While Washington's weather is highly seasonal with extreme variations between summer and winter, it has been known to be unpredictable too. Summers tend to be very hot and humid, the conditions exacerbated in the heart of the city with its concrete and steel. Fall and spring are the best seasons, when chilly but bright, perfect days are the norm. Sudden rain or snowfalls are possible though.
For current weather information, visit weather.gov's Washington, DC page.