Bow echoes in southern Brazil, 2009.

[Ernani Nascimento]

It is severe weather season in subtropical South America, and I am opening this thread to talk about the occurrence of bow echoes in southern Brazil.

Months ago, in the afternoon of September 7th (Brazil´s Independence Day; our late winter), a significant severe weather outbreak irrupted over a fairly extensive region including the 3 southernmost states of Brazil and the region of Missiones in northeastern Argentina. Lots of attention was given to the tornadic storms, but one impressive bow echo also occurred on the far south of Brazil in that same afternoon. Here is a short radar sequence of that storm:

www.4shared.com/file/162840064/da8aebcf/canguu52.html
www.4shared.com/file/162840462/34e0e626/canguu53.html
www.4shared.com/file/162841994/ea7fabea/canguu54.html
www.4shared.com/file/162843269/b50aecf2/canguu55.html

And also pictures of some of the damage caused in a place called Pedras Altas (we performed a damage assessment for that event).

www.4shared.com/file/162850776/15b08497/pedras_altas1.html
www.4shared.com/file/162851109/767f1416/pedras_altas2.html
www.4shared.com/file/162851272/acaae500/pedras_altas3.html
www.4shared.com/file/162851404/e05aa40/pedras_altas4.html
www.4shared.com/file/162852028/203983db/pedras_altas5.html

A METAR/SPECI report from a local airport in that region indicated a 70kt gust during the event, but the strongest part of the squall line did not pass over that station. Our damage assessmet points to surface wind speeds reaching 100kt.

Now, during last night (Nov 29th) and today´s early morning hours (Nov 30th) another strong squall line caused significant damage over a rather extensive region of Rio Grande do Sul state (Brazil´s southernmost state). I am aware of at least one report of wind gusts above 50kt, and a few above 40kt. So I looked at the radar field valid at the time the storm was crossing the Brazil-Uruguay border; the squall line showed bowing segments, as seen below:

www.4shared.com/file/162867973/b5331f7b/2330Z_29112009.html
www.4shared.com/file/162867169/42ee969c/0000Z_30112009.html
www.4shared.com/file/162868206/de2e8284/0115Z_30112009.html

The AWS stations affected by this line showed the typical pressure "jump" and (significant!) temperature drop at the time the strong gusts were reported.

I examined bow echoes and derechos in North America during my PhD studies at the University of Oklahoma and since then I´ve been interested in documenting the SouthAmerican counterparts of such events. In South America these bow echo events seem to be less frequent than the tornadic supercells. Lots of work ahead to better understand the local environment conducive to bow echoes and derechos in this part of the world.