Ask the weatherman: They call it thundersnow
copyright the Chronicle February 19, 2014
Question: At around 3:30 a.m. Friday, out of the corner of my eye, I saw a flash of lightning in the middle of a snowstorm. I have since Googled it and now know it wasn’t an alien taking our photo (my other theory at that hour, since I didn’t think lightning during a snowstorm was possible.)
Answer: Between 3 and 4 a.m. on the morning of this past Friday, February 14, some people were awakened by lightning and/or thunder. Reports were received from Barnet, St. Johnsbury, and Barton. Very heavy snow fell from then until about 7 a.m. Snowfall rates of 2-3 inches per hour were noted.
It doesn’t happen often, but winter thunderstorms, producing snow, are a part of the weather spectrum across the United States, parts of Europe and Asia, and likely in the southern Andes. The ingredients needed are the same as for warm-season thunderstorms: moist, unstable air and something to give it an initial boost upward. The only difference is that the temperature is at or below freezing all the way to the Earth’s surface, so that snow reaches the ground rather than melting on its way down and arriving as rain, as it does in the warm season.
There are several ways to make the air unstable enough for thunderstorms, and to give it the initial upward boost. In this case, there wasn’t a cold front to produce the lift, nor was there heating by the sun or the warm waters of a large lake. Instead, what most likely happened was this: a tongue of relatively mild air at mid-levels was wrapped around the northwest side of the deep low moving across Cape Cod at the time, by its strong circulation. At the same time, a pool of very cold air a little higher up in the atmosphere, driven forward by strong jet stream winds at that level, overtook the warm tongue. The same jet stream ventilated the atmosphere, like opening the damper on a stove. So both the cold air layer and the relatively mild layer beneath it were drawn upward. Once the mild layer began to poke up into the cold layer, it accelerated upward, since milder air is less dense than colder air. Thunderstorm cells formed within the more general area of snow, and the rest is history!
Widespread thundersnow last occurred in Vermont during February 2011. In that case, spectacular thunder and lightning ahead of a strong front continued for 60-90 minutes as multiple bands of thunderstorms blossomed along the front and then propagated eastward, ahead of it. Some of the thunderstorms were accompanied by hail as well as snow — a genuine rarity.
Fairbanks Museum and Planetarium
Editor’s note: Send your weather questions to [email protected]. Steve Maleski will answer one each week. To read a past Chronicle story on thundersnow, which appeared in the February 9, 2011, edition, click here.