Mobile Bay faces up to 7 feet of storm surge as Isaias heads for the northern Gulf

Seven feet is the top of the National Hurricane Center’s storm surge forecast for Mobile Bay, measured as water depth above ground if the peak surge lands at high tide, and the range of 5 to 7 feet is the same one assigned to the open coast from Ocean Springs, Mississippi, to Indian Pass, Florida. Those numbers come from Advisory 5, issued at 4 p.m. CDT on Wednesday, Oct. 7, and Intermediate Advisory 5A at 7 p.m. carried them forward unchanged.

The National Weather Service office in Mobile expects the surge to peak Friday night and recede by Saturday morning, after Isaias, forecast to be a hurricane by then, makes landfall on the northern Gulf Coast.

Segment-by-Segment Surge Heights in Advisory 5

The advisory’s surge table splits the Gulf shoreline into pieces, and Mobile Bay gets its own line. Along with the bay and the Ocean Springs to Indian Pass coast at 5 to 7 feet above ground, the table lists 3 to 5 feet from the mouth of the Mississippi River to Ocean Springs, 4 to 6 feet from Indian Pass to the Steinhatchee River, 2 to 4 feet from the Steinhatchee River to Yankeetown, and 1 to 3 feet in Lake Pontchartrain. A storm surge watch spans the mouth of the Mississippi to Yankeetown, which means life-threatening inundation is possible somewhere in that stretch within about 48 hours.

Rain adds a separate flood threat. The advisory calls for 3 to 7 inches from far southeastern Louisiana to the Florida Panhandle from Friday through the weekend, with localized totals up to 10 inches.

The wind behind those numbers is still building. Advisory 5 had Isaias at 65 mph, and the discussion by forecaster Blake peaks the storm at 95 knots, or 110 mph, on Friday evening before landfall, with almost all guidance showing a Category 1 or 2 hurricane when it arrives. Surge scales with that intensity, with the storm’s size and with the angle at which it crosses the coast, and Blake wrote that track and timing depend on how well the storm holds together against shear.

Concave Coastlines and Bay Geometry

NOAA’s storm surge overview explains that surge runs higher on concave coastlines, which curve inward, than on convex ones, and that bays, inlets and barrier islands change how water flows. It also defines storm tide as the combination of surge and the astronomical tide, which is why the advisory’s numbers hinge on whether the peak coincides with high tide. The same page calls storm surge the leading cause of hurricane fatalities.

Geometry alone does not settle the outcome, and the three storms below show how far the readings can swing. The direction of the wind relative to the bay decides whether water is pushed in or out, and Mobile Bay has recorded both extremes within a generation.

Seven Feet Above Ground, Not Above a Gauge

The NHC glossary defines inundation as the flooding of normally dry land, and that is the quantity in the table: depth over ground that is dry on an ordinary day, if the peak surge and high tide line up. A tide gauge reports something different, a water level against a datum such as mean higher high water. The distinction matters when forecast numbers are set beside past gauge readings.

The Mobile forecast office applied the 5 to 7 feet of potential inundation to the entire coastline of coastal Alabama and the northwest Florida Panhandle in its 6 p.m. Wednesday update, and said it expected the surge watch to be upgraded to a warning overnight.

Frederic, Katrina and Sally on Mobile Bay

Hurricane Frederic came ashore on Sept. 12, 1979, with its northern eyewall crossing Dauphin Island shortly before 10 p.m. as a Category 4. The Weather Service’s Frederic summary records 12 to 15 feet of surge on the Gulf beaches and 8 to 10 feet in northern Mobile Bay, a central pressure of 943 millibars and $2.3 billion in damage. Katrina, on Aug. 29, 2005, pushed 8 to 12 feet of surge along the northern and western shores of the bay and 10 to 15 feet across Mobile County, including Dauphin Island, according to the National Hurricane Center’s Katrina report.

Sally in 2020 made landfall near Gulf Shores with 110 mph winds, yet the gauge at Dauphin Island peaked at only 3.1 feet above mean higher high water, and most other bay stations stayed between 1 and 2 feet. The NHC’s Sally report puts 5 to 7 feet of inundation on Baldwin County, Alabama, and the Escambia and Santa Rosa county coasts, east of the bay. After the wind turned northerly, the Coast Guard Sector Mobile gauge fell to 7.0 feet below mean lower low water, a drop of about 10.6 feet in roughly 20 hours that exposed parts of the bay floor on Sept. 16.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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