re: Help me understand this Storm Surge map

Surge really doesn't travel upriver farther than roughly Nola...BUT...the river does rise in places as far upriver as BR because when the Gulf is elevated due to surge, the water coming from upriver has nowhere to go and piles up on itself. Carrollton gage has risen by ~14' in 24 hours during a storm.

-nerd alert-
Secondly, none of these maps reflect a single storm on a single track. They all employ the same basic idea: there is a relatively small historical record of hurricanes. So, a large amount of "synthetic" storms that are representative of the range of possible intensities and tracks are run
In computers. Then statistical analysis with fancy names such as Monte Carlo Analysis is performed where the distribution of affects across all storms at all locations is binned into what we would all call a 100 year storm, etc.

Another way to describe all that noise is this: you have a pair of dice. The historical record of dice rolls is only comprised of a few rolls, therefore, you really can't compute an accurate probability of roll results with that record. You then build a computer model to simulate hundreds and hundreds of rolls to build a synthetic set of results in a short time so you can then accurately assign probabilities to roll results.

Any one pixel on these maps represents a given probability of a certain set of results over a large sample of simulated events.
-end nerd alert-