Having grown up on the Gulf Coast, yellow clouds are always BAD! I've always seen them with "black streamers" clouds that are low, fast, and angry looking. They happen before sunset and usually indicated that you will have two or more thunderstorms back-to-back heading from west to east. You also will get three or more sets of lightning in those situations. One round of lightning at the head of the storm, one round in much later, and the final round as the storm passes.
I have seen yellow clouds a couple of times in the AM, but it is rare. When I've seen them in the AM the end result was and hour for more of very heavy rain (3 to 4 in per hour) with little wind. The AM storms in the Gulf are spawned by the warm water and generally have less wind, but a lot of rain. They drift from south to north from 3AM until about 8 or 9 AM.
I've heard descriptions of yellow and green clouds in the midwest before tornadoes but have never seen them myself.
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