Idealized simulations of the 20 April 2004 Utica, IL supercell

The 20 April 2004 tornadic event over northern IL is intriguing partly because of the forecast challenge that it presented but also because a retrospective analysis of the event reveals that the source of air entering the observed storms is not obvious. This event was notable because of its severity: approximately 16 tornadoes were reported in northern and central Illinois, including the F3 “Utica” tornado that produced significant damage and was responsible for 8 deaths in Utica, IL.

The objective of this work is to identify the likely source of air into the 20 April 2004 Utica, IL tornadic supercell. The proximity of this storm to a retreating northwest to southeast oriented warm front suggests that parcels entering the updraft could have originated on either side of this boundary. A composite sounding of the airmass just to the south of the warm front in the vicinity of the Utica supercell is characterized by a modest convective available potential energy (CAPE) of 1214 J kg^-1 and a moderate 0-3 km storm-relative helicity (SRH) of 248 m² s^-2. Nearly all of the streamwise vorticity present in this composite sounding was located in the lowest 1 km, yielding a 0-1 km SRH of 204 m² s^-2. In contrast, the airmass just on the cool side of the boundary was characterized by both larger CAPE (1520 J kg^-1) and larger helicity (the 0-3km SRH was 585 m² s^-2 and the 0-1 km SRH was 340 m² s^-2). Further behind the warm front the airmass was characterized by lower CAPE (781 J kg^-1) and even larger helicity (the 0-3 km SRH was 634 m² s^-2 and the 0-1 km SRH was 405 m² s^-2).

Ostensibly, the airmasses just behind and just ahead of the warm front should easily support supercells and strong low-level circulations however, it is not clear if the modest potential instability and strong shear of the airmass further behind the front will yield persistent deep convection. Given the very large environmental streamwise horizontal vorticity, should this airmass be capable of supporting persistent deep convection it could also be capable of generating the strongest mesocyclones.

Numerical experiments underway aim to test the character of deep convection simulated in each of the three (homogeneous) environments described above. To evaluate the likelihood that a given airmass dominated the inflow into the observed storm, qualitative comparisons will be made between the structures and evolutions of the simulated and observed storms. Preliminary simulations using the composite warm sector sounding produce a quasi-steady supercell with cyclic low-level mesocyclones.