(4:00PM - 6/20/21) A storm system moving through the region today will bring a severe t'storm risk to a large portion of the state, from this afternoon through tonight.
A storm system moved through the Plains last night into this morning, and will move through the Midwest and Great Lakes today, and eventually into the Northeast by Monday. This storm system will bring a severe t'storm risk to a large portion of the state, this afternoon through tonight. With this update we have made a few significant changes based on real time observations... We have removed the level 3 severe t'storm risk across Northeast and East-Central Illinois. We have also shifted gradients around a bit based on the latest available data.
Two lead disturbances moved from Iowa and Missouri this morning, and have been moving across Southern Wisconsin and Illinois this afternoon. What was originally expected to happen was there was to be regeneration or new development of showers and t'storms across portions of Northern and Central Illinois. However, as of mid afternoon no development had occurred as of yet. The NWS released a RAOB weather balloon at 2PM, and data from this showed that a cap (warm air aloft) was in place ahead of the two disturbances moving across the state. Thus, that is one of a few reasons the activity had developed, and essentially has shut down the threat with this first wave across the state.
Further west along the cold front, showers and t'storms have started to develop across Southern Minnesota, Iowa and Northern Missouri. This activity will spread eastward across a large portion of Northern and Central Illinois, this evening into tonight. An environment supportive of a severe t'storm threat will be in place. This environment will feature moderate to high instability of 1,500-4,000J/KG, dew points in the mid 60's to low 70's, high shear and strong forcing. This environment will be supportive of upwards of an isolated to scattered severe t'storm risk (level 2-3) across much of Northern and Central Illinois. Damaging winds, large hail and a few tornadoes will be the main severe weather threats.
Below is the severe t'storm forecasts for this event.