Big Snowflakes and Other Observations from Friday Evening's Snow

January 17th @ 6:20 pm Radar Image

Radar Image

   

    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:

Radar Loop

 

    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.


Snowfall Reports

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

  



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