This is happening right now. The blue sky that appears gets quickly
gobbled up. These thin rainbowy clouds GROW and the different
sections connect, soon forming (right before my eyes) extremely large
billowy masses. The different sections seem to be attracted to each
other and move towards each other to close gaps. then the hazy gray
appears over it all and it moves over the sun, blocking it. The whole sky
soon is covered with these masses, the sun isn't allowed to reveal
itself for long. The masses are moved, i've seen one move west & 1 move
east at the same time. They will move east and veer north to cover sun.
Sometimes a thin layer will scoot along so fast! while the rest of the clouds
stay still. It's so scary how fast they can do this, without the appearance of
a plane (i did see a tiny white one scoot between clouds tho) and moving
purposefully the way they do. I don't know the techniques they use, but it
is amazing how quickly these rainbowy and white tendrils grow and cover.