ZCZC SPCSWOMCD ALL ACUS11 KWNS 272050 SPC MCD 272050 INZ000-MIZ000-ILZ000-WIZ000-IAZ000-272245- Mesoscale Discussion 0163 NWS Storm Prediction Center Norman OK 0250 PM CST Tue Feb 27 2024 Areas affected...Northern IL and far northwest IN Concerning...Severe potential...Tornado Watch likely Valid 272050Z - 272245Z Probability of Watch Issuance...80 percent SUMMARY...A few supercells are expected to develop over parts of northwest Illinois and then spread east-northeast across northern Illinois into far northwest Indiana through early evening. Very large hail to around baseball size will be the primary initial threat. A conditional strong tornado scenario may develop later over northeast Illinois into far northwest Indiana. DISCUSSION...20Z subjective surface analysis placed a 990-mb cyclone near the Quad Cities along a pronounced cold front sweeping southeast across the Upper MS to Lower MO Valleys. A bent-back plume of relatively richer surface dew points from the mid to upper 50s lies immediately ahead of this wave, along the northwest periphery of the broader warm-moist sector across the OH Valley and lower Midwest. Within the exit region of an intense mid-level jet shifting east from the southern Great Plains to the Lower OH Valley, continued ascent should yield sufficient weakening of MLCIN during the 22-23Z time frame. Initial storm development is most likely in the immediate vicinity of the surface low. The environment will be favorable for discrete supercells forming despite initially modest low-level shear. Amid very steep mid-level lapse rates, large to very large hail production is expected, with peak intensity around 2 to 3.5 inches possible. The primary uncertainty is with how convection evolves downstream given the relatively narrow warm/moist sector where storms develop. There should be a tendency for convection to spread into a drier boundary layer towards the WI border. It is plausible, within a scenario highlighted by various 12Z MPAS members and occasionally by later WoFS/RRFS/HRRR runs, that a longer-tracked supercell or two may develop into the broader warm-moist sector across northeast IL into far northwest IN. Low-level shear within this region will be increasing towards and after dusk, which will conditionally support potential for a strong tornado into the EF2-EF3 range (peak estimated gusts around 120 to 150 mph). ..Grams/Hart.. 02/27/2024 ...Please see www.spc.noaa.gov for graphic product... ATTN...WFO...IWX...IND...LOT...ILX...MKX...DVN... LAT...LON 41339107 41889019 42428914 42648804 42478749 41728666 41188637 40588664 40008714 40078770 40758927 40549065 41339107 NNNN