While attending Iowa State University I witnessed one of the greatest thundersnows of all times. The snowstorm hit one night, the snow was wet, the skies filled with lightning and thunder for a couple of hours. The snow fell at an incredible pace. It was, to say the least, phenomenal! The snow was wet, the visibility was near zero, I will never forget that THUNDERSNOW!
Usually thunder and lightning occur with spring and summertime storms. It takes special circumstances for snow and thunder to coexist. Much of the eastern seaboard saw this phenomenon today. The air at the surface must be relatively warm and very moist but still cold enough to support snow as it falls to the surface. The layers of air above must be substantially colder leading it great vertical instability. Cooling the middle levels of the atmosphere play an important role, A strong lifting mechanism, like a strong low pressure system adds to the convective nature of the storm and increases the threat of thunder and lightning occurring with snow and sometimes sleet. Snowfall rates are usually quite high when thundersnow is in the area. Thundersnow rates are usually in the 1"-4" of snow an hour.
Philadelphia, PA 1/26/2011
Cantore (Weather Channel) December 7th, 1996 Worcester MA
Baltimore MD February 6th 2010
Madison WI February 24th, 2007