There’s lots of theories coming out including somewhat related to what Lowry said. I have a former coworker working at the SAB branch at NCEP or whatever they are called now. He’s talked to Klotzbach, as well as some internal people.
In addition to the African monsoon trough being too far north, we’ve had lots of capping or stability over the MDR. I know Don in here has been all over that. Indeed the mid levels are above normal for temps and heights. It’s not a massive ridge really, just a wide area of above normal heights. Some have theorized that the warmer MDR SSTs may have a feedback into above normal heights in the mid levels. You do need heights near or just a bit below normal to sustain tropical convection which don’t always have the vertical acceleration that convection in the Plains has for example. Combine that with the lack of big waves or “seeds” for tropical activity and it may offer at least a partial explanation.
The atmosphere is not linear and small effects like that can really throw things off. I think sometimes we as a community forget about that.
I can tell you many tropical experts are dumbfounded, but also taking this as a learning opportunity.