SPC AC 191958

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0258 PM CDT Wed Mar 19 2025

   Valid 192000Z - 201200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS
   AFTERNOON/EVENING FOR PARTS OF CENTRAL/NORTHERN ILLINOIS AND WEST
   CENTRAL INDIANA...

   ...SUMMARY...
   Scattered severe thunderstorms are expected this afternoon/evening
   from central/northern Illinois to western Indiana.  A few tornadoes,
   large hail, and damaging wind gusts will be possible.

   ...20Z Update...
   The primary change with this update was a southward extension of the
   Slight risk area to the TN/KY border. Current expectations are for a
   broken band of thunderstorms to track eastward into this area this
   afternoon into the evening. Here, filtered diurnal heating amid
   gradual boundary-layer moistening should yield enough low-level
   destabilization for a threat of scattered severe wind gusts and an
   embedded tornado risk -- given favorable low-level hodograph size
   and curvature. Elsewhere, severe probabilities were trimmed from the
   west behind the eastward-advancing dryline. 

   For details on the near-term severe risk, reference Tornado Watch
   #57 and MCD #245.

   ..Weinman.. 03/19/2025

   .PREV DISCUSSION... /ISSUED 1059 AM CDT Wed Mar 19 2025/

   ...IL/IN vicinity into early tonight...

   A deep (990mb) surface low over northwest MO this morning will
   continue to shift east into northern IL through late afternoon, and
   into IN/MI and southern Ontario overnight. Meanwhile, the exit
   region of a 90-100+ kt 500 mb jet will overspread IL/IN this
   afternoon into early evening. 

   Cloud cover and elevated convection this morning across IA and
   northern IL, along with lofted dust moving over the region may
   inhibit heating to some degree into the afternoon. Furthermore,
   boundary layer moisture is expected to remain modest, most likely in
   the 51-54 F range given 15z upstream observations in the low 50s F.
   If any stronger heating occurs, mixing may further limit dewpoints
   from climbing into the mid/upper 50s F as suggested by some forecast
   guidance. Nevertheless, cold temperatures aloft (-17 C on 12z RAOBs
   from DVN) will support steep midlevel lapse rates and modest
   destabilization into the early afternoon. A dryline is noted across
   central MO at 15z. This feature will continue to sweep east into the
   afternoon and become a focus for stronger thunderstorm development
   across IL by 18-19z. 

   Vertical shear will be favorable for supercells given elongated
   hodographs with enlarged, looping low-level hodographs. These
   vertical wind profiles are already evident in VWP data from DVN and
   LOT where a west to east oriented surface warm front is roughly
   aligned across east-central IA/northern IL. Evolution of modest
   boundary layer moisture will be key for determining the tornado risk
   this afternoon across IL/IN. If dewpoints can reach 55-56 F, a
   localized area of enhanced tornado potential may develop. While some
   risk for tornadoes will still exist with lower dewpoints, the risk
   would be more muted and likely less intense. Given the conditional
   risk for a strong tornado or two, will maintain the 10 percent SIG
   tornado risk. Otherwise, somewhat low-topped supercells will also
   pose a risk for damaging gusts of 60-70 mph and isolated large hail
   up to 1-1.5 inches in diameter. Isolated convection may develop
   southward along the dryline/cold front into western/central KY by
   late afternoon before convection weakens after sunset.

   ...TN/MS/AL vicinity this afternoon/evening...

   Dewpoints have gradually increased into the mid 50s late this
   morning. Continued warm advection ahead of a wind shift this
   afternoon should allow dewpoints to reach the upper 50s to low 60s
   F. While forcing for ascent will be weak compared to further north,
   it should be sufficient to erode capping in the persistent warm
   advection regime. Instability will remain modest, generally
   near/less than 500 J/kg MLCAPE. While vertical shear will be quite
   strong, this should limit updraft intensity/longevity. A broken band
   of storms may still pose a risk for a few strong gusts.

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