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Mesoscale Discussion 1717 NWS Storm Prediction Center Norman OK 0346 PM CDT Mon Aug 12 2019 Areas affected...Central/southeast IA...Northern IL...Far northeast MO Concerning...Severe potential...Tornado Watch likely Valid 122046Z - 122215Z Probability of Watch Issuance...80 percent SUMMARY...Thunderstorms capable of primarily damaging winds and a few tornadoes are possible late this afternoon into the evening. Tornado watch issuance is likely prior to 23Z. DISCUSSION...An MCV is currently moving through the upper Mississippi Valley, with a weakening area of primarily stratiform rain noted across northeast IA into northern IL. In the wake of the MCV, clearing has been noted across central into southeast IA, with cumulus gradually increasing along an outflow/differential heating boundary from west-central IL into southeast IA. Lapse rates are generally weak across this region, but continued heating of a very moist low-level airmass will result in the development of moderate to locally strong buoyancy (MLCAPE of 1500-3000 J/kg) from southeast IA into northwest/central IL. Forcing for ascent will be somewhat nebulous in the wake of the leading MCV, but scattered thunderstorm development is eventually expected in the vicinity of the surface boundary and a weak surface low across central IA. Area VWPs and the 20Z ILX sounding depict sufficient deep-layer shear for organized convection, and initial development will include the potential for a few discrete supercells. Some increase in low-level flow/shear is expected with time into the evening, and a tornado threat will evolve with any supercell, particularly in the vicinity of the surface boundary which may move slowly northward over the next 2-3 hours. A strong tornado will be possible if any supercell can become sustained in the zone of enhanced low-level helicity near and just north of the boundary. Otherwise, locally severe wind gusts and marginally severe hail will be possible with any discrete convection. Most HREF members depict some sort of upscale growth this evening into portions of central IL, which would pose an increasing risk of more widespread damaging winds and potentially QLCS tornadoes. Tornado watch issuance by 23Z is likely in order to cover these threats, though earlier watch issuance is possible if deep convective initiation appears imminent.