https://phys.org/news/2018-09-nasa-noaa-satellite-tropical-storm-isaac.html

In general, wind shear is a measure of how the speed and direction of winds change with altitude. In order to understand how it affects a tropical cyclone or hurricane, think of a tropical cyclone as a series of vertically stacked tires, all rotating. As you go up from the ground, each tire represents the rotation of the storm's center at a higher level in the atmosphere. The different levels of rotating winds in the center of tropical cyclones need to be stacked on top each other to strengthen. If there are winds higher up that push some of the tires askew near the top, it affects the balance and rotation of the tires below. That's what happens when vertical wind shear pushes against a storm. It pushes the center and weakens (or wobbles) the rotation of all of the tires.

The National Hurricane Center or NHC noted "Satellite imagery indicate that Isaac remains a poorly organized tropical cyclone, with the low-level center mostly exposed during the early morning hours." Isaac is being battered by strong vertical wind shear.