Good morning! It's dawn on Labor Day, and that means it's time for Hurricane Gustav to come ashore in the Gulf Coast.

Thankfully, the storm's course has kept it cruising along at a pretty good rate of speed, and on a trajectory which keeps the bulk of the bad weather to the southwest of New Orleans. The other good news is it's "only" a Category 3 storm, thanks to it drifting over colder sections of the Gulf (unlike Katrina). This map shows this well.

I've been following the tidal gauges displayed online at NOAA's "Tides and Currents" website, where the tidal gauging stations noted in the map above share their data with you, the web browsing citizen. Consider this graph, from Cedar Key, Florida:

The blue sine wave is the predicted tidal fluctuations based on the position of the moon and sun (tides are caused by gravitational tugs from those massive bodies). But superimposed upon that trend over the past fifteen hours is something making the sea a bit higher than it would otherwise be predicted to be: the red line shows us this observed water level. The green line shows the difference in water level between what's predicted and what's observed. What's causing the extra height to local sea level? This is storm surge, where the howling force of the hurricane's winds push the sea up in front of them. Because of the way hurricanes spiral in the northern hemisphere (counterclockwise, due to the Coriolis effect), these wind-induced storm surges are greatest in the area in front of the storm's path and to the right: the "passenger seat" position as it moves ashore. This is why storm surges were so great in Mississippi during Hurricane Katrina, and New Orleans was spared the worst of it (the eye of the storm passed east of the city then).

Today, the eye of the storm will pass west of the city, which really worried me when I first read the projections, because that put New Orleans in the path of the highest storm surge. The next six hours will be critical on this front. Here's the data from Pilots Station East, Louisiana, which is out on the Mississippi River's Bird's Foot Delta, in prime storm surge position for Gustav's path:

There isn't a lot of tidal variation in this location (low amplitude to the blue line), but you can see that the local water level is leaping upwards, 4 feet above normal at the time I'm writing this. How high will it eventually go? During Katrina, the highest storm surges were somewhere around 27 feet above normal (in Mississippi, not New Orleans). I think this graphic will continue to track the changes over the day, and it may be more updated than my text by the time you read this.

Good luck to everyone on the Gulf Coast. Stay dry.

Labels: hurricanes, news