Meso-scale discussion for thunderstorm activity forming northern Illinois and southern Wisconsin-

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.