Severe storm threat continues… Severe thunderstorm watch #138 in effect until 10pm
The latest thinking on Severe Thunderstorm Watch #138 from the Storm Prediction Center
Mesoscale Discussion 0477 NWS Storm Prediction Center Norman OK 0325 PM CDT Mon Apr 10 2017 Areas affected...Portions of IL...extreme east-central MO...IN...southern WI...southwest Lower MI...southern Lake MI...far western OH Concerning...Severe Thunderstorm Watch 138... Valid 102025Z - 102230Z The severe weather threat for Severe Thunderstorm Watch 138 continues. SUMMARY...The risk for severe thunderstorms continues across Watch 138. An isolated severe risk may extend farther south, though additional watch issuance is unlikely. DISCUSSION...Isolated to scattered thunderstorms continue to develop near a front from far south-central WI southward to west-central IL and into east-central MO. The greatest coverage of this activity lies within western portions of Watch 138, in proximity to the strongest deep ascent, and will continue tracking east-northeastward. Surface-based convection encountering the strongest buoyancy will extend across northern parts of central IL east-northeastward into northwest Indiana -- characterized by 1000-2000 J/kg of MLCAPE aided by a narrow corridor of dewpoints around 60F ahead of the front beneath 8.0-8.5 C/km midlevel lapse rates. Semi-discrete cells evolving near and south-southwest of the Peoria area will ingest this stronger buoyancy and likely be the most intense across the Watch area -- from the general area around Peoria east-northeastward toward the Pontiac area and farther east-northeast toward the far south suburbs of Chicago and adjacent far northwest Indiana. With effective shear around 45-55 kt, supercell structures will remain possible. The Lincoln IL VAD wind profile indicates slight veering of winds with height above the ground, though with ample low-level speed shear, supporting around 200-250 m2/s2 of 0-1-km SRH. An isolated tornado could occur with the most intense storms, though large hail and damaging winds should be the primary risk. Otherwise, the risk for severe hail and locally damaging winds will continue spreading across Watch 138, with the severe risk diminishing west of the front. South of Watch 138 near the front, storm coverage should remain more isolated owing to weaker deep ascent, though an isolated severe hail/wind risk may evolve from the Saint Louis vicinity into southern parts of central IL. Convective coverage is presently forecast to remain too limited for additional watch issuance. Also south of Watch 138, from east-central IL into central IN, a separate quasi-linear band of convection is evolving, and will continue developing eastward/northeastward during the next several hours. This activity will be supported by gradually moistening inflow (surface dewpoints rising into the lower 50s) yielding weak buoyancy. Effective shear around 30-45 kt, combined with 30-40 kt of flow around 1-2 km above ground, may support semi-organized convective structures capable of locally damaging wind gusts. Weak buoyancy should minimize severe coverage such that additional watch issuance is unlikely.