The trend late Sunday into early Monday has continued to have Isaac an increasing threat to the central Gulf Coast and especially Louisiana. Some of the tools used to forecast the track are called ensemble forecasts.
Rather than use one "deterministic" simulation of the atmosphere, ensemble forecasts use the uncertainty we know exists in forecasting and then generate a variety of solutions to find an average or more confident solution. Ensembles also generate a spread or show us the likely error in relying on any one deterministic physical simulation of the future weather. Here is a link for the latest ensemble position of Isaac.
Here is the latest forecast of confidence from the National Hurricane Center. Forecasting the track of tropical storms has become more accurate over the years but forecasting rapid changes in intensity remains difficult. The "fan" or forecast envelope is really a 66% probability that the path will be within that range, so no one should think of the center or the center track being more likely than a path near the edge of the envelope.
The trend is to keep the path of Isaac moving toward the west in the Gulf of Mexico but over 85 degree water. If this continues, the treat to landfall in the Florida decreases, but then increases for southern Mississippi to Louisiana and New Orleans.
That is the reason authorities have already begun to take action along the central Gulf. There is NO indication right now of any significant impact from what is left of Isaac on the D.C. area. We will sure keep you posted as to local impacts if any throughout this week