A powerful winter storm that brought thunderstorms to southern California and snow to Las Vegas moved across Illinois and Indiana late Thursday night and Friday morning December 18-19. Sleet and freezing rain developed across northern Illinois south of I-80 between 7 pm and 10 pm Thursday night with generally snow, sleet, and freezing rain north of I-80. Across far northern Illinois, from Rockford to Belvidere to Crystal Lake to Highland Park and points northward, precipitation fell almost entirely as snow throughout the event resulting in the highest snowfall totals... ranging from 7 to 10 inches.
South of this line, but north of Reagan Tollway (I-88) and the Eisenhower Exp. (I-290), precipitation started as snow and ended as snow, but fell mainly as sleet through most of the night, resulting in significantly lower snow and sleet accumulations (from 2 to 5 inches). Even so, the liquid equivalent that fell from the sky was nearly the same as that over areas of far northern Illinois that saw the heavy snow. This is because sleet is a much more concentrated hydrometeor, holding a greater degree of water equivalent but compacting down much more than snowflakes.
South of the I-88/I-290 corridor, but north of U.S.-30 precipitation throughout the night and Friday morning consisted largely of sleet and freezing rain, resulting in sleet accumulations from 1.5 to 3 inches, but also ice accumulations up to a quarter of an inch.
Another discontinuity in precipitation type occurred along and south of U.S.-30 where some sleet but mainly freezing rain fell during the majority of the storm. While sleet and snow accumulations were only around an inch or less, ice accumulations from 1/2 to 1 inch were common south of U.S.-30 and north of U.S.-24. This icing resulted in numerous downed tree branches and power lines.
The image below shows that while the snow and sleet accumulations varied greatly from north to south across northern Illinois, the amount of water equivalent that fell from this storm was fairly uniform with most locations ranging from 0.75 to 1.25 inches.
The map below shows observed snowfall for 24 hours ending between 7 am and 11 am on Friday, December 19. Again note that for areas south of a line from Rockford to Highland Park but north of I-88/I-290, sleet contributed significantly to accumulations. In the area south of I-88/I-290 but north of U.S.-30 primarily sleet and freezing rain fell. South of U.S.-30 precipitation mainly fell as freezing rain resulting in significant icing. Tens of thousands of utility customers were reported without power in the areas of significant icing.
Not all values on these maps represent a final precipitation amount, however most do.
A closer view...
Predicting precipitation type can be extremely challenging, because very slight differences in temperature at various levels of the atmosphere can mean all the difference between 10 inches of snow and half an inch of ice, as shown by this storm. The figure below shows the various thermal profiles associated with different precipitation types during the heart of this event.
The lines shown on this chart above represent the temperature (and dewpoint) at five different locations through the lowest 16,000 feet of the atmosphere. The five locations are: Gary, IN (yellow); Rockford, IL airport (orange); DuPage County Airport (purple); Midway Airport (blue); and O'Hare Airport (green). At the surface, all locations are below the freezing mark (indicated by the red line). In at least some portion of the layer between 5,000 and 13,000 feet above the surface, the temperature at all locations was above freezing. Above this layer, all locations were below freezing and likely were initiating precipitation in the form of snow.
The temperature at Gary (in yellow) was warmer than the other four locations throughout the entire vertical column, and Gary is also the location that received the most freezing rain versus the other four points. Snow falling into the relatively warm layer above Gary melted almost completely and continued falling as rain, which then froze when it hit the ground.
The temperatures at O'Hare (green) and Rockford (orange) are generally the coldest throughout the vertical column. These are the two northernmost locations nearest the band of heavy snow that was reported near the Wisconsin border. Snow falling into a layer just barely above freezing will experience very little melting and will fall all the way to the surface as snow, as was the case near these locations.
Midway Airport (blue) and DuPage County Airport (purple) received a mixture of sleet, snow, and freezing rain throughout this event. Temperatures (as well as accumulations) at these two locations fell somewhere between the other three. Some snow falling over these locations remained frozen through the moderately warm layer and reached the surface as snow. Some snow melted completely into rain and froze again upon contact with the cold ground. Some snow melted partially and then re-froze before hitting the ground as sleet.
This example shows how temperatures vary over a very short geographical distance within a large metropolitan area, and how even just a degree or two can make a tremendous difference in the type of precipitation that occurs.
Pine cone and pine needles encased in ice - near Morris, IL
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