Thundersnow Part One: Defining a Phenemenona

This winter, residents in northern Illinois have barely seen one flake of snow since the middle of December.  While this may have most people feeling joyous – no shoveling, no trudging through deep piles of snow or worrying if you will trip and fall on ice, some people may actually miss this wet white fluffy stuff.  Perhaps recounting the Groundhog’s Day blizzard back in early February 2011 that blanketed the Chicago area with up to 21 inches of snow (recorded at O’Hare airport, which is the official reporting site for Chicago) will bring back fond memories of a snowy winter scene?  Whatever you like, snow or no snow, this initial post for this site will be dedicated to the snow lovers and will feature a fascinating but complex weather feature – thundersnow.

What is thundersnow?

Before I take a retrospective look back at the Groundhog’s Day blizzard, where thunder snow played a prominent role in producing the massive amounts of snow, I will explain what thundersnow is.  The best and most simplistic definition of thundersnow is that it is simply a thunderstorm that produces snow rather than rainfall.

Ingredients for a thunderstorm

cropped-cloud-8075__340.jpg

A thunderstorm cloud, or Cumulonimbus cloud, at the beginning stage of development

However there are other factors in play that make this to be such a complex and rare weather phenomena. Thunderstorms need three essential ingredients to develop: lift, moisture, and instability. (Instability refers to warmer air at the surface and colder air above this layer of warm air. Think of how warm air rises.  If a parcel of warm air is actually colder than the air above it, it will sink and not aid in the development of a thunderstorm).  So if one of these three ingredients is missing, a thunderstorm simply would not develop.  A thunderstorm is characterized by a rising updraft of warm moist air and downward moving rain cooled air. Thunderstorm clouds, or cumulonimbus clouds, can grow upward to 40,000 feet or higher. The air at the surface is typically well above freezing leading to rainfall.

Putting the two together

Snowstorms, by contrast, consist of an extensive area of relatively flat shallow cloud.  Upward and downward air motions within this cloud deck are minimal and temperatures at the surface are below freezing.  Thundersnow is a little like combining the two; there is still an expansive cloud deck but there is rising motion within the cloud deck creating “bumps” of cloud material of up to near 5,000 feet.  Since the air at the surface is cold enough to support snow in the first place, what is the deciding factor to generate thundersnow?  Simply you need some sort upper level system that will not only provide the forcing needed for convection to develop, but it also supplies the colder air in the middle levels of the atmosphere to increase the instability needed for the cloud to develop vertically and to create an environment suitable for charge separation, or lightning.

When was the last time thundersnow occurred in northern Illinois?

As I previously mentioned, thundersnow is not a common weather phenomenon.  In fact the last time a thundersnow event impacted northern Illinois was a little over six years ago, January 31-February 2, 2011.  This storm was nicknamed “The Groundhog’s Day Blizzard”.  The second part of this post will be solely dedicated to provide interesting information on this storm.

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