Major winter storm bearing down on the Chicago area-Change over from rain and sleet to heavy snow expected this afternoon and evening

 Mesoscale Discussion 1660
   NWS Storm Prediction Center Norman OK
   1154 AM CST Sun Nov 25 2018

   Areas affected...Portions of eastern Iowa...northern Illinois

   Concerning...Heavy snow 

   Valid 251754Z - 260000Z

   SUMMARY...A mix of sleet and snow falling across parts of eastern
   Iowa into northern Illinois, is expected to transition to mainly
   snow over the next several hours. Very heavy snow, with accumulation
   rates of at least 1-2 inches/hour, is likely.

   DISCUSSION...A narrow, west-to-east oriented band of mixed wintry
   precipitation is ongoing from an area along a Keokuk County IA to
   Cook County IL axis. Current KDVN and KLOT dual-polarimetric radar
   data suggest that mainly sleet is falling within this band of
   heavier precipitation.

   The aforementioned band of precipitation has been sustained
   predominantly by 700 mb WAA/frontogenetic forcing, which should
   continue for several more hours. As a deepening surface low
   approaches the area later this afternoon/evening, large scale lift
   should only increase with time. Meanwhile, mesoanalysis indicates
   relatively strong 925 mb CAA impinging on the horizontal
   precipitation band to the north. Latest RAP point forecast soundings
   also suggest that cooling of the low-level troposphere will
   transpire during the early afternoon hours. As such, a change over
   of sleet/mixed wintry precipitation to snow is expected. With very
   strong lift expected within a saturating/deepening dendritic growth
   zone, very heavy snowfall, with rates of at least 1-2 inches/hour
   should prevail through the afternoon hours.
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