Tuesday, September 4, 2018

Gordon to Generate Storm Surge Flooding along Northern Gulf Coast

Tropical Storm Gordon continues to track towards the Northern Gulf Coast....as of 0700 Eastern Time this morning, maximum sustained winds were 65 mph and movement was west-northwest at 15 mph. This means overnight Gordon strengthened and the forward speed slightly decreased. Gordon's forward speed is still faster than average for this region, but is forecast to continue decreasing through landfall.

GOES EAST Infrared Satellite Imagery shows that Gordon's convection has increased overnight. Darker colors indicate colder temperatures and higher cloud tops.

Gordon is a relatively small tropical cyclone, with tropical storm force winds extending 80 miles from the center of circulation. The National Hurricane Center's surface wind field map, depicted below, shows the extent of Gordon's tropical storm force winds (shaded orange) - showing us that these winds cover a relatively small area. Small storm size and relatively fast forward speed will reduce the extent of storm surge flooding. 

The National Hurricane Center forecasts storm surge inundation potential of 3-5 feet above normally dry ground from Shell Beach, Louisiana, through Dauphin Island, Alabama. This includes the entire Mississippi Coast. To the east, from Dauphin Island through Navarre, Florida, including Pensacola, storm surge potential is 2-4 feet. The same level is forecast from Shell Beach through the mouth of the Mississippi River.

Given Gordon's small size and forecast track, I would be surprised to see storm surge levels exceeding 3 feet east of the Florida/ Alabama line, and I expect any coastal flooding in the western Florida Panhandle, including Pensacola, to be minor.

Storm surge inundation could reach 3-5 feet from Shell Beach, Louisiana through Dauphin Island, Alabama, including the entire Mississippi Coast. 

While Gordon should not generate an extensive storm surge, in terms of geographic extent, it is forecast to become a hurricane, with the center of circulation approaching the Mississippi Coast, one of the most efficient areas in the world for storm surge generation. Coastal residents should take caution- storm surge can build very rapidly in this region- particularly in Southeast Louisiana, like near Shell Beach, and the entire Mississippi Coast. Where hurricane-force winds are blowing on shore, storm surge levels can increase rapidly.

A word of caution about Southeast Louisiana, from near the Rigolets to mouth of the Mississippi River....when hurricanes strike the Northern Gulf Coast, we generally think of onshore wind direction as blowing from south to north, however, in extreme southeast Louisiana, the coastline shape throws a wrench in this generalization. Where the Mississippi River Delta juts out into the Gulf of Mexico, strong northeast or even north winds are actually blowing "onshore" and displacing water from Mississippi Sound to wetland areas and small towns along the Mississippi River levee.

As of 0700AM Eastern Time on Tue Sep 04, storm surge levels have not exceeded 2 feet at any coastal location along the U.S. Gulf Coast. Visit the U-Surge Project for updates.

As Tropical Storm Ida made its closest approach to Mississippi Sound in November 2009, water levels at Shell Beach, Louisiana, actually peaked as winds turned from the north. The coastal geography in this area exposes Shell Beach to a north wind, pushing water into town.

So friends in Southeast Louisiana- don't let your guard down if Gordon's center of circulation passes east of you....you can still generate quick storm surges on the "left" side of the storm.

Last night I looked into storm surge history for Lake Pontchartrain and I'm not concerned about substantial storm surge entering the Lake. We would need a slower moving storm centered south of New Orleans to push substantial water into the Lake, and Gordon's forecast track, small size and rapid forward speed will all serve to keep storm surge levels relatively low in this area.

As of 0700AM Eastern Time, no coastal locations were observing a storm surge exceeding 2 feet along the U.S. Gulf Coast. I will keep this map updated on the U-Surge website as Gordon approaches the coast.

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