SPC AC 272000

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0200 PM CST Tue Feb 27 2024

   Valid 272000Z - 281200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS
   NORTHEAST IL AND FAR NORTHWEST IN THIS EVENING...AND ACROSS PORTIONS
   OF THE LOWER AND MIDDLE OH VALLEY THIS EVENING THROUGH TONIGHT....

   ...SUMMARY...
   Severe thunderstorms, associated with large hail, damaging gusts,
   and tornadoes, some of which may be significant, will be possible
   from late this afternoon into the overnight, across portions of
   northeast Illinois and northwest Indiana as well as the Lower and
   Middle Ohio Valley.

   ...20Z Update...

   ...Northern IL/Southern WI This Evening...
   Recent surface analysis places a low near the IA/MO/IL border
   intersection. A dryline arcs southeast from this low to near STL and
   then back southwest into north-central AR. Dewpoints behind this
   dryline are in the 40s, while dewpoints across much of IL are in the
   upper 50s/low 60s. A warm front also arcs northeastward from this
   low to just north of the ORD, then eastward across southern Lake
   Michigan, and southeastward across northern IN into far west-central
   OH. 

   General expectation outlined in the previous discussion remains
   valid, with convective initiation anticipated near this low, with
   the resulting storms then tracking northeastward in the vicinity of
   the warm front. It remains uncertain whether or not these storms
   will be on the warm side of the boundary. Given the kinematic
   fields, any surface-based storms would likely be supercellular and
   capable of all severe hazards, including tornadoes. Recent guidance
   include the 18Z and 19Z WoFS as well as the 12Z MPAS runs all
   suggest an increasing likelihood for surface-based, warm sector
   storms. Given these trends, expanded the Enhanced southeastward to
   include more of northeast IL and far northwest IN and also
   introduced a 10% significant tornado probability. 

   ...OH Valley This Evening and Overnight...
   Overall expectation outlined in the previous outlook remains
   unchanged for tonight across the OH Valley. As the night
   progresses, activity will likely evolve into fast-moving bowing
   structures with an enhanced risk of damaging wind gusts and a few
   tornadoes. Given the strengthening low-level flow, destructive wind
   gusts around 75 mph are possible, with a strong tornado or two
   possible as well.

   ..Mosier.. 02/27/2024

   .PREV DISCUSSION... /ISSUED 1026 AM CST Tue Feb 27 2024/

   ...OH Valley...
   A fast-moving southern-stream shortwave trough over the central
   Plains will track eastward today and begin affecting parts of MO by
   late afternoon or early evening.  Most 12z guidance suggests that at
   least isolated thunderstorm development will begin around/after dusk
   as the large-scale forcing overspreads the northern extent of
   moist/unstable air mass from southwest MO into southern IL.  These
   storms will intensify and track eastward overnight roughly along the
   OH river, moving into parts of OH/WV/PA by early morning Wednesday. 
   Initial storms may be discrete, with sufficient vertical shear for
   supercells structures capable of all severe hazards.  As the night
   progresses, activity will likely evolve into fast-moving bowing
   structures with an enhanced risk of damaging wind gusts and a few
   tornadoes.  A strong tornado or two cannot be ruled out.  Have added
   an ENH risk area for this scenario overnight.

   ...IL/WI...
   A compact surface low is forecast to deepen across eastern IA this
   afternoon, with a narrow warm sector pulling northward into much of
   northern IL.  Strong forcing near the low and along the associated
   warm front will lead to intense thunderstorms across portions of
   northern IL after 22z.  Forecast soundings show steep low and mid
   level lapse rates and sufficient CAPE, along with strong low and
   deep layer shear profiles.  This will be conducive to supercell
   structures capable of very large hail and damaging winds.  There
   remains uncertainty whether these storms will be in the warm-sector,
   or will be undercut by nearby cold air.  Have upgraded a small part
   of this area to ENH risk for hail, but there is also concern for
   tornadoes if the activity remains surface-based.

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