Isaac is still forecast to push on-shore near New Orleans as a Cat. 1 hurricane. The movement has further slowed to a northwest motion at 7 mph. Tremendous rainfall is still expected near, and to the immediate right of its path.
For the past few days, HPC has been forecasting a secondary northeast spoke of heavy rain. Models did not support this fully, and now HPC has pulled back rainfall amounts over most of Alabama and Georgia. Threshold rainfall amounts (amount estimated to start the more significant runoff) is near 4 inches in most spots. Now, it is not likely that this will be reached.
Here is a short term model showing what the radar might look like late today. It shows how slow the movement of Isaac is. Rainfall within 50 miles of the eye will be extremely heavy.
This model does show outer bands associated with Isaac. It is indicating an inflow from the Atlantic, across parts of the Carolinas, and then into Georgia and Alabama. These bands, if the set up over one area for a while, can produce more locally heavy rainfall.
Model showing rainfall as of 8 pm Tuesday
Here is the updated rainfall forecast, The spike of heavier rain over South Carolina and Georgia represents the Atlantic inflow. The track of heaviest rain with Isaac (mainly the red-shaded region) impacts Northwest Florida, Southwest Alabama, South Mississippi, and then northward. I have increased confidence that this will keep the heavier rainfall out of either Alabama or Georgia Power areas.
Here are a couple of MMEFS runs. The first is from North Georgia, showing inflows into Lake Lanier. The vast majority of model runs indicate little, if any, increase in inflows into the lake. This is representative of most of North Georgia.
Here is a MMEFS run for the Coosa River in Alabama. It shows precipitation traces. There will be periods of showers and storms for the next several days, but note that very few of these produce heavy rainfall.