Manassas 10 Day Weather. 1  Thursday:The Manassas forecast for Apr 20 is 72 degrees and Partly cloudy. There is 60 percentage chance of rain and 3 mph winds from the South-Southwest. 2  Friday:The Manassas forecast for Apr 21 is 75 degrees and Sunny. There is 58 percentage chance of rain and 7 mph winds from the West.