Thundersnow Part Two: The Groundhog Day Blizzard as a Case Study

If you recall, my previous post on the topic of thundersnow centered on the meteorological background of this weather phenomenon. Let’s go and take a closer look at the most recent occurrence of thundersnow that impacted northern Illinois, the Groundhog’s Day blizzard that occurred January 31-February 2, 2011.

Fast facts of this blizzard

Chicago O’Hare recorded 21.2 inches of snow over the three day period, making it the third highest producing snowstorm on record.

Rockford recorded 15.1 inches of snow over the three day period, which is also the third highest producing snowstorm on record.

How did other areas across northern Illinois fare from this storm?  Generally one to two feet of snow blanketed northern Illinois and northwest Indiana.  However strong winds made it challenging to accurately measure snowfall, due to the blowing and drifting of the snow.  In addition the convective nature of thundersnow events (convective or convection – when warm air rises causing an upward current in the atmosphere, the process thunderstorms develop) resulted in some variability of snowfall density.


Three day snow totals across northern Illinois and northwest Indiana.  Data source: National Weather Service in Romeoville, IL.

How this storm developed

A low pressure system originating over the Southern Plains strengthened considerably as it lifted northeast into southern Illinois.  Although light snow was reported January 31st, the more intense snowfall did not occur until the afternoon hours on February 1st as the axis of greatest lift, the mechanism for increasing the instability of the atmosphere, positioned itself over the region. Recall from my earlier post that this instability helps any vertical motion rise faster and easier, which then helps with snowfall production and charge separation, or lightning.  In addition moisture was not lacking as this storm was ingesting rich Gulf moisture.

How strong were the winds?

Not only did this strong area of low pressure bring very heavy snowfall across the region, but it also provided a setup for very strong winds which aided in the blinding blizzard conditions.  The juxtaposition of a deepening low pressure system moving in from the south and a strong ridge of high pressure to the north, a strong pressure gradient (a difference in pressure over an area) setup across the Midwest late in the day on February 1st.  These lines of pressure, or isobars, were tightly packed over much of the region, in particular northern Illinois and northwest Indiana.  These tightly packed isobars associated with this strong area of low pressure indicated strong winds in this location.


Weather map depicting the low pressure system (red L), ridge of high pressure (blue H), and the pressure gradient in between the two systems.  The more tightly packed these lines are, the stronger the winds.  Data source: National Weather Service.

Concluding thoughts

The Groundhog’s Day blizzard was historic in many ways, and provided a good example of a classic thundersnow event.  This unique combination of meteorological forces teamed up to blanket northern Illinois with a tremendous amount of snow.  Many people were stranded on Lake Shore Drive for hours – without any heat, gas, and food.  Now that you are a little more familiar with thundersnow, we will make sense of what all these terms and weather systems mean.


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