Severe Weather Outbreak in Brazil

[LucasMarcomini]

Last week a severe weather outbreak happened in southern states of Brazil. The storms were caused by a surface low and a mid/upper level through that were almost stationary over the area. The storms produced severe hail, a tornado and at least 4 waterspouts.

HPC/NCEP's model discussion from a couple of days

MODEL COMPARISON (VALID FROM 00UTC DEC 12): NOTE: GLOBAL MODELS
SHOW DEEP MID/UPPER LEVEL LOW DOMINATING THE MESO/SYNOPTIC FLOW
PATTERN ACROSS MID SECTIONS OF SOUTH AMERICA DURING THE NEXT
SEVERAL DAYS. AS COLD AIR ALOFT ADVECTS OVER THE WARM WATERS OF
THE BRAZILIAN CURRENT...THIS WILL TRIGGER SEVERE COASTAL
CONVECTION ACROSS THE SOUTHERN STATES OF BRASIL-URUGUAY...MOST
INTENSE ON DAYS 01...02 AND 04

MODEL COMPARISON (VALID FROM 00UTC DEC 13): MODELS CONTINUE TO
AGREE ON EVOLUTION OF MID LEVEL LOW THAT IS TO MEANDER ACROSS MID
SECTIONS OF SOUTH AMERICA DURING THE NEXT SIX TO SEVEN DAYS. THIS
SYSTEM WILL DOMINATE THE MESO-SYNOPTIC FLOW PATTERN...AND IT IS
EXPECTED TO FAVOR AREAS OF HEAVY RAINS AND POTENTIALLY SEVERE
CONVECTION. CONFIDENCE IN THE FORECAST REMAINS HIGH.

Supercell in São Gabriel, in the state of Rio Grande do Sul

Storm moving over Pelotas, Rio Grande do Sul

Radar image of the storm:

Satellite GIF:

One of the waterspouts:

Images of the hail that fell in Caxias do Sul

Watch video >

Watch video >

Watch video >

It's interesting to notice that the supercells striations indicated a counter clockwise rotation, but in the southern hemysphere it should be clockwise.
There were no conditions for supercell development in the soundings. SRH was generally positive (should be negative down here). My guess is that the low circulation may have favored the formation of this storms and it's supercelular development.