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.