Water can remain liquid in 'icing conditions' i.e. below 0 deg C or 32 deg F but impact with an object (like a main rotor blade or airframe) can cause freezing into a solid. However, the "freezing" does not occur all-at-once. In fact, there is a substantial lag.
I expect you all know this already, it forms part of the knowledge associated with ice detection systems like the Leigh Ice Detector.