Weather
Nepal sees enormous climatic variation between the plains of the Terai, the mid-hills where Kathmandu and Pokhara are located, and the high Himalaya. The monsoon can continue into September, but normally, by the middle of the month, it only rains every few days rather than daily. While temperatures are still hot in many places (in the 70s and 80s Fahrenheit in Kathmandu and Pokhara; higher on the Terai), the humidity subsides by late September. Mountain areas, as always, are cooler.
Crowds & Costs
September is the start of the busy autumn season in Nepal, although things don't get really busy until October. Hotel prices enter the peak-season category. Travelers return to the trekking trails, although even the most popular treks don't get too crowded in September.
Trekking Trails
Trekking trails are still fairly quiet in September, as rain continues, landslides block some access, and trails remain slippery. However, some of the better-developed trails will be accessible and enjoyable in September, such as Poon Hill near Pokhara—one of Nepal's most famous viewpoints. See below for more on the classic Ghorepani Poon Hill Trek (and check out Everest Base Camp in September for more about trekking in the Everest Region this month).
Chat with a local specialist who can help organize your trip.
Where to Go
Well-established trails with good teahouse infrastructure, such as those in the Everest and Annapurna regions, are ideal places to go for a trek in September. Access is easier in such places, and maintenance is higher than in some more remote parts of the country. The monsoon rains refresh Kathmandu and Pokhara, and the climate and air quality are better in September than prior to the monsoon.
What to Do
Treks on better-developed and more accessible trails can be enjoyable in September. Some whitewater rafting and kayaking excursions begin again, but water levels on some rivers can be very high after the monsoon and may have even changed course due to landslides. Later in September (and later in the year) is a better time to hit the rivers than earlier in the month. Cultural activities remain enjoyable and accessible in Nepal's hill towns and cities.
Check out this article for things to do and see in Pokhara, or consider visiting Bandipur, one of central Nepal's hidden gems.
Events in September
Traditional Nepali festivals (whether Hindu or Buddhist) usually follow a lunar calendar. This means that some festivals that fall in September one year may be in August or October the next. Some festivals that may fall in September:
Indra Jatra, Kathmandu Valley. One of Nepal's most prominent street festivals, Indra Jatra, marks the end of the monsoon season and the beginning of the harvest. Expect eight days of colorful processions, masked dances, and deity worship.
Dashain, Nationwide (September/October). Occasionally starting at the end of September, Dashain is the most important festival for Nepali Hindus and runs for 15 days. As a major holiday, Nepalis travel back to their hometowns and villages to spend the festival with their families.
Krishna Janmashtami, Lalitpur. Krishna's birthday is most grandly celebrated at Patan Durbar Square.
Godavari Running Festival, Kathmandu. A trail running competition on the outskirts of Kathmandu.
Traveling to Nepal in September? Check out these great itineraries
Ghorepani Poon Hill Trek - 8 Days. This short trek gives you access to Nepal's favorite viewpoint of the Annapurna range. You'll follow ancient trails that connect local communities, pass through blooming rhododendron forests and rice paddies, and enjoy views of snow-covered mountains. At night, sleep in comfortable teahouses located in small villages along the route.
Explore the Kathmandu Valley - 6 Days. Kathmandu has many unique things to see and do. In six days, you can see many of the Kathmandu Valley's highlights, from markets, temples, and cultural sites to sunsets, mountain views, and hiking.,
More Helpful Information
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Nepal in October
Best Time of Year to Visit Nepal
How Many Days to Spend in Nepal