These are very weak, high based showers without thunder, but with microbursts. Studies have shown that they predominantly occur in the High Plains and western U.S.: particularly in unstable, very dry low level environments with surface temperature-dew point spreads of 30 to 50 degrees and an area of mid-level moisture as a source for the weak showers.
Photograph by: Moller
The cloud on the left is developing, whereas the fuzzy anvil on the right has matured and is producing a trail of virga. Microbursts would be most likely to occur beneath the virga, when the downdraft reaches the ground. Several of these virga showers did produce microbursts in the Lubbock, Texas area.