Here are some links to information about current weather conditions that visitors will experience plus the latest satellite images.

Current Sri Lanka weather

Colombo 5 Day Forecast

Colombo climate history



http://www.tripadvisor.com/Travel-g29...

 

It is hard to tell when to go to Sri Lanka because you will find many things to do any time of the year regardless of season. If you are focussing on specific activity and a targeted area then it is definitely better to follow the below information 

  •  Weather can be rough in monsoon rain period. There are two monsoon in Sri Lanka
       1) Northeast monsoon (December to March) -  Affect North and East and Ancient cities 
      2) southwest monsoon (June to October) Affect South and West including Colombo 
  • Middle of the contry (Kandy and Nuwara Eliya) could affect by rain at any time of the year (Hard to predict) 

  if you are targeting some part of the country then avoid monsoon rain period in that area will give higher chance of success. Although you avoid monsoon rain period... there will unexpected short rain as Sri Lanka is a tropical island. 

Hope this link helpfull  http://www.meteo.gov.lk/ (Probably the best)