I would use the following equation in a sentence immediately preceding the equation. I would use the equation below if there were more text in between.
I would tend to avoid these formulation to be frank, I would re-organise the text to avoid forward references to an equation in general, unless I speak about a celebrated equation. For example The law or large numbers below has first appeared in... , Einstein's equations below describe.... Compare with The following equation is the strong law of large numbers: [Displayed Eq.].
If I refer to an equation above, I number it and refer to the number: In Eq. (xx), $a$ is the acceleration, ...