Significant severe weather is expected tomorrow, that is no longer news to any of us if we've been paying attention. Now lets talk about the timing. Multiple rounds of severe weather will occur, so what you should expect with them and where?
The first round will begin during the overnight and early morning hours ahead of the approaching warm front. This is a common scenario with dynamic systems such as the one we'll be dealing with. These storms are typically elevated due to the surface layer being stable with overnight cooling. Large to very large hail is the primary threat with these storms. We could even see 2" hail or larger as the atmosphere is VERY cold aloft, and the forcing is quite strong.
The most favored corridor for these storms will generally be I-70 through about US-24. Perhaps reaching I-80, but right now it looks like areas further south of 80 are favored. These storms will be quite loud too!
By mid to late morning, most of these storms should be shifting out of Illinois. The forecast starts to get a little iffy after this point, because that first round of storms will affect what happens later. Our current thinking is they will clear out of the area. This will allow the warm front to continue lifting north to I-88 and maybe even into Wisconsin.
Around this time (8am-12pm) there will be showers and thunderstorms hanging around near the front. Some of them could be severe with a hail/wind threat, but overall the ingredients won't be in place yet for the significant severe.
Once these move through, assuming we clear out and destabilize as currently forecast, the stage will be set for the big show. The system will be fully mature, a pacific front will swing into the ripe, unstable warm sector characterized by extreme wind shear and the stage will be set for explosive supercell development.
These should first form somewhere near the MS river and QUICKLY (over 60mph) move northeast across the state. These are the storms that will be capable of producing strong, long track tornadoes in addition to damaging winds and more hail up to baseball size (or perhaps larger?)
This 3rd round, while the most significant of the bunch, is also the one most prone to failing. Cloud cover may hang around too long, or the front timing could be off by an hour - but if neither of that happens - a severe weather event will unfold. These storms could get going as early as 2pm - give or take an hour - and track through the rest of the state into the evening. As they progress towards the eastern part of Illinois and the Chicago metro area, the chance for them to begin congealing into more linear complexes will begin, with a damaging wind/hail threat taking over as the dominant threats, although tornadoes will still certainly be possible.
These storms should mostly be out the state by sunset, except for maybe parts of eastern and southern Illinois, if they manage to develop that far south.
Its going to be a long day, stay vigilant stay prepared! Most of the ISC team will be out chasing, and we'll do our best to provide live updates in the field and our social media feeds!
To see our latest TAM; reference this article here: https://www.illinoisstormchasers.com/forecasts/fri-sat-march-27-28th-severe-tstorm-threat-update-2?fbclid=IwAR1jiAqHFcG8mAI4NtaEf2uCK2Rj7y86yqzQ2C_RaZmxLRBWRqedTMO56dY