The worst tornado outbreak of 2015 so far occured on April 9th with the largest storms in northern Illinois.
The setup for the outbreak:
Excerpts from the Storm Prediction Center’s 2000Z Convective Outlook:
PROGRESSIVE...MULTI-STREAM FLOW WILL PREVAIL OVER THE CNTRL AND ERN U.S. THIS PERIOD...DOWNSTREAM FROM WRN STATES SPLIT. STRONG SHORTWAVE TROUGH NOW EXITING THE CNTRL HIGH PLNS SHOULD ACCELERATE ENE INTO ERN IA THIS EVE...BEFORE FURTHER ACCELERATING AND TURNING NE TO NEAR SSM EARLY FRI AS NRN BRANCH IMPULSE AMPLIFIES SE INTO MN. AT THE SAME TIME...WEAKER LEAD DISTURBANCE NOW CROSSING THE OZARKS SHOULD MOVE NE INTO THE MID-OH VLY BY THIS EVE...AND INTO NY/PA EARLY FRI...AS DEEP SWLY FLOW STRENGTHENS ACROSS THE E-CNTRL AND ERN U.S.
AT THE SFC...NW MO LOW EXPECTED TO DEEPEN NEWD INTO SRN WI BY THIS EVE...AND UNDERGO FURTHER DEEPENING AS IT REACHES THE SSM AREA EARLY FRI. COLD FRONT TRAILING SSW FROM THE LOW SHOULD STRENGTHEN AND ACCELERATE E ACROSS THE MID-MISSISSIPPI VALLEY AND OZARKS BY LATE TODAY...AND INTO THE UPR OH...TN...AND LWR MS VLYS EARLY FRI...WHILE MORE DIFFUSE WARM FRONT ADVANCES N INTO LWR MI.
IN SUMMARY...THE OVERALL SET-UP SUPPORTS A BROAD AREA WITH AN APPRECIABLE RISK FOR SVR WEATHER LATER TODAY THROUGH EARLY FRI...BUT WITH MITIGATING FACTORS THAT LIKELY WILL KEEP THE CORRIDORS OF GREATEST RISK SOMEWHAT FRAGMENTED AND MULTI-MODAL.
...MID-MS VLY TO SRN GRT LKS THIS AFTN THROUGH EARLY TNGT... SEVERAL FACTORS MAY COMBINE TO YIELD A MESOSCALE CORRIDOR OF SOMEWHAT ENHANCED SVR THREAT...INCLUDING THE POTENTIAL FOR A COUPLE STRONG TORNADOES...OVER THE ERN IA/NE MO/NRN IL/SRN WI AREA LATER TODAY INTO EARLY TNGT. PERSISTENT HEIGHT FALLS ACROSS REGION...LARGELY DUE TO AMPLIFICATION OF UPSTREAM...LARGER-SCALE TROUGH...WILL OFFSET SINKING/DESCENT OCCURRING IN WAKE OF LEAD UPR IMPULSE AND EMBEDDED MCV NOW CROSSING THE OZARKS. AT THE SAME TIME...SATELLITE SUGGESTS LIKELIHOOD FOR AT LEAST MODEST SFC HEATING AHEAD OF COLD FRONT AND DEEPENING SFC LOW APPROACHING FROM NW MO.
WITH 700 MB SWLY WINDS INCREASING TO AOA 60 KTS ATOP 40+ KT SSW FLOW AT 850 MB...HODOGRAPHS WILL BE FAVORABLE FOR SUSTAINED SUPERCELLS WITH LARGE HAIL. THE DIRECTIONAL COMPONENT TO LOW-LVL SHEAR LIKELY WILL NOT BE GREAT. BUT COMBINATION OF STRENGTHENING FLOW AND EXPECTED BACKING OF NEAR-SFC WINDS/ENHANCED SRH IMMEDIATELY AHEAD OF SFC LOW SUGGEST THAT A CORRIDOR OF HEIGHTENED TORNADO POTENTIAL COULD EVOLVE LATER THIS AFTN...BEFORE THE STORMS MERGE INTO BROKEN LINES/SMALL CLUSTERS EARLY TNGT. ALTHOUGH CLOUDS AND SHOWERS MAY DELAY NWD ADVANCE OF WARM FRONT ACROSS IL/IND...STRENGTHENING SFC PRESSURE FALLS/TRACK OF SFC LOW FURTHER INDICATE THAT THE SVR THREAT COULD EXTEND NWD INTO SRN WI AND SW LWR MI...WITH DMGG WINDS BECOMING THE DOMINANT THREAT OVER TIME.
SPC Mesoscale Discussions relevant to the Illinois tornadoes:
Here is the GRLevel2 capture of the tornado as it hit Rochelle:
The largest tornado hit Rochelle IL. Here are several videos:
Links: