Breaking down the rare winter phenomenon of thundersnow

We've all heard thunder and seen lightning during thunderstorms that we associate with warmer months, but have you ever heard thunder during a snowstorm?

If so, you've encountered a rare event. The last recorded occurrence was captured by the National Weather Service's St. Louis Office when a lightning flash was documented as snow fell in February 2013 in Jefferson City.

Another event was documented in late February 2011 during the historic Groundhog Day blizzard across the Midwest. Reports of thundersnow were recorded from Joplin through northeast Missouri with that system. Impressive snowfall rates of 2 to 4 inches per hour helped set the conditions for thundersnow each time.

Thunder occurs because of the rapid expansion of air heated by a lightning bolt. The difference between thunder in the winter and in above-freezing/rainy conditions starts with the air masses.

A cold air mass moves into a region of even cooler air, causing the warmer air mass to rise. When this happens, an updraft is created. Large amounts of snow within the clouds begin to collide quickly, causing negative and positive charges to separate. The negative charges in the cloud remain at the bottom of the cloud deck, while positive charges climb to the top.

When there are too many negative charges in the bottom of the cloud, the cloud disperses its negative charges into the ground or nearby clouds. The path that these charges take produces lightning, heating up the air around it, and the result is thundersnow.

On average, 6.3 thundersnow events occur in the U.S. annually. Many times the sound of thunder is muffled by the falling snow, so it can be hard to pinpoint if lightning isn't observed.