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South American Forecast Discussion
 
(Latest Discussion - Issued 1642Z Oct 23, 2017)
 
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SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1242 PM EDT MON OCT 23 2017

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

MODEL DISCUSSION (VALID FROM OCT 23 AT 0000 UTC): THE GLOBAL
MODELS FOLLOW A SIMILAR EVOLUTION THROUGH 108-120 HRS...THEN
DEVELOP PATTERN DIFFERENCES OVER THE SOUTHEAST PACIFIC DURING THE
LATTER PART OF THE FORECAST. AS A RESULT...OVERALL CONFIDENCE IN
THE FORECAST IS HIGH. NOTE THAT ENVIRONMENTAL CONDITIONS ARE TO
BECOME FAVORABLE FOR SEVERE THUNDERSTORMS TO BUILD ACROSS MID
SECTIONS OF SOUTH AMERICA ON WEDNESDAY-THURSDAY...WITH FORECAST
INDICES SHOWING RISK OF LARGE HAIL.

IN A SLOW TO EVOLVE PATTERN...A BROAD MID LEVEL TROUGH DOMINATES
THE FLOW ACROSS THE SOUTHEAST PACIFIC/SOUTHERN CONE OF SOUTH
AMERICA. AXIS IS TO SLOWLY PULL ACROSS SOUTHERN CHILE TO PATAGONIA
IN ARGENTINA EARLY IN THE CYCLE...WHILE ANCHORING ON A CLOSED LOW
NORTH OF THE ANTARCTIC PENINSULA. ON TUESDAY THE WANING LOW MOVES
TO THE WEDDELL SEA WHILE THE TROUGH PULLS ACROSS PATAGONIA TO THE
SOUTHWEST ATLANTIC. THE TROUGH IS TO THEN WEAKEN WHILE RAPIDLY
SHEARING SHORT WAVE ENERGY UNDER A BLOCKING RIDGE TO THE EAST. AT
LOW LEVELS THIS IS TO SUSTAIN A BROAD TROUGH ACROSS THE SOUTHERN
CONE...WITH POLAR FRONTS TO REVOLVE AROUND THIS AXIS.
THESE...HOWEVER...ARE TO CONFINE TO SOUTHERN CHILE-PATAGONIA IN
ARGENTINA DURING THE NEXT 48-60 HRS. OVER TIERRA DEL FUEGO THIS IS
TO FAVOR LIGHT TO MODERATE CONVECTION WITH ACCUMULATION OF
10-15MM...WITH THE LIFTED STABILITY INDEX SHOWING POTENTIAL FOR
DEEP CONVECTIVE CELLS/CUMULONIMBUS CLOUDS FORMING IN ASSOCIATION
WITH THIS FEATURE. ON TUESDAY EXPECTING AN ADDITIONAL RAINFALL
AMOUNTS OF XX-XXMM/DAY. OVER SOUTHERN
CHILE...MEANWHILE...EXPECTING RAINFALL AMOUNTS OF 05-10MM THROUGH
TUESDAY MORNING.

ON THE NORTHERN STREAM...A 500 HPA LOW NEAR 40S 35W IS TO ANCHOR A
SHORT WAVE TROUGH OFF THE SOUTH COAST OF BRASIL EARLY IN THE
CYCLE. THE MID/UPPER LEVEL TROUGH IS TO REMAIN NEARLY STATIONARY
DURING THE NEXT TWO DAYS. LATER ON WEDNESDAY IT WILL START TO
WEAKEN WHILE ACCELERATING TO THE EAST-SOUTHEAST. AT LOW
LEVELS...THE MID LEVEL TROUGH SUSTAINS A DEEP OCCLUDED LOW OFF THE
SOUTHEAST COAST OF BRASIL...WHILE THE ASSOCIATED FRONT STRETCHES
ACROSS RIO DE JANEIRO/NORTHERN SAO PAULO TO PARAGUAY. FRONTAL
BOUNDARY IS TO REMAIN NEARLY STATIONARY DURING THE NEXT THREE
DAYS...WHILE THE OCCLUDED SURFACE LOW GRADUALLY MEANDERS TO THE
SOUTHEAST. MOISTURE IS TO CONVERGE ALONG THE SURFACE FRONT...WITH
SCATTERED CONVECTION OVER RIO DE JA JANEIRO-ESPIRITO SANTO TO
RESULT IN ACCUMULATION OF 20-35MM THROUGH TUESDAY MORNING...AND
10-15MM ON TUESDAY TO WEDNESDAY.

SHORT WAVE TROUGH ON THE NORTHERN STREAM IS TO THEN PULL ACROSS
CENTRAL/NORTHERN CHILE LATER ON TUESDAY...CROSSING NORTHERN
ARGENTINA/SOUTHERN BRASIL ON WEDNESDAY. THIS WILL BE ACCOMPANIED
BY A SUBTROPICAL JET MAXIMA...WITH DIVERGENCE ON ITS RIGHT EXIT
REGION TO ENVELOP THE MESOPOTAMIA VALLEY IN ARGENTINA-SOUTHEAST
PARAGUAY ON WEDNESDAY. AS THE MID LEVEL TROUGH MOVES ACROSS
SOUTHERN BRASIL TO THE WESTERN ATLANTIC ON THURSDAY IT IS EXPECTED
TO INTENSIFY. AS IT MEANDERS ACROSS MID SECTIONS OF SOUTH AMERICA
THE MID LEVEL TROUGH WILL INTERACT WITH SURFACE FRONT OVER
SOUTHERN BRASIL-NORTHERN ARGENTINA...FAVORING CYCLOGENESIS ALONG
THE FRONTAL BOUNDARY. THIS IS TO INTENSIFY ON THURSDAY WHILE
MEANDERING OFF THE SOUTHEAST COAST OF BRASIL. THIS IS TO FAVOR
SEVERE CONVECTION AS IT STREAMS ACROSS NORTHERN
ARGENTINA-PARAGUAY...TRIGGERING HEAVY CONVECTION WITH ACCUMULATION
OF 50-100MM. STRONG DYNAMICAL FORCING WILL LIKELY RESULT IN LARGE
HAIL STONES. ON THURSDAY THE FOCUS OF THE HEAVY CONVECTION SHIFTS
TO SOUTHERN BRASIL...WHERE IT IS TO TRIGGER MAXIMA OF 35-70MM.

NORTH OVER THE CONTINENT...AT 200 HPA...BROAD RIDGE IS TO ENVELOP
CONTINENTAL AREA TO THE NORTH OF 20S. THIS PATTERN IS TO GRADUALLY
INTENSIFY THROUGH FRIDAY EVENING...WHEN MODELS AGREE ON A CLOSED
HIGH TO FORM OVER AMAZONAS IN WESTERN BRASIL. THE BROAD UPPER
LEVEL RIDGE PATTERN IS TO VENT DEEP CONVECTION ACROSS NORTHWEST
SOUTH AMERICA. OVER AMAZONAS IN BRASIL-AMAZONIA IN SOUTHERN
COLOMBIA AND THE NORTHERN JUNGLE OF PERU THIS IS TO FAVOR MAXIMA
OF 20-45MM. THROUGH WEDNESDAY THE DAILY MAXIMA DECREASES TO
20-35MM. OVER MATO GROSSO-SOUTHERN PARA AND RORAIMA IN BRASIL
EXPECTING SCATTERED CONVECTION TO BUILD IN A DIURNAL
PATTERN...WITH ACCUMULATION OF 15-25MM/DAY. SLIGHTLY HIGHER
AMOUNTS ARE INITIALLY EXPECTED ON THE CENTRAL-SOUTHERN JUNGLE OF
PERU...WHERE UNDER THE INFLUENCE OF AN UPPER LEVEL TROUGH THE
DAILY MAXIMA IS TO PEAK AT 20-35MM/DAY. BUT AS THE RIDGE BUILDS
OVER THE CONTINENT THIS IS TO DISPLACE THE TROUGH FARTHER
OFFSHORE. AS A RESULT THE DAILY MAXIMA GRADUALLY DECREASES TO
15-20MM ON TUESDAY-WEDNESDAY.

VANNUCCI...SMN (ARGENTINA)
RIVAS...SENAMHI (PERU)
DAVISON...WPC (USA)