES TO THE
EAST-NORTHEAST...CENTERING ON A CLOSED LOW OVER THE TROPICAL
ATLANTIC. THROUGH WEDNESDAY EVENING THE LOW RETROGRESSES TOWARDS
THE NORTHEAST STATES OF BRASIL...AND LATER ON FRIDAY IT WILL MOVE
FARTHER WEST ACROSS MARANHAO/EASTERN PARA. BEST DIVERGENCE ALOFT
IS TO REMAIN ALONG THE UPPER LEVEL RIDGE TO THE SOUTH AND
WEST...TO VENT DEEP CONVECTION BETWEEN PERU-AMAZONAS/ACRE IN
WESTERN BRASIL TO SAO PAULO-MINAS GERAIS/ESPIRITO SANTO. ACROSS
AMAZONAS-ACRE/RONDONIA-NORTHERN BOLIVIA TO CENTRAL-NORTHERN PERU
AND EASTERN ECUADOR DIURNAL CONVECTION IS TO RESULT IN RAINFALL
AMOUNTS OF 15-30MM/DAY...WHILE OVER MATO GROSSO/MATO GROSSO DO
SUL-PARA-MINAS GERAIS THE MAXIMA IS TO PEAK AT 30-60MM. UPPER
TROUGH PATTERN IS TO CONTINUE FAVORING A CONVERGENT/SUBSIDENT
PATTERN ACROSS THE NORTHEAST STATES OF BRASIL. THIS WILL INHIBIT
ORGANIZED DEEP CONVECTION.

CAROCA...DMC (CHILE)
DURAN...SENAHMI (PERU)
DAVISON...WPC (USA)
GALVEZ...WPC (USA)