Water at subzero temps
Here's where the much earlier comment about supercooled water could come into play. As I understand it, the term refers to water that has not passed from the liquid to solid state despite being below the freezing point. This can happen when cooling takes place very quickly, and in the absence of both mechanical forces AND particulate matter such as dust, around which crystals would normally begin to form. I've long since forgotten the physics, maybe something to do with the expansion and subsequent contraction of the droplet which would normally happen during "graceful" cooling.
BAM, the supercooled droplet undergoes mechanical stress, upsetting the delicate supercooled state, and it solidifies upon contact with the AC.
From here, I get lost in the thermodynamics of sublimation, melting and
eventual equilibrium. It strikes me, however, that a relatively large mass
of supercooled water (now ice) forming VERY suddenly might take a few
or many seconds to clear, even with fairly powerful heaters - remembering that latent heat of fusion thing....