@sledger good question, and I don't have an authoritative answer...

@sledger
good question, and I don't have an authoritative answer to it. I find it quite normal that after a lengthy consolidation in a range-bound price interval, volume will drop off when both sides hope "the other will blink first". I can only guess which side that will be until a break does get underway. Once that finally happens, I take a closer look at the volume. If it happens still on small volume, the first move will often turn out a "Head Fake" (see Bollinger Bands) and a bigger move on large volume jumps in the opposite direction.
Look at the precedent, albeit on a smaller scale, last November: Large volume supported the breakout, then both price and volume petered out into consolidation. Over Christmas, it moved nowhere fast, and suddenly broke out on Jan 5th with high volume. Now, I consider us to be back at the equivalent to mid-end December.

I know some disciples of VSA (Volume Spread Analysis) have some more specific and in-depth ideas. Maybe one Wyckhoff student would like to expand on it.
I am happy where we're at right now and intend to trade as per above. Most importantly: Have a Plan B