About 5 pm on 21 July, 2015, I went to my front steps to watch a thunderstorm roll in. The storm was moving west to east, and there were two layers of cloud preceding the actual rain. Clouds in both layers were turbulent and broken. Lower layer was thin and covered about 50%. Upper layer was heavy cumulus clouds, but not so thick as to be really dark. Both layers seemed to be moving at about the same speed, meaning that the lower layer was traversing my field of view more quickly than the upper layer. A couple of minutes before the rain hit, I saw a cluster of about 8 dark spots moving in the same direction as the storm, but faster. They were at about 55 degrees up from the horizon, and in the southwest sky. My first thought was that it was a small flock of blackbirds outrunning the storm. As my attention was drawn to the cluster, I noted that the dark spots were blurry, and that there was no flapping of wings, or relative motion between the spots. They moved in unison from west to east as if they were one fixed object, always maintaining the same appearance, the same speed and same direction. They then passed behind some of the lower clouds. I got the impression that they were just at the height of the bottom of the upper clouds. This impression was probably due to their blurriness, which I surmised to have been caused by being barely within that cloud layer, and slightly obscured by thin cloud. However, when the cluster was not behind the lower cloud layer, its appearance was always the same illumination and blurriness. This leads me to believe that it was not just within the upper clouds, as that would surely have caused a variability of appearance. The cluster was traversing my field of view about 3 or 4 times faster than the lower cloud layer. That would have put them at 6 to 10 times faster than the storm speed. My guess is that they were at least 500 feet up, but no higher than 3000 ft. Based on the height estimate, the cluster diameter would be 25 ft to 100 ft. There was no noise, and no apparent light emission. There did not appear to be any geometric pattern to the dark spots. When the cluster passed closest to me, directly to the south, it appeared about 65 degrees above the horizon. I lost sight of the cluster after about 25 seconds, when it was in the southeast at about 50 degrees elevation from the horizon. It became obscured behind clouds in the lower layer. Based upon my estimates of 3000 ft elevation and 25 seconds, I would estimate its speed at 80 miles per hour. The grey color that was between and around the dark spots seemed to always match the sky color, supporting the premise that it was a cluster of separate objects. However the constant appearance, speed and direction of the cluster gave it the feeling of one unified object, which might have been cloaked somehow.I had no emotional feelings during or after the sighting. The cloudburst shortly afterward was not extremely severe, but it was a good heavy rain for about 10 minutes. What struck me as most unusual was that while conventional aircraft avoid storm clouds, this craft seemed to be within a storm system by choice. The storms that day were patchy summer smaller storms that would have been easy to avoid. Also, it would have been much safer to fly at lower altitude and avoid the possible turbulence at the lower edge of the heavy cloud. The speed of the cluster was apparently sufficient to be able to avoid being within the storm system. If the object had been on the same trajectory before I spotted it, it would have been within or very near to the rain squall.I did not get a photo, as I spent the entire time trying to characterize what I was seeing. Its appearance was pretty subtle, so it might not have shown up very well in a photo anyhow.