Early Season Significant Tornado: Southern Wisconsin/Northern Illinois

 A frontal wave that was handled well by model guidance several days in advance progressed east across the Midwest/Great Lakes Region on 8 February 2024. Dewpoints in the 40s to low 50s spread north in advance of a surface cold front, and MLCAPE values up to 500 J/kg developed during peak heating. Strong boundary layer destabilization also occurred along and in the wake of the cold front, and high based convection developed across the frontal zone as large-scale ascent increased across the region, with activity then deepening and becoming surface based as it encountered greater moisture east of the front. Wind profiles were very favorable for supercells across the warm sector. A couple of those rotating storms produced tornadoes, including a 39 km pathlength tornado with a duration of 36 minutes that produced high-end EF2 damage in the vicinity of Evansville, Wisconsin. The tornado pathlength/duration algorithm that I have been testing yielded a predicted 25 km pathlength with a 31 minute duration using input from NAMNEST forecast soundings across southern Wisconsin. Algorithm output can be found below within the hodograph inset.

Possible key features impacting this event: 1) Based on the WPC surface analysis, the tornadic storm in southern Wisconsin occurred near the intersection between a warm front and cold front. 2) The shear profiles were exceptionally favorable for rotating storms. 3) Steep lapse rates and a fully destabilized boundary layer airmass compensated for the seemingly marginal low dewpoint surface airmass. 4) Storm mergers in southern Wisconsin may have assisted in storm augmentation and subsequent tornado activity.

 

Reflectivity imagery during the time of the southern Wisconsin significant tornado.

Long-lived tornadic rotational velocity couplet moving northeast across the Evansville, Wisconsin region.

SPC storm reports.

Detailed summary of the tornado damage survey for the EF2 Evansville, WI tornado.

Favorably difluent left exit region of a 250 mb jet spreading east across WI/IL/Great Lakes region during the late afternoon of 8 February 2024.

925 mb thermal wave moving east toward WI/IL during the time of significant tornado activity.

Steep 0-3 km lapse rates spreading east in tandem with the progressive surface cold front.

SBCAPE values generally not exceeding 500 J/kg across the warm sector.

WPC surface analysis during the time of tornadic activity. Note the warm front/cold front intersection near the WI/IL border.

SPC objective surface analysis of temperature, dewpoint, and MSLP. Surface dewpoints were very low (40s to perhaps low 50s) across southern WI and northern IL.

NAMNEST proximity sounding taken from the time and place of significant tornado activity in southern WI.

Tornado pathlength and longevity guidance shown within the hodograph plot inset using input from NAMNEST proximity soundings in southern WI. These predicted values compare favorably with the damage survey conducted for the Evansville, WI EF2 tornado.