weather, weather safety/preparedness

Nate Targeting Louisiana-Northwest Florida Coast (H. Michael Mogil, CCM, CBM, NWA-DS*)

Hurricane Nate is nearing peak intensity (strong Category 1 hurricane with sustained winds of 90 miles per hour) as it races toward landfall on the Mississippi-Alabama coast tonight (Fig. 1). However, with the strongest winds and the greatest push of water toward the coast on its eastern flank, places from extreme eastern Louisiana to northwest Florida stand to take the greatest beating.

At 4:00 p.m. C.D.T., pm, the National Hurricane Center (NHC) had posted a hurricane warning from Grand Isle Louisiana to the Alabama/Florida border, Metropolitan New Orleans, and parts of the Lake Pontchartrain shoreline. NHC has also posted storm surge warnings from Grand Isle, Louisiana to the Okaloosa/Walton County line in Florida (Fig. 2).

NHC expects maximum flooding of seven to 11 feet above ground level is expected in portions of southeastern Louisiana and along the Mississippi coast. A similar surge (or greater) could affect parts of Mobile Bay and the rivers that feed the Bay from the north, as well (Fig. 3). Note that surge values are for locations at or near the interface of land and water. The surge can extend across inland locations, but its depth will be tempered by how much the elevation rises. As a result, there may be significant variations of storm surge depth across small distances.

While sustained winds are expected to remain at category 1 levels as the storm makes landfall, it is the push of water onto the land, the dreaded storm surge, that is, perhaps, the most ominous part of the forecast.

Several factors are at play this evening and overnight.

First, the storm is expected to push water across coastal areas from extreme eastern Louisiana to the Florida panhandle. However, Mobile Bay, with its relatively wide Gulf opening (even with barrier islands and peninsulas offering some protection), narrows to a small region just east of Mobile, where several low-lying rivers occupy a broad flood plain. This region lies in the path of the expected surge.

Meteorologists and other scientists know that when a fluid is constrained horizontally, it must rise vertically. This means that a 10-foot coastal surge could grow many feet as it moves northward through Mobile Bay, before it reaches the city of Mobile and the rivers to the north and east of Mobile.

The fast forward speed of the storm will also enhance the push of water onto land.

And, it now appears that the storm will be making landfall close to the time of high tide (shortly after midnight C.D.T.).

Add heavy rainfall, a large area of tropical force winds well inland, and the risk for tornadoes (mainly on the eastern side of the storm’s path) and a large swath of the southeast and middle Atlantic can experience stormy conditions during the next 48 hours.

There are numerous web sites, TV station meteorologists and NWS forecasters that have been sharing (and are continuing to share) needed weather safety information in the face of this tropical onslaught. If anyone reading this article is located in high or moderate risk areas associated with this storm, your window of opportunity to take action is rapidly narrowing. Get to a safe place (above expected storm surge levels) quickly.

© 2017 H. Michael Mogil

Originally posted 10/7/17

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