What in the world is "thundersnow"?

What in the world is "thundersnow"?
SEATTLE - It's rare to get thunderstorms around the Puget Sound area. And it's even more rare to get snow.

But is it possible to get lightning and snow at the same time? Yes, it is, as evidenced by the thunder-snowstorm that moved through the South Puget Sound area Monday evening. (And really, the way this month has gone, would we expect anything else?)

The basic thunderstorm occurs when you have strong updrafts in a building storm. As the water droplets bustle around inside the clouds, they can build up an electric charge -- much like how if you wear socks on a carpet, you build up a charge if you scuffle them around. When the charge gets big enough, and it finds a release, that electric current is what you see as lightning.

To get those strong updrafts, you generally need much colder air moving in at the upper levels of the atmosphere. Since warm air rises, having much colder air aloft allows the air to rise farther, making for a bigger storm.

It is more difficult to get thunderstorms when temperatures are very low, but there's no real maximum temperature required to form a thunderstorm; what you need is a large relative difference in temperature between the ground and the upper levels.

In other words, as a rough example, having it be 28 on the ground and, say, 0 degrees above would work in a similar way as a 68 degree reading on the ground and a 40 degree reading above (again a rough example. It's not really that linear in true life due to other factors, but the idea is the same.)

Thunderstorms are already somewhat rare here because of our temperate climate. We normally don't get the big temperature differences to trigger thunderstorms. But with the arctic air moving in, we were able to get a decent difference in temperature.

But that was just one part of it. A second factor, the Puget Sound Convergence Zone also greatly helped in the creating favorable ingredients.

We had those arctic winds from the north colliding with the typical warmer winds from the south. Where those winds collided on the ground, they were forced up into the air -- helping trigger a strong updraft. The colder air moving in allowed those updrafts to get even higher and create the heavy snow and lightning.