I've been saying this over and over on Twitter, but I can't stress it enough: if you're coming to Porto Alegre, make sure you bring a good jacket. Really. You don't need to bring winter apparel, the type you'd find in Wisconsin or Minnesota, but don't be fooled by the fact that Brazil is a tropical country. Well, it is a tropical country, but not one-hundred percent tropical.
Bring a jacket. Or a light sweater, even if you're just like Elsa.
The climate in Porto Alegre, in fact, is subtropical (within a temperate zone). Winter starts later this month, and the temperatures at the moment vary between the 50s and low 70s (and the tendency is to get colder). It went down to 44 F two or three days ago. The mornings tend to be very foggy, like Mists of Avalon foggy. At night, you'll want to sit next to the heater and drink quentão or chimarrão or whatever hot beverage within reach.
It's possible to wear a thin T-shirt during the day and feel comfortable, but at about 4 PM you'll wish you had a jacket on you, just saying. . . As it is now, we're going through that type of weather that if you wear two layers, you get too hot; and, if you wear no jacket at all, your hands go numb and stiff.
Well, I guess that's all for now. Feel free to add me and @ me on Twitter, if you have any questions, if you find yourself lost or in need of a translator / Brazilian friend / moral support.
All the best and safe travels!