A quick half to one inch of snowfall fell in parts of the Chicago metro late Friday afternoon into mid eve. To the left is a radar snapshot from ~6:20 pm.
This snow was the result of an upper level disturbance moving over the area. This system was actually weakening and the amount of lift with it to develop snowfall was quite marginal. However, snowflake size was quite large and that was key.
This was due to the temperatures in which saturation was happening, that is where snowflakes were developing. With the atmosphere being supersaturated with respect to ice as indicated by model analysis, this was ideal for large snowflake growth. Meteorologists refer to these larger snowflakes that originate within this temperature range as dendrites or plates.
Here is a look at the upper atmosphere during the time of the snowfall, where one can note the saturation at this ideal temperature zone for plate or dendrite growth, and it extends entirely down to the cloud base.
Also of interest on Friday evening were a handful of semi-circle shaped reflectivity enhancements seen within the snow. A loop of these from the NWS Chicago 0.5° reflectivity can be seen below:
It is suspected that these may have been a result of aircraft exhaust acting as condensation nuclei into this already primed atmosphere for snow growth. These echoes were generally seen between 1,500 ft and 5,000 ft, which was within the cloud layer or just below. Based on information about aircraft path into ORD and the wind speeds in place, such a curved path makes sense for this location.
PRELIMINARY LOCAL STORM REPORT...SUMMARY NATIONAL WEATHER SERVICE CHICAGO/ROMEOVILLE IL 835 PM CST FRI JAN 17 2014 ..TIME... ...EVENT... ...CITY LOCATION... ...LAT.LON... ..DATE... ....MAG.... ..COUNTY LOCATION..ST.. ...SOURCE.... ..REMARKS.. 0600 PM SNOW OHARE AIRPORT 41.98N 87.90W 01/17/2014 M0.4 INCH COOK IL OFFICIAL NWS OBS 0600 PM SNOW ROCKFORD 42.27N 89.06W 01/17/2014 M0.5 INCH WINNEBAGO IL OFFICIAL NWS OBS 0715 PM SNOW CLARENDON HILLS 41.80N 87.96W 01/17/2014 E1.0 INCH DUPAGE IL PUBLIC 0755 PM SNOW ST. CHARLES 41.92N 88.30W 01/17/2014 M1.0 INCH KANE IL TRAINED SPOTTER 0830 PM SNOW OAK BROOK 41.84N 87.95W 01/17/2014 M1.2 INCH DUPAGE IL CO-OP OBSERVER