A major winter storm slamming much of the Northeast is creating something called "thundersleet."
The unusual weather phenomenon has been reported in Virginia and is now heading to the Washington D.C. area, according to a Facebook status posted by the Virginia Weather Network.
Thundersleet is exactly what it sounds like. It happens when thunder and lightning occur at the same time as sleet or freezing rain. The mash-up of weather events is extremelyrare, according to a 2009 article from Scientific American which notes that "less than 1 percent of observed snowstorms unleash thundersnow, according to a 1971 NSW study."
Meteorologist John Fuller of KSDK.com has more on how thundersleet forms:
Thunder sleet is similar to thunder snow and thunder showers in the sense that convection is taking place within the cloud. That is, rapid updrafts and downdrafts are causing friction within the cloud producing strong positive and negative parcels within the cloud. When these oppositely charged parcels collide, you get lightning and hear thunder. When the rain falls from the cloud, through a layer just above the ground that is below 32 degrees (today it's is 25-29 degrees), the droplets freeze into granules that look like diamonds. If the rain aloft that falls is warm enough, it may not freeze until it hits the sub-freezing surface resulting in thunder freezing rain and a glaze at the surface.. The key in having any 'thunder' precipitation is convection or strong updrafts and downdrafts causing enough friction and charged particles to produce lightning. Thunder is the sound resulting from the lightning that super heats the air around the bolt.
Here's a video, uploaded by YouTube user Foxfang, demonstrating the phenomenon. You can hear thunder in the background as sleet is raining down.
Another video, uploaded by storm chaser Dan Robinson, shows a car losing control during a 2008 winter storm in Charleston, West Virginia, that involved thunder, lightning, and sleet.