In this test I added a ground plane so I know where to place the two fluids with the ground. Then I widened the ground dust to match the storm better. However, it still looks like the dust from the storm is getting underneath the ground dust.
In this test I moved the ground dust down so that it is flat on the ground plane and not hovering above it. It still appears that the dust from the storm is somehow getting underneath the ground dust.
In this test I significantly widened the ground dust so that the storm will not pass it. Right now it looks like there is too much ground dust, but I think that it might look different when there are objects in the scene that will block line of sight to the edges of the fluids.