Thundersnow: An unusual winter thunderstorm accompanied by snow instead of rain. (Sometimes spelled thunder snow.)
Thundersnow is most likely to occur in a coastal area, because storms can form over the comparatively warm ocean or lake water and then move inland, where they encounter much colder weather. That's what happened earlier this month when a storm dropped six to nine inches of snow on the Puget Sound area. The Seattle Post Intelligencer reported on Dec. 18:
Thursday's storm started off with a bang as two bands of air barreled across the region from opposite directions and collided, producing a rare combination of thunder and lightning, with driving snow that quickly whited-out much of the city.
According to David Schultz, formerly a meteorologist with the National Severe Storms Laboratory in Norman, Oklahoma, and now a professor in Helsinki, only 0.07 percent of recorded snowstorms are associated with thunder. Part of the problem lies in documentation:
Shultz points out that heavy snow has a way of obscuring sound and light — the telltale signs of a thundersnow.
"Where you might hear a regular thunderstorm from four to five miles away, you may not hear or see a thundersnow from a mile away," he said.
Here in the San Francisco Bay Area, and in much of coastal California, thunderstorms are exceedingly rare at any time of year; our summers and autumns are dry. I'm always skeptical when I see movies set in San Francisco or Los Angeles in which thunder and lightning are added for dramatic effect.