First, let me address one item before we dive into the meteorology of Isaac. Many are saying that the media is over-hyping Isaac. To this I say, ABSOLUTELY NOT.
My point is that rainfall and subsequent flooding from ANY TROPICAL SYSTEM regardless of the wind speed can be deadly. We can never underestimate or under report any tropical storm! Yes, Isaac is no Katrina. The characteristics of Isaac are far different than Katrina. The direction of moving is different. By Thursday, the flooding will no doubt be as destructive for many of the same areas hit by Katrina exactly 7 years ago.
Now to the weather. Why has Isaac slowed down?
The high pressure ridge north of the gulf coast centered over the mid west and the Great Lakes is pushing south and east deflecting the movement of Isaac west. The steering currents are not favorable for rapid movement inland. So Isaac continues a Northwest crawl at 6 mph.
Wind direction around the storm and into the shoreline is from the southeast and east. A report of 110 mph gusts Bayou Gauche from St. Charles Parish EOC. This direction funnels gulf moisture right into New Orleans and surrounding areas making the flooding threat greater even with the lack of Katrina-like winds.
A close up of the wind field around Isaac really tells the story on how the southeasterly winds pull gulf moisture right onto the shoreline.
Here is a great map showing the rainfall as of 8AM Wednesday which is approaching 10 inches.
New Orleans more than any city in the US is the most susceptible to flooding because it sits below sea level. Here are a few streets elevation compared to how they sit relative to the sea. The map is not to scale.