Something I've noticed over the past 2 days of runs, the low-level axis has been tilted NE to SW currently, and we're left with two areas where air flow becomes divergent (or a NE lobe and a SW lobe attached to the wave axis).
In the short term, we'll have to see if the SW lobe of the wave becomes the dominant feature. You can see currently there's a bit of a boxing match going on at the low levels between these two lobes:
As the axis approaches the Caribbean, if one of the lobes does not become the dominant feature, you'll notice the axis (blue) becomes more N to S oriented (natural process as these tropical waves enter the Caribbean due to low-level flow). Low-level easterly flow is cutting straight through the eastern Caribbean (red) which reduces the chance of the southern lobe developing. This easterly flow is also on the northern axis of the wave, but due to the proximity to the upper-level trough, the wind direction change from lower to upper levels is favoring development on the northern side of the axis now.
Once we establish genesis, intensity becomes important. Levi does a great job of explaining this on his video (
https://www.tropicaltidbits.com/), but essentially you can see the trough axis (yellow gradient) directly to the NW:
Through diabatic outflow, a tropical system can actually impinge on this trough axis, and essentially move it out of the way (or in some cases, even create ideal upper-level outflow, as we saw with Dorian last year). Previous runs that showed a more developed system were essentially strengthening here due to the storm having a protective bubble and impressive outflow. If you remove those conditions, a weaker tropical system will be sheared by the trough axis (as some model solutions now show).
