Tuesday, August 28, 2012

A closer look at Georges (1998)

Ezra Boyd, a good friend and faithful follower of the storm surge blog, posted the following question just a few minutes ago:

The current forecast has a trajectory just to the west of Georges. Do you have any thoughts on how that impacts surge heights?

This is a great question, Ezra!

I spent some time today creating maps that depict the hourly position/ intensity of Georges, as well as observed storm surge and storm tide levels. I've pasted two versions of this map below- a full Gulf map and a zoomed in version.

 Zoomed out map of Hurricane Georges (1998)

Zoomed in map of Hurricane Georges (1998)

Some notes: Hurricane Georges tracked across the Gulf of Mexico as a category-2 hurricane, before making landfall on the Mississippi Coast. The peak observed storm surge was 9.6 feet at Pascagoula, MS, while storm tides of 12.14 feet were observed on the MS Coast and 10.83 feet on the AL Coast. High water levels in LA generally were 4-6 feet in Lake Pontchartrain, 6-8 feet on east side of New Orleans. The peak surge level in LA was 8.9 feet at NE Bardene Bay, which is 13 miles ESE of Pointe a la Hache.

A few substantial differences are seen with the current storm, as Isaac is tracking farther west, and, perhaps more importantly, Isaac is less intense. Although the NHC is still predicting that Isaac attains hurricane strength, it is still a tropical storm as it approaches the coast. This is important because research by Jordan and Clayson (2008) and Needham and Keim (in press), has indicated that pre-landfall winds correlate better with surge heights than wind speeds at landfall. So this means that a storm like Georges, which traverses the Gulf of Mexico as a category-2 storm will often be able to push more water than a storm which develops into hurricane strength (even if attaining cat-2 status) just before landfall. However, we must always be careful, because other factors, like storm size, also play into the storm surge equation.

That said, I wanted to include these graphics of Hurricane Georges for comparison sake, and also so we can see historic surge heights relative to each other. It is interesting to see some distinct patterns on the Georges map, such as a 4-6 foot surge range consistent along Lake Pontchartrain. Not exactly sure how the more westward track affects surge heights, but one may presume, all things being equal, that this would serve to increase heights in LA, and perhaps lower heights in AL and FL.

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