Whoa ice! Following last night’s rain roads across interior Connecticut became a skating rink right around midnight as temperatures dipped to near freezing. Take a look at the observations around midnight, though, and you might be surprised that the roads were already starting to ice up.
KHFD 140453Z 00000KT 1/4SM FG VV001 01/01 A2986 AO2 SLP111 T00110011 400281078
KBDL 140451Z 32003KT 10SM FEW025 02/01 A2985 AO2 VIS 2 1/2 RWY 6-24 SLP108 T00170006 400171111
As some slightly drier air worked in after midnight the dew point dropped to right around 0ºC. Notice at BDL the dew point at 11:51 pm was 0.6ºC. Even though the temperatures at 2 meters were above freezing, clear skies and light winds (0 knots at Hartford and 3 knots at Bradley) led to radiational cooling and allowed the surface temperature to drop a degree or two below the 2m temperature. This is the same reason why you can get frost when the 2m temperature is at 35ºF or 36ºF because the temperature near the earth can drop several degrees below the 2m level.
In fact, on my drive home from work in West Hartford, the car thermometer read 34º the entire way while the roads were a sheet of ice. I think radiational cooling (clear skies and light winds) cooled the temperature right near the surface as did a bit of evaporational cooling to produce widespread black ice. A real mess and not terribly easy to forecast!