The guide books lead one to believe that one can still visit the Simiens in the rainy season, because the rain is limited to short downpours which only affect part of any given day. This was not our experience in July - it rained torrentially all day, and the Simien park was completely shrouded in thick cloud meaning we could see very little of the marvellous landscape. The only animals we saw were the gelada baboons (who were amazing - envied them their thick coats in that sodden climate!), and we got no more than about a third of the way into the park, because the rain had washed away part of the only road, making it impassable, meaning we had to return to Debark without reaching the higher Afro-montane landscapes. All in all a largely wasted day, followed by an extremely disappointing evening in Debark itself.
The park guides gave us to believe we were simply very unlucky, but I am not so sure. In conversation with out tour guide (who did his best under the circumstances), it became clear that the park authorities don't advise people to visit in the rainy season, but allow people to buy tickets so they can make the money.
Bear all this in mind when planning your trip.
If you own or manage Simien National Park, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.