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Severe Storm Outlook Narrative (AC)
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309 ACUS11 KWNS 231804 SWOMCD SPC MCD 231803 OHZ000-MIZ000-INZ000-ILZ000-WIZ000-232030- Mesoscale Discussion 1408 NWS Storm Prediction Center Norman OK 0103 PM CDT Sun Jul 23 2017 Areas affected...Portions of IL...IN...OH...southern Lower MI...southern WI Concerning...Severe potential...Watch unlikely Valid 231803Z - 232030Z Probability of Watch Issuance...5 percent SUMMARY...Thunderstorms will increase in number and intensity through the afternoon and continue into the evening, posing some isolated severe risk. Watch issuance will be unlikely. DISCUSSION...Air-mass modification continues in the wake of an MCS that crossed the Ohio Valley vicinity, owing to diabatic heating of residual moist air characterized by surface dewpoints in the lower 70s. The more convectively unperturbed air -- farther north toward the lower Great Lakes region -- is similarly moist and is also experiencing diabatic surface-layer heating. The net result of these factors is moderate destabilization, with MLCAPE increasing to 1500-2500 J/kg. This buoyancy will support an uptick in convective coverage and intensity amid an assortment of residual outflow boundaries deposited by the aforementioned MCS, differential-heating zones peripheral to MCS-related cloud debris, and subtle/weakly convergent surface baroclinic troughs from southern WI to southern Lower MI. Effective shear around 25-35 kt may support a few organized, sustained convective elements capable of marginally severe wind/hail moving eastward to southeastward through the early evening. However, water vapor imagery implies that air with a history of subsidence -- reflected by a notable dry intrusion within the base of an approaching mid-level cyclonic perturbation -- will tend to suppress convective-scale upward accelerations owing to statically stable layers aloft. This will limit convective intensity. Furthermore, the lack of stronger deep shear and surface convergence suggest that severe coverage will be limited. ..Cohen/Grams.. 07/23/2017 ...Please see www.spc.noaa.gov for graphic product... ATTN...WFO...CLE...ILN...DTX...IWX...IND...LOT...ILX...MKX... DVN... LAT...LON 41328243 40918226 40208342 39798443 39408603 39578818 40718940 41678987 42498969 42728859 42118668 41568300 41328243
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