By September in Laos rainfall has peaked, river levels are high and temperatures have dropped to a more comfortable mid 20’s in Luang Prabang (this is ‘winter’ after all).
Towards the end of September the rains are usually beginning to recede and decline in intensity. Afternoon downpours can still be expected during September however.
A week touring Laos in September will almost certainly be accompanied by 3 or 4 days with a sudden afternoon downpour, usually preceded and followed by clear blue skies.
Laos receives fewer visitors in September and hotels in Laos have lower prices and better availability.
September is not the best time to head off into rural Laos where the infrastructure is poor and roads can become impassable.
September sees river levels high, the country carpeted in lush green and waterfalls at their most spectacular.