Mesoscale Discussion 0720 NWS Storm Prediction Center Norman OK 0552 PM CDT Mon May 15 2017 Areas affected...Northern Illinois and southern Wisconsin Concerning...Severe potential...Watch possible Valid 152252Z - 160045Z Probability of Watch Issuance...40 percent SUMMARY...Isolated thunderstorms in the region will pose a risk of hail and gusty winds, and an isolated tornado cannot be ruled out. The severe threat should be isolated enough to preclude a WW issuance, although convective trends will be monitored. DISCUSSION...An isolated surface-based severe thunderstorm has developed in northern Illinois in the past hour on the southern flank of a remnant MCS that is currently weakening in Wisconsin. The surface pattern in this region is fairly complicated by a series of low-level boundaries in the region, including a remnant stationary front/outflow boundary just north of the Wisconsin/Illinois state border and another westward-moving boundary just west of the Chicago Metropolitan area. Although synoptic forcing for ascent is negligible across the area, continued convergence along these surface boundaries amidst a moderately unstable and weakly capped airmass may foster additional isolated development in an environment marginally supportive of updraft rotation. Hail and isolated wind gusts will be the primary threat with this activity, although an isolated tornado cannot be completely ruled out especially with any brief updraft/boundary interaction. Convective coverage is the primary uncertainty with regard to weather watch issuance over the next 1-2 hours in advance of an organizing linear complex over north-central Iowa that should reach the discussion area after about 00-01Z.
