Probably. Environment Canada will make the official determination, but the radar evidence from last night suggests a small strong F0 and perhaps a weak F1 one was quite likely. A compact but intense cell crossed into Southwestern Ontario east of Sarnia about 7PM in the evening July 23, 2011.

We watched the cell develop strong verticality over the next couple of hours with high hail densities evident high in the updraft. We noted an MDA event (“mesocyclone detection algorithm” – an automated detection of updraft rotation in a thunderstorm, converting it from an ordinary thunderstorm to a supercell) over this cell about an hour after it crossed the lake shore. The reflectivity radar trace indicated a large upper hail core.

The overall presentation of the cell didn’t suggest a classic tornadic mesocyclonic cell. However, when we looked at the storm relative velocity profile on the lowest tilt we found the couplet (yellow arrow) shown in fig. 1. The couplet didn’t show on further upward tilts, indicating some degree of rotation was occurring near or at the base of the cell. Since the distance between KDTX and the location of the couplet is about 80 miles, the SRV couplet indicated a velocity disturbance around 5000 feet. Since the beam is narrow, and above the ground, it cannot determine how low the disturbance went.

Figure 1. Storm Relative Velocity couplet, July 23, 2011, Lambton Co.

A velocity couplet in a storm relative velocity profile indicates a rapid change in the direction component of the velocity vector (velocity, by definition, is the combination of speed and its direction context). Storm relative velocity profiles measure changes within a wind profile, and therefore will show when there is a significant change in direction of wind, like occurs in a tight rotating air column).

Though the scale is not shown in fig. 1, the wind shift in the couplet can be measured. The light green portion of the couplet measures to -44 knots, to +15 knots for the immediately adjacent red bin. This translates to a wind shift (rotation) occuring at approximately 120 km/h. This measured amount puts the event at the high end of the F0 scale (64-116 km/h and the low end of the F1(117-180 km/h) range. The couplet doesn’t indicate whether or not any rotation actually reached the ground. Damage evaluation will confirm further if the duration was enough to consider a full fledged low power tornado. The SRV couplet survived on the radar scan for more than one cycle, indicating the event would possibly have been in play for several minutes.