FLHurricane.com: WHAZZAT?

When you look at the visible satellite and see a semi-circular line of lower clouds spreading out from thunderstorms, what is that??? It seems like it would be clouds propelled by a downburst situation, but not low clouds, probably mid-level judging from the apparent temperature. Would it look like a roll cloud from the ground? There are a few of them out there right now... they move pretty rapidly out from the center of convection and sometimes persist quite far away from it so then it wouldn't look like part of the storm. And they don't appear to be affected by prevailing low or mid-level winds...
Just curious,
TIA
ADDS satellite TPA :
[url=http://adds.aviationweather.gov/satellite/displaySat.php?region=TPA&isingle=single&itype=vis]

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