Flood Science Storms

The Alabama-Florida Hurricane of 1932 May Provide Insights on Laura’s Flood Potential

|

August 21, 2020

Hurricane Laura threatens to impact the northern Gulf Coast next week. While uncertainty for Laura’s track and intensity remains relatively high, it could impact locations from southeast Louisiana through the Florida Panhandle with hurricane conditions. The latest forecast from the National Hurricane Center has moved the track slightly west, but we can expect considerable changes in the track forecast over the next several days.

National Hurricane Center track map for Laura. The storm is forecast to become a hurricane in the Gulf of Mexico early next week. Source: National Hurricane Center, Friday, August 21, 2020 at 4PM EDT.

As we look to the past for guidance on the future, we may find helpful insights from a hurricane that made landfall along the Alabama Coast on September 1, 1932.

The 1932 storm has much in common with Laura. It passed north of Hispanola and Cuba on a northwestward track, before crossing Florida’s Everglades as a strong tropical storm. Laura is forecast to pass through the Florida Straits with similar intensity on Monday

Track of the 1932 Hurricane. The storm crossed the extreme southern tip of Florida as a tropical storm (green) and then strengthened into a hurricane in the eastern Gulf of Mexico. Hurricane tracking map from NOAA Historical Hurricane Tracks. Link: www.coast.noaa.gov/

Once in the eastern Gulf of Mexico, the 1932 storm continued tracking northwest, while strengthening into a hurricane. It made landfall as a category-1 hurricane near Dauphin Island, Alabama, in the first hours of September, 1932. The storm continued to track to the northwest, passing just west of Mobile, Alabama.

Although Laura’s forecast remains somewhat uncertain, its track, intensity and angle of approach to the coast may have similarities to the hurricane of 1932.

FLOOD IMPACTS IN 1932

As the 1932 Hurricane tracked to the northwest, noticeable flooding began impacting the Florida Panhandle and Big Bend region. Storm surge levels in Apalachicola quickly rose to 4.5 feet by 200PM on August 31, before leveling off (HURDAT 2013). According to the St. Petersburg Times (September 1, 1932), “Flood tides inundated several streets near the Apalachicola waterfront and property in the vicinity of the docks was extensively damaged.”

More than six hours later, the worst storm conditions struck Pensacola, where 3.0 feet of salt water rushed into the bay (NOAA Tides and Currents 2020). Fortunately, property damage was limited because of the city’s high shore line.

Apalachicola’s waterfront flooded during the 1932 Hurricane. The shallow water depth and concave coastal shape makes this section of coast vulnerable to salt water flooding.
Image: https://www.floridamemory.com/items/show/57597

The careful observer will note that storm surge levels in Apalachicola exceeded those in Pensacola, even though the storm tracked closer to Pensacola. This observation fits with the long-term pattern of excessive flood vulnerability near the eastern end of Florida’s Big Bend and Apalachee Bay, caused by the concave shape of the bay and shallow water depth.

However, closer to the storm’s landfall location, along the Alabama coastline, damage was more even more extensive. The Mobile Register’s news from September 2, 1932, has much to say about this storm.

Mobile, the largest metro area along the Alabama Coast, observed flooding from heavy rainfall. It reports, “Street car traffic in Mobile was crippled last night, due to high water from the heavy rains. Hip-deep water was reported in many sections of the city.”

Drainage was likely impeded by storm surge in the bay, a phenomenon known as compound flooding. The Mobile Register continues, “Water from Mobile River began backing up early Thursday morning when it began slapping about the wharf.”

Substantial flooding was also observed farther south, along the western shore of Mobile Bay. A store in Bayou La Batre reported eight inches of water in it, while another store in Coden also reported flooding. Water reached three feet above the ground in a village called Herron Bay, north of Dauphin Island (Mobile Register 1932). We estimate that storm surge levels in the western portion of Mobile Bay reached approximately 6.5 feet, based on this observation of three feet of inundation above ground level at Herron Bay.

High water also impacted marine and transportation infrastructure along Mobile Bay. Wharves in the Bay were washed away and flood waters washed over the causeway of the Cochrane Bridge.

The Cochrane Bridge. Image source: https://bridgehunter.com/al/mobile/cochrane/

CONTEXT FOR HURRICANE LAURA

Comparing flood potential for an active storm to a historical storm can provide valuable insights and sometimes reveal weaknesses in the forecast models. However, we must temper our conclusions with a few important points:

1) Laura is still a developing tropical cyclone in the northern Caribbean, and uncertainty in the track and intensity forecasts remains relatively high. Considerable forecast changes could occur between now and the time of landfall. Generally, the most vulnerable areas to storm surge flooding along the north central and north east Gulf of Mexico include the Mississippi Coast west of Biloxi, Apalachee Bay, east of Apalachicola, and several of the large bays in between, including the interior portions of Mobile Bay and Pensacola Bay.

2) Sea levels have risen noticeably since 1932. The NOAA Sea Level Trends website provides a historical, long-term trend of 1.29 feet of sea level rise per 100 years at Dauphin Island, Alabama. Considering this rate of change, sea levels near the mouth of Mobile Bay would be 1.14 feet higher than they were 88 years ago, adding extra inundation if a carbon copy of the 1932 Hurricane hit today.

The NOAA tide gauge at Dauphin Island measures long-term sea level trends along the Alabama Coast. Image: NOAA Tides and Currents website.

3) Storm characteristics, like forward speed and storm size, also impact storm surge levels. While the forward motion of the 1932 Hurricane may have been typical for hurricanes in the northern Gulf, the storm size was smaller than average, according to the account from The Monthly Weather Review (1932). Smaller storms push less saltwater than large storms, so Laura’s size should be considered before making comparisons.

Storm surge map for 1932 hurricane

Nonetheless, historical comparisons bring past storms to life and help convince people of their flood risk because they can look back at a flood event that actually happened. If the 1932 Hurricane put hip-deep water in Mobile and three feet of water above ground level along the western shore of Mobile Bay, surely Laura has at least the same potential, given heightened sea levels and warmer waters that feed more rainfall.

Given the track and intensity forecast, coastal residents from New Orleans to Panama City, should monitor the progress of Laura and prepare for potential hurricane impacts next week.

REFERENCES

HURDAT, (2013): Master Metadata file for HURDAT. Link: http://www.aoml.noaa.gov/hrd/hurdat/metadata_master.html

Mobile Register, September 2, 1932, Mobile, Alabama.

Monthly Weather Review, 1932: Link: aoml.noaa.gov/general/lib/lib1/nhclib/mwreviews/1932.pdf

National Oceanic and Atmospheric Administration (NOAA) Sea Level Trends Website. Link: tidesandcurrents.noaa.gov/sltrends/.

NOAA Tides and Currents, 2020: National Archive of Coastal Water Levels. Link: www.tidesandcurrents.noaa.gov.

St. Petersburg Times, September 2, 1932, St. Petersburg, Florida.