sneaky giant cloud

In the back yard it was still and cold and dark like before a snow. I went to the front and saw this:


There's a small local dark grey cloud over the mountains, and perfectly blue sky to the east, but the WHOLE rest of the sky, in every direction and way, way behind me is a high, thick solid snow cloud, slowly moving from the west. Kinda creepy, like when a giant spaceship obliterates the sky. To understand the effect, note that the transmission towers on the mountain, a mile up, are still clearly visible. The cloud isn't right on us, it's way up there, higher than that local cloud which is also really high.