"Winter Storm Lucian" will impact the greater Chicago area this evening (Tuesday, Feb 5) and into the early morning hours of Wednesday (Feb. 6), bringing significant ice accumulations of up to 0.40 inches.
An Ice Storm Warning was issued earlier this morning (Tuesday, Feb. 5) for the event. Ice Storm Warnings are issued when confidence is high in ice accumulations of 0.25 inches or more. To show how rare of an event this is for Chicago, I've generated a plot below (Image 1, above) that shows the number of days since the NWS Chicago (KLOT) has issued an Ice Storm Warning -- It's been 3328 days, which is over 9 years.
The last time Chicago saw an Ice Storm Warning was Dec. 26, 2009. During that event, a mix of ice and sleet preceded a "surprise" snow storm where up to a foot of snow fell. With the combined ice and heavy snow, travel was extremely difficult. There were 21 deaths recorded from the storm.
Exactly one year prior to that event on Dec. 26, 2008, a significant ice storm took place in the Chicago area, as well. During that event, around 0.50 inches of ice accumulated (Image 2, right). The reason for the rarity of significant ice events in a particular area, such as Chicago, is due to extremely small changes in the atmosphere that affect precipitation type, and the amount of moisture available.
In winter, all precipitation starts out as snow high in the atmosphere. If all of the air between there and the ground is below freezing, it will fall as snow. If there is a small "wedge" of warm air just above freezing, this snow will partially melt into an ice ball, and will fall to the ground as sleet or ice pellets. In the case for freezing rain, a larger wedge of warm air reaching 3 degrees Celsius causes the snow to melt entirely and fall as rain. In special occasions, the temperature at the surface may be at or below freezing even when temperatures aloft are warmer.
When this occurs, the precipitation that falls as rain will instantly freeze once hitting the ground, power lines, cars, or anything else in its path. Check out the diagram (Image 3, above) for how these precipitation types generate.
Beth Carpenter is a co-founder and meteorologist at Thermodynamic Solutions, based in Indianapolis. You can reach Beth at email@example.com.