Hi Paul,
I'm impressed. What you say is true with regards to the limits of daytime heating, but because we have a large amount of moisture currently present in the atmosphere, it easily compensates for this loss. If we get even small amounts of sun appearing elsewhere, as is the case in Eastern Ontario, it does also provide assistance to the already buoyant airmass. Notice the cells starting to already emerge, even though we're into the late afternoon hours.
Regardless, with cold fronts, they simply wedge up the warm moist air ahead of it and condense it rapidly into showers and possibly powerful thunderstorms. However, because of the fact that we're now getting into sunset, and therefore losing the sun, it limits thunderstorms from reaching severe criteria, or at least the chances of them reaching that level.

This isn't always the case, though, because severe weather can still emerge during the evening/overnight hours depending on both the strength of the front and the amount of moisture out ahead of it (this is one prime reason behind frequent lightning). Cold fronts, in general, enhance instability because they supply a continuous flow of cool air at mid levels of the atmosphere, so this does generate persistent thunderstorms/thundershowers and convective rain cells to emerge pretty easily. In this case, this is a fairly strong cold front (not exceptionally strong, but strong nevertheless) and it can still generate some decent cells to develop.
As for the forecasts, I know they've been fairly pessimistic, but when we work with thunderstorms, it is often mostly associated with a "chance" or "risk", even if they say straight thunderstorms and what have you. This is because it is often difficult to predict precisely where thunderstorms will hit, as compared to straight showers. Notice that forecasts are often more accurate in the Winter in predicting snowstorms as compared with the unpredictable nature of thunderstorms. For me, predicting Canada's forecast was pretty easy merely because of the movement of the system.
And the cloud cover we've been seeing for most of today is not so much to do with the front itself - it is more of a "pre-frontal" system.
Edit: I'm sort of in an opposition with some of the forecasts mainly because of the sporadic behavior of these cells, and hence my issuing of the
risk of showers and thunderstorms for tonight, as opposed to them being persistent. I'm expecting those winds to become moderate this evening as well at times.
Trav.
