Around 4:30 pm on Tuesday afternoon. I watched this thunderstorm pass by to the northeast of Watershed Farm. There was enough wind shear to encourage supercells to develop although the stronger dynamics were over the Ottawa Valley. In fact a tornado did occur in Gatineau just across the Ontario Quebec border.
This particular cumulonimbus was still developing but there was evidence that there was already a separation of the up and down drafts. The forward flank downdraft and associated rain was producing a shallow arch single rainbow to the right. The shallow angle the rainbow made with the horizon revealed that sunset was still hours away - almost 4 hours in fact. There looked to be a bit of a shelf cloud toward the middle of the cloud base and I'm not sure if this was with the forward flank or rear flank downdraft. From the orientation of the shelf fingers, this shelf cloud is more likely to originate from the forward flank downdraft. The flanking line of the CB on the left edge of the painting is short and steep. This storm had a lot of development left to do.
This storm was destined to rumble along the Lake Ontario lake breeze. Apparently damage was reported near Peterborough and this could have been the cumulonimbus responsible.
Although there is no secondary rainbow in this view, the sky between the primary and secondary bows is noticeably darker than elsewhere. Alexander of Aphrodisias first described the effect in 200 AD and it now carries his name. Light rays undergoing a single reflection in raindrops form the primary rainbow or brighten the sky inside it. Rays reflected twice are deviated to form the secondary bow or brighten the sky outside. Raindrops along lines of sight between the two bows cannot send light to your eye and so the sky is darker there - a good explanation.
As I was painting this, I was thinking of Lawrence Nickle's reference to the Group of Seven's treatment of clouds as "boulders in the sky". This cumulonimbus was a boulder :>)