12AM Update for July 13th Event
NAM forecast highlights a scenario where
greatest tornado risk is near IA/IL border.
The models were virtually all wrong with the southward extent of convection Sunday evening. Models, generally, blew up storms over far southern MN into IA, but there’s been pretty much nothing there. There’s one cell over SD, but even the HRRR shows that fizzling. There was some speculation that this cell would blow up into an mesoscale convective system (MCS) over IA. I don’t see that happening.
With this said, although some convection may eventually fire over portions of IA, I think the main player is the MCS over MN/WI. On the current trajectory, that should affect much of central/southern WI and probably northern Illinois. The result is little to no air-mass disruption south and west of US-52, which would support extreme instability across eastern IA and central/NW IL by afternoon. I think Chicagoland may have some convective debris to somewhat limit destabilization. The morning convection should leave an outflow boundary somewhere southwest of Chicago, perhaps close to US-52? Probably a bit southwest of there. The HRRR/NAM seem to favor a boundary/wind-shift across far eastern IA into central IL.
The next question comes to the timing of afternoon initiation. Latest HRRR runs show initiation across south-central to SE IA by 16-17z (11 a.m. to noon), but I’m a bit skeptical. I suppose this is possible. That could lead to a mixed and messy storm evolution, depending on how quickly storms can become surface-based and if any storms latch onto a front/boundary.
Assuming that overnight/morning convection is limited (SW of US-52) AND new storm initiation is held off until mid to late afternoon, that’s going to lead to an ugly scenario with extreme instability coinciding with strong shear. I am not 100% sold on this, as rarely does a setup (especially this year) take full advantage. As we’ve seen with some progs (and thinking back to previous events in the IL vicinity this year), even ongoing convection in the morning won’t completely rule out the late-day event, as I think we’re at a point of no return where there will be intense storms tomorrow. I doubt anything will change that at this juncture, given trends and obs.
Final thoughts:
I would tend to favor a solution where some spotty shower activity or weak convection manages to get going in the morning across E IA/central IL, but does not have much of an effect on destabilization. It’s probable that some rogue storms fire early in the afternoon and pose somewhat of a severe risk, but the better chance of severe, some significant, should be from mid/late afternoon into the evening hours. I think we’re looking at a corridor slightly west of the previous SPC outlook, targeting E IA into much of central into NW IL, but primarily SW of US-52. You can’t write off a threat in Chicagoland with this sort of setup (small margin of error could have major implications on a large population), although I think the strongest storms stay southwest of the city.
