Severe weather threat increasing- Northern Illinois and portions of the Chicago area targeted for possible tornado watch later this afternoon

The Storm Prediction Center in Norman, Oklahoma has just issued a mesoscale discussion highlighting the potential for a tornado watch issuance across northern Illinois and portions of the Chicago area later this afternoon.

Despite the fact, that it’s still February, atmospheric dynamics are strong and temperatures are quite springlike with Illinois temperature’s ranging from the 50s north to the 70s south.   Thunderstorms are expected to develop rapidly later this afternoon and could quickly become severe, producing large hail, strong damaging winds and even tornadoes. Heavy rainfall is also possible with these storms.

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
   IA.

   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.