Prognostic Meteorological Discussion
Issued by NWS

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4
000
FXSA20 KWBC 211543
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1143 AM EDT TUE OCT 21 2014

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

MODEL DISCUSSION (VALID FROM OCTOBER 21 AT 00UTC): ON THE NORTHERN
STREAM THE MODELS FOLLOW A SIMILAR EVOLUTION THROUGH 132
HRS...THEN START TO DIVERGE ON WHEN A TROUGH OVER THE EASTERN
PACIFIC IS TO MOVE INLAND ACROSS CHILE TO ARGENTINA. BOTH EUROPEAN
MODELS FAVOR A FASTER EVOLUTION THAN WHAT THE GFS SUGGESTS. IN
THIS AREA THE OVERALL CONFIDENCE IN THE FORECAST IS HIGH THROUGH
THE MEDIUM RANGE PERIOD. ON THE SOUTHERN STREAM...TO THE SOUTH OF
50S...MODELS AGREE ON PATTERN EVOLUTION THROUGH 72-78 HRS. THEY
THEN DIVERGE ON AMPLITUDE/SPEED OF SHORT WAVE TROUGHS ENTERING THE
BELLINGSHAUSEN-WEDDELL SEAS. IN THIS AREA VARIABILITY AMONG THE
ENSEMBLE MEMBERS IS LARGE...AND CONFIDENCE IN THE FORECAST IS LOW.

CONSISTENT WITH PREVIOUS FORECAST...PROGRESSIVE MID LEVEL SHORT
WAVE TROUGH IS TO MOVE ACROSS THE BELLINGSHAUSEN SEA/ANTARCTIC
PENINSULA TO SOUTHERN CHILE EARLY IN THE CYCLE. AS IT MOVES ACROSS
THE ANTARCTIC PENINSULA TO THE WEDDELL SEA THIS IS TO CLASH WITH A
RIDGE TO THE EAST. AS THEY COLLIDE...THIS IS TO THEN FORCE THE
TROUGH TO SHEAR SHORT WAVE ENERGY TO THE SOUTH-SOUTHEAST...LEADING
TO SLOW/ GRADUAL EROSION OF TROUGH PATTERN AS THE CYCLE
PROGRESSES. ANOTHER PERTURBATION IS TO CLOSELY FOLLOW...STREAMING
ACROSS THE DRAKE PASSAGE/ANTARCTIC PENINSULA BY 60 HRS...INTO THE
WEDDELL SEA BY 72 HRS. THIS IS TO ALSO CLASH WITH THE RIDGE TO THE
EAST...BUT BY THEN THE RIDGE WILL WEAKEN ENOUGH TO ALLOW THE
TROUGH TO CONTINUE FARTHER EAST ACROSS THE SOUTH ATLANTIC. AT LOW
LEVELS...THESE ARE TO FOCUS SUCCESSIVE POLAR FRONTS ACROSS EXTREME
SOUTHERN PORTIONS OF CHILE/ TIERRA DEL FUEGO. THESE ARE TO TRIGGER
SCATTERED CONVECTION WITH MOST INTENSE EXPECTED EARLY IN THE
CYCLE...WITH MAXIMA OF 10-15MM. OTHERWISE...EXPECTING RAINFALL
AMOUNTS OF 05-10MM IN ISOLATED  CONVECTION.

ON THE NORTHERN STREAM...ALSO AT MID LEVELS...THE MODELS CONTINUE
TO FORECAST A SHORT WAVE TROUGH TO MOVE TO THE EASTERN PACIFIC
EARLY IN THE CYCLE...WITH AXIS TO EXTEND BETWEEN 110W-95W TO 18S.
A LOW IS TO CLOSED ALONG THIS AXIS...TO CENTER NEAR 38S 95W BY 48
HRS. TROUGH IS TO THEN REMAIN NEARLY STATIONARY THROUGH 120-132
HRS. AS IT MEANDERS TO THE WEST...THE DEEP TROUGH IS TO INDUCE THE
SOUTHWARD AMPLIFICATION OF A RIDGE ACROSS ARGENTINA. THE RIDGE IS
TO EXTEND NORTH-TO-SOUTH ALONG 60W TO LA PAMPA IN ARGENTINA. AT
LOW LEVELS THE TROUGH ALOFT IS TO SUSTAIN A FRONTAL WAVE TO THE
WEST OF JUAN FERNANDEZ ISLAND...WHERE IT IS TO GENERALLY REMAIN
THROUGH THE EXTENDED RANGE FORECAST PERIOD.

AT 200 HPA...A SHORT WAVE RIDGE EXTENDS FROM WESTERN BRASIL WHILE
CROSSING MATO GROSSO TO SOUTHEAST BRASIL. A TROUGH LIES TO THE
EAST...WITH AXIS NORTHWEST ACROSS BAHIA TO THE NORTHEAST STATES OF
BRASIL. THE BEST INSTABILITY ALOFT IS EXPECTED BETWEEN THE TROUGH
AND THE RIDGE...TO ENHANCE CONVECTIVE DEVELOPMENT FROM TOCANTINS/
GOIAS TO PARA/NORTHERN AMAZONAS TO THE WEST. IN THIS AREA...
CONVECTION IS TO GRADUALLY INTENSIFY/BECOME BETTER ORGANIZED AS
THE CYCLE PROGRESSES. THROUGH 72-84 HRS THE MODELS SHOW MAXIMA OF
20-40MM IN DEEP CONVECTION...INCREASING TO 20-45MM BY 84-108 HRS
WHILE BUILDING SOUTH ACROSS MATO GROSSO/MATO GROSSO DO SUL TO
NORTHERN BOLIVIA AND PORTIONS OF PARAGUAY. OTHER CONVECTION TO THE
WEST IS TO BUILD ACROSS ACRE IN BRASIL TO CENTRAL/NORTHERN
PERU...WITH MAXIMA IN THIS AREA TO PEAK AT 15-30MM OVER THE NEXT
FEW DAYS. OVER ECUADOR...MOST INTENSE IS TO CLUSTER ALONG THE
ANDES/WEST COAST...WHERE ONSHORE FLOW IS TO GENERALLY FAVOR
RAINFALL AMOUNTS OF 15-20MM/DAY.

ALSO AT UPPER LEVELS...A SHORT WAVE TROUGH IS TO ENTER NORTHERN
CHILE BY 24 HRS...TO THEN STREAM ACROSS THE NORTHERN PROVINCES IN
ARGENTINA/PARAGUAY. A HIGHER AMPLITUDE TROUGH IS TO FOLLOW...TO
REACH NORTHERN CHILE/SOUTHERN PERU BY 48 HRS. THIS IS TO CONTINUE
EAST ACROSS THE ANDES INTO NORTHWEST ARGENTINA/CENTRAL BOLIVIA BY
72-84 HRS. AS THEY SPILL ACROSS THE ANDES...THESE ARE TO INTERACT
WITH THERMAL LOW OVER NORTHWEST ARGENTINA...SUSTAINING AN INCREASE
IN DIURNAL CONVECTION. THROUGH 48 HRS THIS INCREASES TO
20-30MM/DAY...AND BY 72 HRS IT IS TO PEAK AT 20-40MM. THE TROUGHS
ALOFT ARE TO ALSO ENHANCE CONVECTION ACROSS SOUTHERN SIERRA OF
PERU/ ALTIPLANO OF BOLIVIA...WITH MAXIMA OF 10-15MM IN SCATTERED
DEEP CONVECTION. HIGHER AMOUNTS...HOWEVER...ARE FORECAST ACROSS
THE SOUTHERN JUNGLE OF PERU/NORTHERN BOLIVIA...WHERE THE TROUGH
ALOFT IS TO FAVOR RAINFALL AMOUNTS OF 20-30MM.

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





USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.