In a typical summertime thunderstorm, warm, moist air rises rapidly into the atmosphere, where it hardens into ice crystals, which then collide and cause a buildup of electrical charges. At a certain threshold, the electrical charges are released onto the earth’s surface in the form of lightning strikes.

NEW HAVEN, Conn. (WTNH) – Parts of Connecticut Sunday could experience a possibly once in a lifetime winter weather phenomenon called thundersnow. Sunday’s snowstorm is expected to ramp up as it continues into the afternoon,

Dozens of schools were closed on Friday after Storm Goretti brought thundersnow and travel disruption to Wales. An amber weather warning for heavy snow was in force for the south Wales valleys, mid and north Wales, until 09:00 GMT on Friday.

The storm system’s dangerous mix of ice, heavy snow and Arctic cold has halted some transit systems and closed schools in many cities. Millions of Americans are stuck inside, possibly for days. More than 12,000 flights were canceled this weekend, according to FlightAware.