Drizzle falls from clouds and freezes on contact, but why?

MILWAUKEE (WITI) -- Another humbling forecast surprise: pardon the geekiness of this post. As a meteorologist, it's always good for me to look back and see why a forecast did not verify. Thursday, December 18th is a good example. I did not anticipate drizzle falling from the clouds.

The image below is an atmospheric sounding showing the temperature and dewpoint profile for the lower atmosphere over Milwaukee Thursday morning. An inversion (rising temperature with height) has been trapping moisture below 5000 feet for the past few days. Thus the overcast.

 

But on Thursday, there was just enough lift in the lower atmosphere to squeeze out moisture. The air is warmer aloft than it is at the surface. So drizzle is falling on surfaces below freezing. The result is icy roads and sidewalks.