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Mesoscale Discussion 219
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   Mesoscale Discussion 0219
   NWS Storm Prediction Center Norman OK
   0203 PM CST Tue Feb 28 2017

   Areas affected...Portions of northern IL...northern IN...southeast
   IA...extreme northeast MO

   Concerning...Severe potential...Tornado Watch likely 

   Valid 282003Z - 282230Z

   Probability of Watch Issuance...80 percent

   SUMMARY...The area is being monitored for increasing
   severe-thunderstorm potential, including the risk for tornadoes,
   severe hail (possibly significant), and damaging wind gusts. The
   issuance of a Tornado Watch will likely be required during the next
   few hours.

   DISCUSSION...A northwest-southeast-oriented warm front extending
   from surface low pressure in northeastern IA to southern IN will
   continue to develop northeastward, as the leading edge of a
   high-level speed maximum also spreads northeastward. Recent water
   vapor imagery indicates a notable surge of midlevel drying advancing
   through central IA, likely associated with an impulse embedded
   within the flow aloft. As forcing for ascent associated with these
   features continues to overspread the northeastward-developing warm
   sector (where modest pressure falls are observed), convection should
   increase in coverage/intensity during the next several hours.
   Already, isolated elevated convection has been intensifying
   northeast of the warm front in east-central IL, with additional
   convection increasing in the open warm sector across south-central

   Despite the return of only modest boundary-layer moisture (surface
   dewpoints in the middle 50s to around 60F), steep midlevel lapse
   rates associated with a well-established elevated mixed layer
   overlying the area will support sufficient buoyancy for intense
   updrafts. With 40-60 kt of effective shear, rotating updrafts will
   likely be capable of producing severe hail -- possibly significant.
   Damaging wind gusts and tornado potential will exist, especially
   near and south/southwest of the warm front, where effective inflow
   layers will extend to the surface. Significant tornadoes could even
   occur owing to ample streamwise vorticity in the boundary layer
   (effective SRH around 200-400 m2/s2), and especially in proximity to
   the warm front given pre-existing vertical vorticity.

   There is some uncertainty regarding the timing of the more robust
   increase of surface-based convective risk. Regardless, the risk for
   all severe hazards will be increasing, likely warranting Tornado
   Watch issuance during the next few hours.

   ..Cohen/Hart.. 02/28/2017


   LAT...LON   42058871 41908772 41368627 40518662 40308817 40309071
               40599215 41179228 41749123 41959054 42058871 

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