Weather in Prague
The chart below gives a reasonable indication for Prague weather from the averages for sunshine, temperature and rainfall in Prague in historical almanacs but, Prague weather is so changeable as to make them not useable for a two week holiday etc.
Prague weather forecasters like Weather Underground (see current conditions below) give a very accurate Prague weather 5 day forecast in Celsius and Fahrenheit along with other info. This site also has a lot of data on Max, Min and Average temperatures and historical data which is searchable by using the "history and almanac" search box. Prague weather during the summer months means most people intent on travel to Prague would bring a waterproof jacket and a light jumper along with their regular stuff.
Current Prague Weather Forecast
Allowing for Prague weather conditions, the decision on when to come here will rest largely on what you want to do when you arrive. Most hotels in Prague have 3 seasons loosely based around the following dates;
Prague in Low Season - 3rd January to March 29th and 4th November to December 27th
Wunderground (English Language link)
Prague in Mid Season - 29th March to November 4th
Prague in High Season - 27th December to January 3rd
Rates change in Prague in bank holiday weekends so be careful when booking dates close to the borders of the seasons and check with the hotel about their seasons. Some hotels include Christmas in their HIGH Season dates whilst others include in their standard rate. Prague in it's "Top" seasons would be for Christmas, New Year and possibly Easter.
Prague in January and at the end of the year are about the same i.e. Prague weather is generally cold but not wet and it can snow at any time. Before Christmas i.e. Prague in it's low season, October/November generally ties in with the Opera/Ballet season in Prague. Although you will find performances all year round, the season brings the more established Czech companies and internationally renowned troops to Prague. The big advantage is that you can see these events in some spectacular locations, at a fraction of London prices and with a greater choice. The disadvantage is that Prague in Winter has weather which can be bitterly cold. If it is culture that you want, I would advise a visit to Prague in November as it is not yet very cold, Prague accommodation is reasonable and Prague tickets are still available on the day. After Christmas the temperature will be zero or less for long periods.
Prague in Spring i.e. March/April until the end of October is a bit more stable after Easter. All Prague hotels move to a common price level (some call it Prague in mid-season, others high season). In the spring months the Prague weather is changeable from cool and clear in March to drizzly in April/May. The weather in Prague in the summer months when the city is at it's most crowded can be very hot. The cobbles radiate heat. I always recommend Prague in Spring and Autumn to visit when the weather in Prague is warm but not hot and when the wind is fresh but not chilling. If you have the opportunity for a quick getaway to Prague at the end of February or early March and the Prague weather is good then you can get high quality Prague accommodation for half of what you would pay for Prague in peak times.