By September the downpours of the monsoon are beginning to abate and towards the end of the month it's peak tourist season in Nepal. Earlier in the month there's a high chance of rain, but as the days progress, the weather clears up and days are dry and clear. Temperatures are pleasantly warm, typically in the low 20s, which is perfect for exploring. With the dust and pollution washed away by the rains, those mountain views are at their most epic.
Autumn is the ideal time of year to visit Nepal. In early September you might still encounter the odd shower, but the skies are usually clear and the visibility excellent. The landscapes are green and full of colour, so expect to be taking plenty of photos. Temperatures are pleasantly warm without the heat and humidity of the monsoon months and exploring eons-old temples and bustling markets is a breeze. Some hiking trails remain muddy and damaged by the monsoon rains and if you want to visit higher altitude trails around places like Namche Bazaar you might want to wait until October.
As the month progresses, the season kicks into full gear and the trails and teahouses become much busier. After being wet and difficult to reach over the summer, the national parks are accessible again. This is a great time of year to go on a rhino safari and birdwatchers flock to see the many migrating species that rest up here. It's harvest season, so you're likely to see people working hard in the bright green paddy fields. Festivals abound too with the colourful Hindu celebration of Dasain taking centre stage in many communities.