The weather in september is fine it starts gettin towards winter built-up around end of Sept. You should be able to visit rohtang pass during your visit around this time. Yes, you will be able to see lots and lots of snow at rohtang pass.
Late September is generally a good time to visit Shimla and Manali. The weather is pleasant with mild temperatures, making it suitable for outdoor activities and sightseeing. However, it is advisable to check the weather forecast as conditions may vary and carry some warm clothing for cooler evenings.
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