The best time to visit Assam is November to March when the heat and humidity are tempered by the cool winds blowing in from the Himalayan foothills. June brings the onslaught of heavy rains and it lasts until September. Guwahati has a fairly good drainage system leaving communication relatively unaffected by the rains. Air traffic does get affected though. So it may be best if you avoid visiting here during the monsoon season.
Assam Tourism Development Corporation Ltd.
Assam, India Website - www.assamtourism.org