This is a very cool effect!!! The mechanics of what's happening is really kind of neat IMO.
So, as a rule storms suck up - I like to think of it as "breathing in" - warm and moisture-laden air which is pulled high up into the sky, and they then "exhale" much much cooler air that isn't really that much less humid. This particular storm is starting to become outflow-dominant - a big gush of the cooler air has fallen out of the storm and hit the ground, and is pushing out in front of the storm. As it pushes out, it displaces the still warm-air on the ground right in front of it, making that warm air rise very quickly and the moisture in that warmer air also starts to condense very quickly because of the interaction with the cold air. So you get these rising bright streaks of newly-forming cloud, that basically outline the front "surface" of the bubble of cool air that's pushing forward behind them.