Every spring and summer, heavy rains can cause mudslides in the mountains above Rio de Janeiro which includes Petrópolis. In December, 2001, between my first and second visits to Alexandre in Petrópolis. I read on the internet from Buenos Aires about mudslides caused by heavy rains. The December, 2001 mudslides killed 52 people in and around Petrópolis. Alexandre quickly e-mailed me that he was okay. However, if I had wanted to return to Petrópolis much earlier than I did, he wouldn't have been able to meet me at the Rio airport. There were more mudslides in January, 2003 that killed another 17 or so. Most of the deaths and injuries occurred in the favelas (shanty villages) precariously constructed on the mountainsides. However, Alexandre lost a friend to mudslides in 1988. Whenever the rain starts, return to your hotel or a sturdy building until you're told it's safe to go outside.
It is less a warning, but more of a caution... be aware that many Brazilians (particularly those from Rio) come to Petropolis in the winter months. What this means is that Friday afternoon and Sunday return traffic can be a bit slower. However the biggest issue to plan for is hotel vacancies.
I spent several hours stopping into and calling hotels, motels, hostels, B&B's and pousadas. After trying every single one that was listed or given as a reference, I was disappointed to learn that everyone was fully booked. Later it was explained to me that this is a common phenomenon during the winter months. It is best to book ahead, particularly if your visit will be over a weekend.