Afternoon Update: June 22 Severe
The evolution of today’s severe weather threat was assumed to be complex from the start and it’s certainly unfolding that way. A potent mesoscale convective complex (MCC) developed over the Dakotas late Sunday night and tracked southeastward through Siouxland. The complex brought damaging winds to portions of eastern Iowa with significant wind gusts, before it started to fade across northern Illinois and southern Wisconsin.
While there are still many variables to consider with today’s severe weather threat, a few things are certain. First, a cold pool behind the MCC has resulted in limited instability across southern Wisconsin, northeastern Iowa and northwestern Illinois. The axis of best overlap between instability and shear has nudged south and currently resides across southern Iowa. Considerable cloud debris and leftover shower activity continues across northeastern Illinois. A severe weather threat, including tornadoes still exists today, but has trended south from earlier model forecasts.
The focus point shifts toward southern Iowa and the upstream environment located across adjacent northwestern Missouri. By 12 p.m. CDT, mesoanalysis was showing 3000-4000 J/kg MLCAPE in this region. A subtle surface low was objectively analyzed near the Iowa/Nebraska border with a front extending southwest. Considerable theta-e advection, as well as increasing instability and steep mid-level lapse rates, should continue to lift through central and eventually eastern Iowa. Capping remains evident south of I-80, although with time this afternoon that cap should be eroded.
With surface winds still locally backed and an outflow boundary draped near I-80, the apparent target for initial storm development, likely severe, should be from south-central to east-central Iowa by mid to late afternoon. This scenario is supported by the latest runs of the HRRR. Assuming the cap holds and the environment in eastern Iowa improves, supercell thunderstorms should fire and quickly become severe. If the storms remain discrete, a threat for large to very large hail and isolated tornadoes will exist. While additional backing of winds would further enhance the tornado threat, it is not completely necessary to support a few tornadoes. The threat of a strong tornado is still in question, but cannot completely be ruled out.
Watch for the thunderstorms to move into northwestern Illinois late this afternoon and the threat should transition to more of a damaging wind event into the evening. If the air-mass over northern Illinois can also recover adequately, there could be fairly widespread damaging winds as well. Chicago is east of the enhanced tornado threat, but a couple of isolated tornadoes remain possible from southeastern Wisconsin into northeastern Illinois, northern Indiana and lower Michigan.
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