NOAA's National Weather Service Weather Forecast Office

Isolated thunderstorms that initially extended from Troy to Springfield, Missouri formed rapidly into a nearly solid line of severe thunderstorms before noon on October 24, 2001.  The severe storms formed along a progressive strong cold front which extended from northern Illinois to northern Oklahoma. Unseasonably warm, moist, and unstable air from the south colliding with the colder air from the north, and strong winds aloft, combined to create a favorable setting for severe weather.  One storm which developed near Troy, Missouri at 10:30 am CDT (40 miles northwest of St. Louis - Lincoln county storm) took on the shape of a supercell (see reflectivity images below).  It produced hail and damaging winds over parts of Greene and Macoupin counties in southwest Illinois before exiting the St. Louis county warning area. Strong rotation (a mesocyclone) near the pendant or hook-shaped echo are frequently observed with supercell storms. Usually the largest hail and damaging winds occur in the southern part of the supercell, while tornadoes may occur in the vicinity of the hook. The Lincoln county storm had a very long lifespan, and was traced over three hours, spawning tornadoes in east-central Illinois (northwest of Champaign Illinois) and into northwest sections of Indiana. Some supercells evolving in environments like October 24, 2001 will frequently have extended lifespans covering over 200 mile distances. Other storms along the line became supercells as they moved into the St. Louis metro area after 1130 am cdt. Many of these storms were responsible for producing large hail and damaging winds across the metro area, including the damage at Portage Des Sioux.

After 2:15 pm CDT, a severe storm took on the shape of a bow echo over Perry county Missouri and traveled at speeds of 60 mph across southern and eastern Randolph County in Illinois. A mesocyclone, showing strong rotation, was detected by the WFO St. Louis Doppler radar along the forward - northern side of the bowing segment. Bow echoes are indicators of damaging winds and sometimes tornadoes form along the northern - forward side of the bow. The mesocyclone with this bow echo spawned a weak tornado (F0 - F1) along the southern sections of Percy, Illinois after 2:30 pm CDT.

Over 55 reports of damaging winds and large hail were received at the NWS office in St. Louis on October 24th, making it one of the busiest severe weather days of 2001 across our area.

The Local Storm Report

Damage Survey (text)

A map of the tornado track

Some pictures from our storm survey

At 11:45 am CDT, a line of severe thunderstorms moves across the Bi-State area.  Notice the severe thunderstorm (Lincoln County Supercell) just northwest of Carlinville, Illinois.  This supercell developed as it crossed the Mississippi River into Illinois.  It stayed intact all the way into northwest Indiana, causing extensive damage along the way.  The Franklin County Supercell produced large hail and damaging winds over Franklin and St. Louis County. 

At 12:09 pm CDT, three supercells were identified across the Bi-State region.  The St. Charles County Supercell was responsible for the damage at Portage Des Sioux.  Hail and damaging winds continued to be reported with the Franklin County Supercell as it entered the far western sections of St. Louis County at this time.  A new storm, exhibiting supercell characteristics is observed over northern Crawford County.

At 12:30 pm CDT, the three supercells continued to remain intact as they moved to the northeast at speeds of 55 to 60 mph.  The three storms did not spawn any tornadoes over the area.  

At 2:27 CDT, a bow echo embedded within the larger squall line moved east-northeast at 60 mph across southern Randolph County in Illinois and northern Perry County, Missouri.  

A closer view of the bow echo at 2:27 pm CDT, shows a spearhead reflectivity pattern along the leading edge suggesting accelerated storm outflow.  The corresponding storm relative velocity pattern shows a strong tornadic circulation (bright green 100 knots inbound velocities) adjacent to a region of darker red (27 kts outbound).  This circulation, referred to as a tornadocyclone and located at the top of the spearhead echo pattern, was responsible for spawning the Percy, Il tornado