Tornado outbreak sequence of March 9–13, 2006

Beginning on March 9, the Storm Prediction Center (SPC) issued a moderate risk, spanning across northern and central portions of Dixie Alley, reaching into western Tennessee and western Kentucky. A slight risk was given to the surrounding area, reaching into the Ohio Valley, and to the east, including sections of north-central Georgia and the Florida Panhandle. While wind shear patterns and instability were conductive for the development of supercells and possibly strong tornadoes, the event was forecast to materialize as a major derecho event. Still, the elevated helicity values reaching 300 m²/s², and elevated dew points reaching into the 60s, aided the issuance of a large 15%, hatched risk for strong/EF2+ tornadoes throughout the moderate risk area. However, an even larger 45% hatched risk for damaging winds was placed throughout many of the same areas, forecasting the risk for the serial derecho to materialize.[3] As the event unfolded, multiple tornadoes were reported, including a damaging EF2 that caused significant damage in the Morton and Fair Oaks towns of Arkansas. However, the main event was a widespread derecho that prompted hundreds of severe damaging wind gusts throughout the main risk area.[4] Tens of thousands of people also lost electricity throughout the region. Two non-tornadic deaths happened on March 9, one by damaging wind, and another that was killed after a lightning bolt ignited a house fire.[2][5]

Tornado Watch 64, placed along a region which would soon experience multiple strong to intense tornadoes on March 11.

A slight risk was issued for many of the same areas on March 10, though only a 2% risk for tornadoes was issues, mostly in two corridors in the Ark-La-Tex region extending into Missouri, and another one mostly in Mississippi and west-central Alabama. The main risk was expected to be hail, and as the evening advanced, dozens of large hails reports came in from the aforementioned areas. However, no tornadoes touched down in this area, though one F0 did occur all the way in California, causing damage near the town of Encinitas in San Diego County.[6][7]

A much more substantial tornado event materialized on March 11. The SPC again issued a moderate risk for severe weather, primarily driven by a 45% hatched risk for large, destructive hail, centered in Missouri and Arkansas, though it extended into extreme eastern Oklahoma and southwestern Illinois. However, elevated wind shear valued ranging from 50 to 70kt, and steep mid-level lapse rates meant the environment was favorable for supercells, so a 10%, hatched risk for strong tornadoes was introduced for the area, as any supercell that could remain discrete would be capable of sustaining and producing strong tornadoes. The most favorable window for severe weather was expected to be overnight in the 00z to 06z timeframe, meaning that any tornadoes that could develop would end up being difficult to see.[8] As the night progressed, multiple supercells popped up in Oklahoma, eventually advancing throughout the risk area. Hundreds of damaging hail reports occurred on this area. However, as the supercells entered the more favorable environment for tornadoes, multiple of them became tornadic, resulting in multiple strong to intense tornadoes. One particular long-tracked supercell produced a tornado family in southeastern Missouri, which included a deadly F3 that killed two people in Perry County during its 53.5 mi (86.1 km) path, which crossed into Illinois, before dissipating near the town of Tamaroa. The supercell had produced a long-tracked, damaging F2 previous to this one, as it travelled through Reynolds, Iron, and Madison counties. Another supercell near this one produced another intense F3 in the town of Festus, Missouri, before crossing into Illinois. The tornadoes were accompanied by very large hail, some as large as softballs, before the day came to a close.

March 12, 1630UTC Convective Outlook (above), and respective tornado probability contour (below).

The next day, on March 12, conditions became even more favorable for the development of severe weather. The SPC, in its 1630 UTC outlook, introduced an incredibly rare high risk for many of the same areas from the day prior, with the main corridor being placed in eastern Kansas, most of Missouri, southeastern Iowa, and central Illinois.[9] The high risk was driven by the elevated probabilities of long-tracked, strong to intense tornadoes, all along the main high risk corridor. A 30% hatched risk for EF2+ tornadoes was given to this corridor as a result. A 45% hatched risk for large hail was also placed throughout the risk area, as any supercells capable of tornadoes would also produce large hail on their track. A potent upper-level trough, progressing along a strong 100-120 kt jet stream, in an area deepened by a progressing surface low in Oklahoma and Kansas, was the driving factor for the intense convective activity later in the evening/overnight hours. Additionally, sufficient moisture aided by dew points in the 60s, convective available potential energy (CAPE) values in the 1,500–3000 J/kg range, and very favorable low-level wind shear further destabilized the atmosphere, making it even more favorable for sustained supercells, capable of all hazards. However, a capping inversion in place for most of the day limited convective initiation at first, keeping all the necessary ingredients at bay. However, after the cap moved in the evening, multiple lines of powerful supercells began to form, soon entering the primed environment for tornadoes. As such, two PDS tornado watches were issued for most of the risk area, highlighting the elevated probabilities for intense tornadoes to develop in these areas.[10][11] As the morning advanced, severe weather, spearheaded by severe wind, affected the Kansas City Metropolitan Area, with significant wind and hail damage to many homes and businesses, but only two tornadoes in the far northwestern part of the area.[12] Significant damage was also reported at the University of Kansas in Lawrence and at the Great Wolf Lodge Indoor Water Park. Numerous airplanes also flipped over at the Kansas City Downtown Airport.[13]

F2 tornado damage to a home in Bentonville, Arkansas.

As the evening advanced, the main line of supercells began to take shape, eventually becoming a widespread line, being responsible for many of the tornadoes throughout the remainder of the day. One exceptionally long-lived supercell thunderstorm was responsible for many of the tornadoes on March 12. This storm began in the morning over northern Oklahoma before embarking on a nearly 800 miles (1,300 km) journey northeast across southeastern Kansas, Missouri during the afternoon and early evening, into Illinois during the mid-to-late evening, through Indiana by the late evening, and into lower Michigan where it lost severe characteristics more than 17.5 hours after inception. This supercell lasted the longest and travelled the furthest than any other in history. The city of Springfield, Illinois saw two strong tornadoes track directly through the city from this storm. It also did damage near Sedalia, Columbia, and Mexico, Missouri. The deadliest tornado from this outbreak sequence came from another supercell; an F3 which killed 4 people near the town of Renick, Missouri. The strongest tornado, spawned by this same supercell, was a rare double tornado rated F4, but fortunately it remained over mainly rural countryside. Multiple other tornadoes came from this intense line, including multiple strong to intense tornadoes in the western region of the main risk area, in Oklahoma and Arkansas, where F3 tornadoes struck the towns of Twin Oaks, Oklahoma, and areas southwest of Bentonville, Arkansas. The storms progressed into the overnight hours and into early morning on March 13, before the main of storms finally came to an end.

A moderate risk for severe weather was issued by the SPC on March 13, as a severe damaging wind event was expected to materialize in the Ohio Valley area extending into the Great Lakes region. A 45% hatched risk for damaging wind gusts was issued for this whole area, with an accompanying 10% unhatched risk for tornadoes.[14] An slight risk for severe weather, alongside a 5% chance for tornadoes extended into Mississippi, Alabama, and Tennessee. As the day progressed, the expected wind event did not materialize, though some weak tornadoes did occur in the southern regions of the risk area. Fifteen more tornadoes were reported on March 13 in Alabama and Mississippi before the system finally weakened and the severe weather came to an end.[15]

Extremely strong winds were recorded throughout northwestern Illinois and eastern Iowa. A record wind gust of 107 mph (172 km/h) was recorded at the Quad Cities Airport in Moline. The wind measurement was verified with backup equipment from the FAA after the main instrument failed.

The winds, which were equivalent to a strong Category 2 hurricane, caused severe damage to numerous houses, along with countless trees and power lines. The damage was reported to have been caused by a microburst.

A microburst containing winds between 85 and 100 mph (137 and 161 km/h) was reported in Bridgeview, a south suburb of Chicago. The microburst hit the area just after midnight on March 13, and may have been part of a supercell to hit the southern suburbs just before the thunderstorm complex exited the area at 3:00 a.m. CST. Roofs were ripped off apartments along a three block length, and seven garages were damaged or destroyed, according to the Chicago Tribune'.[163] Harlem Avenue in Bridgeview (Illinois Route 43) in the vicinity of 77th Street was closed for four hours while debris was cleared from the road.

Flash flooding closed U.S. Route 45 around U.S. Route 30, but major flooding was not reported on any of the area highways during the rush hour. 15,000 customers were left without power as the thunderstorms passed. The number had been reduced to 2,000 by daybreak, but strong winds knocked a total of 17,000 customers offline as of midday Monday.[164]

Strong winds also forced the closure of parts of downtown Evanston when building material blew off a 29-story building, slightly injuring a construction worker. Four more construction workers were injured, one seriously, in Antioch when a roof being constructed for a new church collapsed due to the wind. A Wind Advisory was issued by the National Weather Service, indicating sustained winds of at least 30 mph (48 km/h) and gusts of up to 50 mph (80 km/h).

Both Ford County, Illinois and Iroquois County, Illinois recorded damage due to microbursts. In Ford County along Illinois Route 9, power poles were snapped and damage to vehicles was recorded and attributed to a microburst which had winds between 85 and 100 mph (137 and 161 km/h). In Iroquois County, most of a cattle building was blown into a field just west of Illinois Route 1 near Milford. This microburst had reported winds of 90 mph (140 km/h).

Damage around Crescent City and Interstate 57 in Iroquois County was likely caused by straight-line winds. [165]