The circle is the bright reflectivity associated with debris from the tornado in St. Louis Co.
This is headed toward I-70 where it intersects with I-170 and the north part of the City of St. Louis along with Bridgeton Terrace, St. Ann, and Lambert Field.
If you know how to read aviation METAR reports +FC = tornado on the ground. At :12 the tornado was just west of the airport and there were gusts to 50 mph at the airport. Debris ball now (8:18) was near Ferguson.
NCAR-RAP Real-Time Weather Data
Output produced by METARs form (23 April 2011 01:17 UTC)
|KSTL 230112Z 29029G42KT 1SM R30R/2800VP6000FT +FC -TSRA BKN020 OVC040CB 22/19 RMK TORNADO B10 W MOV E AO2 PK WND 29042/0112 CONS LTGICCGCC ALQDS TS ALQDS MOV E P0001|