In the early to mid-afternoon, the storms that formed along the boundary were pretty much stationary. But by around 4:00, a cell in Warren County - the first to go SVR warned - began to push east. When this storm merged with another in St. Charles Co. around 5:00, the show was on, and this storm then continued to push ESE and eventually SE all the way to the southern tip of IL.
When the SVR was issued for St. Louis city, I decided to head down I-55 to Collinsville to have a look. In the last few scans before I left, the velocity scan on NEXRAD was showing very strong winds in west and north St. Louis County and pushing into the city, and another cluster of high winds to the SE in IL along the line of Monroe and St. Clair Co. By the time I got under way and had the WX radio or KMOX on in the car, this storm was also SVR warned.
As I approached Collinsville, the shelf, rather high, surged overhead and I could see a lot of CG in the precip area. Just before I exited I-55, outflow wind kicked up a huge cloud of dust from a construction area SW of the I-55/70 and route 157 interchange. I exited there, as planned, and found a spot in a parking lot of one of the restaurants a little up the bluff where I could get a wide view of the storm moving across the American Bottom flood plain in front of me. The storm was totally outflow dominant at this time, but still close to severe level, producing wind of 40-55 mph across St. Clair Co., and a lot of CG lightning. I watched for a while and videotaped until the precip area was very close, then retreated back up I-55 to the Troy exit, where I stopped and watched some more. At both locations, I would estimate the wind gusted around 40 mph and I observed a lot of CG lightning, violent cloud motion, and some blowing dust. Clearly the brunt of the storm was surging to the SE through St. Clair Co., and much of the strongest wind was along the gust front ahead of the precip.
As I returned home, a new cell formed almost overhead and a lot of CG lightning zapped down all around me near the east edge of Edwardsville. Had I stayed put in Troy, I would have been in the core of the new storm, with torrential rain and, from the appearance of it, probably some small hail. Not bad for never going more than 10 mi. from home.
The storm was severe on the MO side, producing winds of up to 65 mph in St. Charles Co. and 50-60 mph wind across west St. Louis Co. and the central and southern part of St. Louis city. Also many reports in these areas of penny to nickel size hail. It weakend slightly in IL, producing 40-55 mph wind and, probably, some small hail. Some areas also received 2-3 inches of rain, according to TV reports. The worst damage was around Wentzville and O'Fallon in St. Charles Co. While watching radar before heading out to intercept the storm, I noticed that in this area, where the merger occurred, the storm briefly took on an HP appearance on reflectivty, and storm-relative velocity indicated some rotation for a couple scans. Hence, it is not surprising that the worst damage was in that area. At the peak, about 40,000 customers lost power - frustrating, I'm sure, to those who had recently gotten it back after doing without for days following last week's storms.