Prognostic Meteorological Discussion
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FXSA20 KWBC 281557
PMDSA

SOUTH AMERICA FORECAST DISCUSSION - INTERNATIONAL DESKS
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1156 AM EDT TUE MAR 28 2017

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

MODEL DISCUSSION (VALID FROM MARCH 28 AT 0000 UTC): MINOR
CORRECTIONS TO PREVIOUS FORECAST PACKAGE...WITH MODELS REMAINING
IN GOOD AGREEMENT THROUGH DAY 07. THIS IS WELL SUPPORTED BY THE
GLOBAL ENSEMBLE MEMBERS...LEADING TO A HIGHER CONFIDENCE IN THE
FORECAST.

A MID/UPPER LEVEL LOW OFF THE SOUTH/CENTRAL COAST OF CHILE REMAINS
THE SYSTEM OF CONCERN. THIS IS A SLOW TO EVOLVE SYSTEM THAT IS
EXPECTED TO PERSIST DURING THE NEXT FEW DAYS. AS IT MEANDERS OFF
THE COAST OF CHILE...THE TROUGH IS TO SHEAR SHORT WAVE VORTICES
ACROSS THE CENTRAL/SOUTHERN PROVINCES IN ARGENTINA. THE BEST
INFLOW OF SHORT WAVE VORTICITY IS EXPECTED LATER TODAY AND DURING
THE DAY ON WEDNESDAY...WHEN A STRONG VORTEX SPILLS ACROSS THE
ANDES INTO LA PAMPA/CENTRAL ARGENTINA. THIS WILL SUSTAIN HEIGHT
FALLS OF 50-125GPM...WITH MOST INTENSE FOCUSING OVER LA
PAMPA/NORTHERN PATAGONIA. AT 250 HPA A NORTHERN POLAR JET WILL
MOVE INLAND ACROSS THE SOUTHERN CONE THROUGH MIDDAY ON
WEDNESDAY...WHILE A SUBTROPICAL JET STREAMS ACROSS NORTHERN CHILE
TO MENDOZA/CORDOBA. AS THEY MEANDER OVER THE CONTINENT...THESE
JETS ARE TO COUPLE ON THEIR DIVERGENT SIDES...TO SUSTAIN A BROAD
AREA OF UPPER DIVERGENCE ACROSS CENTRAL PROVINCES IN ARGENTINA. AT
LOW LEVELS...THE TROUGH ALOFT SUSTAINS A SURFACE FRONT THAT
MEANDERS EAST ACROSS PATAGONIA-CUYO/CENTRAL PROVINCES IN ARGENTINA
EARLY ON WEDNESDAY MORNING... WITH A LOW TO OCCLUDE OVER CENTRAL
PATAGONIA EARLY ON THURSDAY MORNING. THE OCCLUDING LOW IS TO THEN
TRIGGER MODERATE CONVECTION ACROSS CENTRAL PATAGONIA WITH
ACCUMULATION OF 25-50MM ON THURSDAY TO FRIDAY. AS IT MEANDERS OVER
THE CONTINENT...THE POLAR TROUGH WILL PRESS AGAINST A BROAD CELL
OF THE SUBTROPICAL RIDGE TO THE EAST. THE TIGHT PRESSURE GRADIENT
WILL THEN FAVOR A STRONG NORTHERLY JET ACROSS
ARGENTINA...ADVECTING WARM/MOIST AIR SOUTH. INFLOW OF COLD AIR
ALOFT WILL THEN DESTABILIZE THE COLUMN...WITH THE ENSUING VERTICAL
WIND SHEAR TO THEN FAVOR LONG LASTING/POTENTIALLY SEVERE
CONVECTION. THE FORECASTED HELICITY VALUES ON WEDNESDAY-THURSDAY
MORNING SUGGEST POSSIBILITY OF MESOCYCLONES FORMING OVER LA PAMPA.
THROUGH WEDNESDAY CONVECTION ACROSS ARGENTINA IS TO BECOME MORE
ACTIVE/BETTER ORGANIZED...WITH MAXIMA OF 30-60MM EXPECTED THROUGH
WEDNESDAY MORNING...AND 50-100MM ON WEDNESDAY TO THURSDAY. NOTE
THAT DURING THIS PERIOD STRONG DYNAMICAL FORCING WILL LIKELY
RESULT IN LOCALLY HIGHER RAINFALL AMOUNTS. ON THURSDAY-FRIDAY
FOCUS OF THE HEAVY CONVECTION SHIFTS TO NORTHWEST PROVINCES IN
ARGENTINA/SOUTHERN BOLIVIA...WITH MAXIMA OF 25-50MM/DAY.

A MID/UPPER LEVEL RIDGE TO THE EAST OF THE CONTINENTAL TROUGH
FAVORS THE NORTHWARD AMPLIFICATION OF A TROUGH OVER THE CENTRAL
ATLANTIC. THE TROUGH IS TO EXTEND NORTH BETWEEN 40W-10W TO 15S
THROUGH WEDNESDAY MORNING. LATER IN THE DAY THE TROUGH SPLITS IN
TWO...WITH SOUTHERN SEGMENT TO RAPIDLY PULL ACROSS THE SOUTH
ATLANTIC WHILE THE NORTHERN HALF RETROGRESSES TO THE
SOUTHEAST/SOUTHERN COAST OF BRASIL. THE LATTER IS TO THEN ANCHOR
ON A CLOSED LOW NEAR 30S 38W. AT LOW LEVELS...THIS SUSTAINS AN
ELONGATED FRONT THAT IS TO MOVE NORTH TO ESPIRITO SANTO/SOUTHERN
MINAS GERAIS-CHACO PARAGUAYO LATER TODAY. ON WEDNESDAY-THURSDAY
THE FRONT IS TO STALL OVER SOUTHERN BAHIA/MINAS GERAIS-MATO
GROSSO. OVER ESPIRITO SANTO/MINAS GERAIS THIS IS TO FAVOR
SCATTERED DEEP CONVECTION WITH ACCUMULATION OF 25-50MM. ON
WEDNESDAY...UNDER FAVORABLE JET DYNAMICS...THIS INCREASES TO
35-70MM...BUT ON THURSDAY TO FRIDAY IT DECREASES TO 20-35MM. IN A
BRISK EASTERLY FLOW...CONVECTION IS TO ALSO CLUSTER ALONG THE
COAST BETWEEN ESPIRITO SANTO AND SAO PAULO IN BRASIL. IN THIS AREA
SCATTERED CONVECTION WILL TRIGGER MAXIMA OF 10-15MM/DAY OVER THE
NEXT COUPLE OF DAYS. THROUGH THURSDAY IT IS EXPECTED TO WEAKEN.

AT 200 HPA...THE SUBTROPICAL RIDGE OVER NORTHERN SOUTH AMERICA
WILL ANCHOR ON A CLOSED HIGH THAT IS TO MEANDER BETWEEN MATO
GROSSO IN BRASIL AND THE NORTHERN JUNGLE OF BOLIVIA. THIS PATTERN
IS TO HOLD DURING THE NEXT FEW DAYS. AS IT HOLDS...THE RIDGE WILL
VENT DEEP CONVECTION ON THE SIERRA OF PERU AND THE ALTIPLANO OF
BOLIVIA. OVER THE SIERRA OF PERU EXPECTING SCATTERED CONVECTION
WITH MAXIMA OF 15-20MM/DAY...WHILE ON THE ALTIPLANO INITIALLY
EXPECTING RAINFALL AMOUNTS OF 15-30MM. THROUGH THURSDAY THIS WILL
DECREASE TO 15-30MM/DAY. THE RIDGE ALOFT IS TO ALSO VENT DEEP
CONVECTION ACROSS EQUATORIAL SOUTH AMERICA. ON THE NORTH COAST OF
BRASIL-AMAPA/AMAZON RIVER DELTA REGION INITIALLY EXPECTING
RAINFALL AMOUNTS OF 20-45MM/DAY SCATTERED DEEP CONVECTION. OTHER
CONVECTION IS EXPECTED ACROSS RORAIMA-AMAZONAS IN BRASIL TO
SOUTHERN COLOMBIA/NORTHERN JUNGLE OF PERU...WITH MAXIMA OF
25-50MM/DAY. ACROSS ECUADOR/NORTHERN PERU THE UPPER LEVEL RIDGE IS
TO SUSTAIN A DIVERGENT PATTERN ALOFT THAT IS TO VENT DEEP
CONVECTION. THIS WILL LEAD TO CONVECTIVE ENHANCEMENT ON WEDNESDAY
WHEN LOW/MID LEVEL WINDS SHIFT TO THE WEST...ENHANCING MOISTURE
TRANSPORT/CONVERGENCE ACROSS GUAYAQUIL TO NORTHERN PERU. EARLY IN
THE CYCLE MAXIMA IN THIS AREA IS TO PEAK AT 15-20MM. ON WEDNESDAY
THIS INCREASES TO MAXIMA OF 75-150MM...WITH POTENTIAL FOR LOCALLY
HIGHER AMOUNTS DUE TO STRONG TOPOGRAPHICAL FORCING. ON THURSDAY
THE MAXIMA WILL PEAK AT 50-100MM...WITH GRADUAL DECREASE
THEREAFTER.

ABDEMUR...FAA (ARGENTINA)
TEPES...DMC (CHILE)
DAVISON...WPC (USA)
$$




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