News 3 Thundersnow Facts: Mysterious storms explained

Thunderstorms—like this one rolling over a Minnesota farm—usually form when warm, moist air near the ground rises, creating unstable air columns.In a rare phenomenon dubbed thundersnow, such conditions can occur in late winter or early spring, sparking thunder and lighting during snowstorms.Researchers are studying how these winter thunderstorms form with snow-imaging technology (above right).Photograph by Richard Olsenius/NGS (left) and courtesy Patrick Market (right)

"Thundersnow" Facts: Mysterious Storms Explained
Christine Dell'AmoreNational Geographic News
March 3, 2009

The late-winter snowstorm that blanketed much of the eastern U.S. on Sunday and Monday packed some serious sound and fury—emphasis on sound.
Along with the snow clouds, a rare and little-known phenomenon known as thundersnow rumbled over parts of Georgia and South Carolina.
Thundersnow—when thunder and lighting occur during a snowstorm—most often appears in late winter or early spring, experts say.
That's because the ingredients for thundersnow—a mass of cold air on top of warm, plus moist air closer to the ground—often come together during that time.
What Causes Thundersnow
Thundersnow starts out like a summer thunderstorm, Market said. The sun heats the ground and pushes masses of warm, moist air upward, creating unstable air columns.
As it rises, the moisture condenses to form clouds, which are jostled by internal turbulence.
The "tricky part" for making thundersnow, Market said, is creating that atmospheric instability in the wintertime.
For thundersnow to occur, the air layer closer to the ground has to be warmer than the layers above, but still cold enough to create snow—a very precise circumstance.
In the recent southern U.S. thundersnow storms, for instance, the atmosphere became unstable enough that thunderstorms with rain developed. Those storms then moved north where the air was below freezing, said Howard Silverman, a National Weather Service senior forecaster in Sterling, Virginia.
The thundersnow events were also coupled with "pretty decent snowfall rates," at the rapid clip of more than two inches (five centimeters) an hour, Silverman said.
Heavier snowfall is usually linked to thundersnow, both experts agreed.
The University of Missouri's Market has done research showing that most of the time 6 inches (15 centimeters) of snow will accumulate within a 70-mile (113-kilometer) radius of a thundersnow event.
(Related news: "Volcanic Lightning Sparked by 'Dirty Thunderstorms,' Study Finds.")
How to See Thundersnow
Experiencing thundersnow requires being in the right place at the right time, said University of Missouri atmospheric scientist Patrick Market, who has received funding from the National Geographic Society's Expeditions Council. (The National Geographic Society owns National Geographic News.)
Even then, he said, you probably won't see anything but white.
"In a really nice thundersnow event, the sky [simply] gets bright. You don't see a lightning bolt. There's nothing for a second or two, and then you hear a rumble of thunder."
The best spots for catching thundersnow in person are Wolf Creek Pass, Colorado, and the eastern shores of Lake Ontario, Market said—two hot spots that he has pinpointed in his research.
Thundersnow can also occur along coasts, experts say, because that's where moisture from warm water can easily evaporate into the colder, drier air above.
(Watch an interactive animation of a thunderstorm.)
Thundersnow to Aid Weather Forecasts?
Market and colleagues track thundersnow storms in the field. After the researchers identify a winter storm, they release weather balloons, which are meant to reveal how the atmosphere becomes unstable.
By collecting data on pressure, temperature, humidity, and wind speed and direction—the five key factors meteorologists use to make predictions—Market and colleagues hope their thundersnow studies can help make future weather forecasts more accurate.