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Mesoscale Discussion 20
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   Mesoscale Discussion 0020
   NWS Storm Prediction Center Norman OK
   0200 AM CST Fri Jan 12 2018

   Areas affected...Southwest IN...southeast IL...western
   KY...southeast MO...western TN...extreme eastern AR...northern
   MS...and extreme northwest AL

   Concerning...Freezing rain 

   Valid 120800Z - 121300Z

   SUMMARY...Rain will change to freezing rain (rates up to 0.10
   inch/hour) across the discussion area, as surface temperatures fall
   below 32 F with the passage of a sharp cold front.  Higher freezing
   rain rates (around 0.20 inch/hour) will be possible with showers,
   with freezing rain persisting the longest across the Mid-South
   (northern MS into southwest TN).  Sleet should become the dominant
   precipitation type between 09-12Z across the rest of the discussion

   DISCUSSION...Trends in surface analyses indicated a sharp cold will
   continue to advance to the east across the lower OH/TN/MS Valleys
   during the rest of the overnight to the early morning.  Given the
   structure of this front, the vertical-thermodynamic profile in the
   lower troposphere will have sub-freezing temperatures below 900 mb
   with the frontal passage, and a warm nose persisting between 700-900
   mb per trends in upstream raobs and forecast soundings.  This will
   result in ongoing rain changing to freezing rain within 1-2 hours in
   the wake of the cold front.  Strong height falls and DPVA attendant
   to a progressive shortwave trough taking on a negative tilt across
   the lower MS/TN Valleys will sustain the swath of precipitation and
   showers developing and spreading from southwest to northeast through
   at least 12-13Z.

   Meanwhile, recent trends across the western extent of the
   precipitation shield in parts of southeast MO and southern IL
   indicated a reduction of the warm nose, such that sleet has become
   the predominant precipitation type.  Given the continued potential
   for embedded convection, the areas that change to sleet should have
   rates sufficient for accumulation, as the vertical-thermodynamic
   profile cools.

   ..Peters.. 01/12/2018

   ...Please see for graphic product...


   LAT...LON   34178838 33929036 33439110 33839155 34459125 34459125
               36828951 38608758 38458645 38338557 38358501 37228608
               36598666 34728765 34178838 

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