030  
FXSA20 KWBC 231642  
PMDSA  
 
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)  

 
 
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