Here is a few links for weather information:

BBC's Weather Info - 5 Day Forecast

Time to visit:

The best time to visit Berlin is May through September . During this season the weather is ideal for  going around the city, cafe sitting, and for leisure walk in the parks in the city.  Berlin is visited by business travellers during the month of march, for the world's top travel event ITB Berlin.  In these peak seasons, hotels in berlin are generally reserved.