The Synoptic setting...
A historically deep low pressure system supported a very sharp surface trough into the Midwest on the morning of the 26th. At 7 am EDT, an anomalously strong upper level jet of 140 knots with an associated 100+ knot mid level jet (approx. 500 mb) was in the process of advecting into the southwest Great Lakes with deep diffluent flow observed.  The upper jet had began to curve cyclonically over the Midwest with the divergent left front exit region basically over the western Midwest and coincident with the sfc trough in place.  850 mb flow was observed at over 75 knots with sfc gusts in excess of 45 knots ahead of the sfc trough per warm air advection.  The dryline/trough was supporting a line of severe thunderstorms with ongoing reports of damaging winds and a few tornadoes.  This system was a perfect text book case of strong cyclogenesis spurred by a powerful upper level jet and associated strong cold air advection with forcing fields lined up perfectly to support mass upper level divergence with low level convergence, in the presence of a negatively tilted upper level trough.

The Mesoscale and Stormscale environment...

The mesoscale environment could not have been more dynamic to support severe thunderstorms.  By 9 am EDT, a severe line echo wave pattern (LEWP) had developed across southern IN, with a broken line of weaker cells across northern IL into NW IN.  One major component this system lacked early on was appreciable convective avaliable potential energy (CAPE), with typically 500 J/kg or less noted. However, backed sfc flow with nearly 50 knots at 1.5 kft,  supported very impressive (nearly off the charts) 0-1 km storm relative helicity values with around 500 m2/s2 early on in the event to over 800 m2/s2 by the end of the event.  Effective shear (cloud bearing) were more than ample to support strong rotating updrafts (meso/misocyclones) and given the confined low level shear values(0-1km) of over 50 knots with SR helicity of 500-800 m2/s2, it would certainly not take much to produce a tornado as was the case.  Even though CAPE parameters were very marginal and pre-squall line lifting condensation level's /level's of free convection were also marginal, one very important thing to note is that much of the CAPE was located in the 0-3 km layer and most certainly the 0-1 km layer, meaning that any low level stretching of ambient vorticity would have been very strong at or around cloud base and very supportive for tornadoes (very similar to the 4-20-04 outbreak).  In addition, strong flow just off the sfc per the synoptic fields would have easily mixed down to the ground given the PW fields (column loading) and the addition of any miso/meso-cyclones.  Conditions became more supportive for severe damaging winds and tornadoes as the upper level jet neared the region (flow increased) and instability increased per diurnal heating.

 

The radar trends and observations...

Radar trends were certainly interesting nonetheless. The aforementioned broken line of cells that initially pushed into the forecast area (FA) over the NW, were full of moderate to strong misocylone couplets, most noticeable on the TMDW radar. The same signatures were not as impressive on KLOT leading one to believe that the couplets were based very low in the cells. (TMWD base level is 0.3 while KLOT is 0.5) The couplets developed quickly and were typically short-lived and at any given time 3-5 were observed. Given the dynamic nature and the environment with the above mentioned parameters in place, a tornado warning with squall line mention was used. The line weakened as it pushed NE into the FA, but concerns on whether this was a sampling issue given the proximity to the radar, supported an issuance of another warning given the environment in place. The LEWP advecting NE into the FA contained numerous strong circulation couplets and again given the environment and the parameters in place a TOR warning with squall line mention was issued.  The TOR with the squall line mention was preferred given the environment, anomalously strong nature of the system, and numerous transient velocity couplets. Numerous reflectively tags were also observed which is typical given the strong upper jet and highly sheared flow, with likely geostrophic adjustment processes. Many of the LEWP inflow notches were coincident with these tags.

 

Significant damage reports including generalized narrative and known injuries/fatalities...

Thirteen tornadoes were produced from 4 separate LEWP segments and 1 low topped supercell. Most damage was on the scale of an EF0 tornado with the most intense damage rated as EF1.  Each misocylone produced widespread 60-80 mph wind around any tornado damage, likely a result of the pressure perturbation associated with the circulation.

  JC


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