Storm Surge on Mobile Bay, Pensacola Bay and the Mississippi Sound

The northern Gulf coast from Bay St. Louis to Destin is a chain of shallow bays and sounds behind barrier islands: the Mississippi Sound with its back bays at Biloxi, Ocean Springs and Pascagoula; Mobile Bay; Perdido Bay; and the Pensacola Bay system with Escambia, East and Blackwater bays and Santa Rosa Sound. Each one handles surge differently, and the National Hurricane Center's post-storm reports show how.

This guide is the evergreen companion to our Charlotte Harbor surge explainer, written for the landfall zone. Every historical number here comes from the NHC Tropical Cyclone Report linked beside it. If a storm is on the map right now, the live numbers are on our tropics tracker and, for Isaias, on our Pensacola-to-Mobile page.

Surge, storm tide and inundation

NHC's reports define three numbers, and reading the wrong one is how people misjudge a forecast. Storm surge is the rise the storm causes above the predicted tide. Storm tide is surge plus tide, measured against a fixed datum. Inundation is how deep the water gets on normally dry ground, in feet above ground level, and it is the number NHC puts in its advisories. At the shoreline, NHC treats ground above Mean Higher High Water, roughly the normal high-tide line, as dry land, which is why its gauge readings are given in feet above MHHW.

None of the three includes waves, and on an open beach the waves can matter as much as the surge. In Opal in 1995, the tide gauge at the Panama City Beach pier peaked at about 8.3 feet above mean sea level, while the debris line at the end of the pier was about 18 feet: NHC's report says breaking waves on top of the surge added about 10 feet. Older reports also use different measures from newer ones, so a number from 1995 or 2004 is not directly comparable with one from 2020. The sections below say which is which.

Why this coast piles water up

NHC lists what sets surge at any spot: storm intensity, forward speed, size, angle of approach, and the shape of the coastline and sea floor. On the last of those it singles out this region. A wide, gently sloping shelf lets wind stack water up instead of letting it circulate down and away, and NHC names the Gulf coast, especially Louisiana and Mississippi, as particularly vulnerable because the ocean floor deepens so gradually offshore.

Then the bays take over. NHC notes that surge depends heavily on local features such as barrier islands, inlets, sounds, bays and rivers, and that it runs higher on a coastline that curves inward. This coast has all of them, in a few dozen miles.

Mobile Bay is a funnel

USGS describes Mobile Bay as about 32 miles long, about 23 miles wide near its opening to the Gulf, narrowing to about 10 miles at the city of Mobile, and remarkably shallow, averaging 10 feet deep. Its long axis runs almost due north. A south wind on the right side of a storm blows straight up that length, and the narrowing, shallow bay stacks the water higher toward Mobile, the Causeway and the Delta than at the mouth.

Zeta in 2020 shows it in gauge readings. The open-coast gauge at Dauphin Island peaked at 2.4 feet above MHHW. Inside the bay, the West Fowl River Bridge gauge reached 5.9 feet and Coast Guard Sector Mobile 5.3 feet, with 4.2 feet at Weeks Bay on the east side. NHC put inundation along Mobile Bay at 3 to 6 feet above ground, reported the U.S. 90 Causeway completely flooded and part of Water Street in downtown Mobile underwater, and found debris lines as high as 7 feet in Daphne and Spanish Fort that likely included waves. Zeta came ashore in southeastern Louisiana; the bay was on its east side.

Nate in 2017 did it again as a Category 1 that made its final landfall near Biloxi. NHC estimated 4 to 7 feet of inundation in Mobile Bay, with 5.6 feet above MHHW at the West Fowl River Bridge and 5.4 at Coast Guard Sector Mobile, against 3.3 feet at Dauphin Island.

Bigger storms push it much further. Katrina came ashore at the mouth of the Pearl River on the Louisiana-Mississippi line, and NHC's report still found 8 to 12 feet of storm surge along Mobile Bay, particularly the northern and western shores, flooding several miles inland, plus 10 to 15 feet along coastal Mobile County including Dauphin Island, where the surge cut a new channel through the island's western end. Georges in 1998, which made landfall near Biloxi, produced preliminary storm surge estimates of 5 to 10 feet in Mobile County and 7 to 12 feet in Baldwin County, with 9.3 feet in west Mobile Bay and 11.9 at Fort Morgan.

And the funnel drains

Put the center east of the bay and the wind on the bay comes from the north, off the land. Sally in 2020 came ashore at Gulf Shores, just east of the bay's mouth. Most gauges around Mobile Bay peaked only 1 to 2 feet above MHHW, and NHC notes that 3 feet of inundation occurred in most of its storm surge warning area except inside Mobile Bay.

Then the bay emptied. Once Sally's winds turned out of the north, nearly every tide gauge around Mobile Bay fell below Mean Lower Low Water, the datum nautical charts use for depth. At Coast Guard Sector Mobile the water dropped about 10.6 feet in roughly 20 hours to 7 feet below MLLW, exposing parts of the bay floor, and some gauges went lower than their instruments could measure.

The lesson for the Eastern Shore and downtown Mobile is that a few miles of track decide whether the bay floods or drains, and the forecast cannot always pin that down two days out. And if you see the bay floor, stay off it. The water is coming back when the wind swings around.

Pensacola Bay, Perdido Bay and Santa Rosa Sound

East of Mobile Bay the shape changes. Perdido Key and Santa Rosa Island shelter a maze of back water: Big Lagoon and Perdido Bay, Pensacola Bay with Escambia, East and Blackwater bays at its head, and Santa Rosa Sound running east toward Fort Walton Beach, all draining to the Gulf through a few passes. Surge pushed in through those passes spreads up the bays, and while it holds the passes high, rain has nowhere to go.

Sally is the clearest example, because the whole system sat on its east side. NHC found 5 to 7 feet of inundation along the coasts of Baldwin, Escambia and Santa Rosa counties, the Pensacola tide gauge peaked at 5.6 feet above MHHW, and NHC's model put the upper reaches of Escambia and Blackwater bays at 6 to 7 feet in places with no gauges. Surge and waves cut three new channels through Johnson Beach on Perdido Key. NHC says the surge kept Sally's rain, up to 29.99 inches at Orange Beach, from draining into rivers, bays and the Gulf, which drove the flash and river flooding that followed. A woman near Innerarity Point died after rising surge overtook her car.

Ivan in 2004 came ashore just west of Gulf Shores as a 120 mph Category 3 with an eye 40 to 50 nautical miles across. NHC reports 10 to 15 feet of storm surge from Destin west to Mobile Bay and Baldwin County. Waves on top of that surge dropped as much as a quarter mile of the Interstate 10 bridge system across Pensacola Bay into the water and heavily damaged the U.S. 90 causeway across the north end of the bay. Perdido Key, NHC wrote, was essentially leveled.

Dennis, ten months later, shows why size matters. It also came ashore at 120 mph, on Santa Rosa Island about 10 miles west of Navarre Beach, but NHC notes its hurricane-force winds covered only a small area near the eye. Its storm surge was 6 to 7 feet near landfall, overwashing Santa Rosa Island near and west of Navarre Beach, and 3 to 5 feet along the Alabama coast. Same wind speed, roughly the same stretch of coast, about half Ivan's water. NHC's surge overview gives the reason: a larger storm pushes on more ocean, for longer.

The Mississippi Sound and its back bays

USGS describes the Mississippi Sound as about 80 miles long and 4.5 to 14 miles wide, but only about 10 feet deep, bordered on the south by Cat, Ship, Horn, Petit Bois and Dauphin islands. The islands break waves; they do not stop surge. A shallow sound that wide gives an onshore wind a long, shallow run to push water toward the mainland and into the bays and bayous behind it.

Katrina is the benchmark. Its final landfall was at the mouth of the Pearl River, putting the whole Mississippi coast on its east side. NHC's report has 24 to 28 feet of storm surge across a swath about 20 miles wide centered on St. Louis Bay, taking in Waveland, Bay St. Louis, Pass Christian and Long Beach, with a high water mark of 27.8 feet at Pass Christian, and 17 to 22 feet from Gulfport to Pascagoula. The surge reached at least six miles inland in many places, up to 12 miles along bays and rivers, and crossed Interstate 10 in many locations. Katrina had weakened from Category 5 to 3 before landfall; NHC attributes the surge mainly to the storm's huge size.

The back bays can run deeper than the beach. In Zeta, a USGS gauge on Old Fort Bayou in Ocean Springs reached 9.5 feet above MHHW and a Biloxi back bay gauge 7.2 feet, and NHC's hindcast put up to 10 feet above ground in the back bay around St. Martin, while the Bay Waveland Yacht Club gauge read 8.2 feet and the Pascagoula NOAA Lab 7.1. Speed can flip that. Nate came through fast, and NHC's report says its short-lived onshore winds probably left the open coast higher than the back bays, with up to about 9 feet above ground along the shore near Biloxi and Ocean Springs.

Two of those storms were only Category 1 at landfall in this area. Nate made its final landfall near Biloxi at 75 mph and NHC found 6 to 9 feet of inundation near and east of it. Isaac in 2012, a Category 1 that came ashore in Louisiana, produced 5 to 9 feet of inundation in Mississippi, with surge reaching north of Interstate 10 in Hancock and Harrison counties.

The right side of landfall

Hurricane winds blow counterclockwise. In a storm moving north, the east side gets wind from the south, straight off the Gulf, and the storm's forward motion adds to it, which is why NOAA says the strongest winds are nearly always on the right side of the track. On this coast that south wind blows straight up north-facing bays. The west side gets wind from the north and east, off the land, which can push water out.

Sally drew the line sharply. NHC found 5 to 7 feet above ground east of its Gulf Shores landfall, along Baldwin, Escambia and Santa Rosa counties; 3 to 5 feet west of it along the Alabama and Mississippi coasts and in southeastern Louisiana; and a drained Mobile Bay in between. Michael in 2018 shows the same split from the other end. It came ashore near Mexico Beach and Tyndall Air Force Base, and NHC estimated up to 14 feet above ground at Mexico Beach but only 2 to 4 feet to the west, from west of Panama City to near Pensacola.

In practice: if the forecast track brings the center ashore to your west, plan on the deep water, even if your town is not in the middle of the cone. If it comes ashore to your east, you may get less surge or a blowout, and the forecast can still shift you to the wrong side in a day.

Timing with the tide

NOAA notes that the Gulf of Mexico is one of the places with only one high and one low tide a day. The tide here is also small: NOAA's great diurnal range, the average difference between the day's high and low, is about 1.2 to 1.3 feet at Pensacola and Dauphin Island and about 1.5 to 1.8 feet at Pascagoula, Biloxi, Bay Waveland, Weeks Bay, Coast Guard Sector Mobile and the Mobile State Docks.

That cuts both ways. A surge of 5 or 10 feet dwarfs the tide, so timing is not the whole story the way it is on coasts with big tides. But NHC's advisory ranges assume the peak surge arrives at high tide, and a foot and a half is still a foot and a half in a living room that sits just above the line. Look up when the high falls at your nearest station while the storm is still days out.

Forward speed matters more. NHC's surge overview says a faster storm builds higher water at the immediate coast while a slower one drives it farther inland, and slow storms hold water up across many tide cycles. Sally crawled ashore, and a USGS gauge on the Aucilla River, nearly 200 nautical miles east of its landfall, climbed for three days to 5.8 feet above MHHW. Long, slow events are also when surge and rain combine, as they did in Sally's back bays.

How to read NHC's surge products

• Storm Surge Watch: life-threatening inundation from rising water moving inland from the shoreline is possible somewhere in the area, generally within 48 hours. Storm Surge Warning: the same danger, generally within 36 hours. Either can be issued earlier if tropical-storm winds will cut short the time to evacuate, and for places that may not flood but could be cut off by flooding around them. NHC uses 3 feet of inundation as a first-cut threshold for these.
• Peak Storm Surge graphic: the inundation ranges from the public advisory, mapped along the immediate coast. Each range is the height water could reach above normally dry ground somewhere in that stretch, if the peak comes at high tide. Colors follow the top of the range. It can change every three hours, with full and intermediate advisories.
• Potential Storm Surge Flooding map: an interactive map of how deep water could get on land, including inland along bays and rivers. It is built from hundreds of model runs and shows the level with about a 1-in-10 chance of being exceeded, a reasonable worst case. It already accounts for the tide and land elevation, but not waves, rain or river flooding. It usually first appears with the first storm surge watch and posts an hour or more after each full advisory.
• National Storm Surge Hazard Maps: the planning tool for quiet weather, showing near worst-case inundation for Category 1 through 5 hurricanes. Use it to learn what your address could see before there is a storm.
• Evacuation zones: drawn by your county around storm surge risk. Escambia County's zone lookup even shows simulated surge for each zone. If your zone is called, the reason is water, and a strong house does not change that.

What to take from the history

• Category is not the surge number. Nate and Isaac were Category 1 storms and put 5 to 9 feet on the Mississippi coast. Katrina weakened before landfall and still produced the deepest surge in any of these reports, because it was huge.
• Surge forecasts move with the track. Sally's first NHC forecast for the Alabama coast was 2 to 4 feet; it rose to 4 to 7 as the track shifted east, and the extreme western Florida Panhandle forecast did not reach 4 to 7 feet until just before landfall. Plan for the range NHC gives you and the possibility it grows.
• In Zeta and Nate the head of Mobile Bay ran 2 to 3 feet higher than Dauphin Island, and in Sally the bay drained when the wind blew offshore. Know which side of the track you are on.
• In the back bays of Pensacola and the Mississippi Sound, surge and rain arrive together. Clear drains early and expect low streets to flood from rain even where surge never reaches.
• Leave the islands and low ground while the bridges are open. Never drive into water over the road.